BERKELEY 

\RY 

UNIVERSITY   OF 
V        CALIFORNIA 


A  TEXTBOOK  OF 
GEOLOGY 


BY 


AMADEUS   W.   GRABAU 

\\ 

S.B.,  MASS.  INST.  OF  TECHNOLOGY;  S.M.,  S.D.,  HARVARD 

PROFESSOR     OF      PAL/EONTOLOGY      IN     THE      GOVERNMENT     UNIVERSITY 

OF   PEKING,    CHINA,    AND    PALEONTOLOGIST   TO  THE 

CHINESE   GEOLOGICAL   SURVEY 

FORMERLY     LECTURER     IN     MINERALOGY     AND     IN     GEOLOGY     IN    TUFTS 

COLLEGE,   PROFESSOR    OF    MINERALOGY    AND    GEOLOGY    IN 

THE  RENSSELAER   POLYTECHNIC   INSTITUTE,   AND 

PROFESSOR     OF     PALEONTOLOGY    IN 

COLUMBIA  UNIVERSITY 

AUTHOR  OF  "PRINCIPLES  OF  STRATIGRAPHY,"  "GEOLOGY  OF  THE  NON- 
METALLIC    MINERAL    DEPOSITS    OTHER    THAN    SILICATES,"   "  NORTH 
AMERICAN    INDEX    FOSSILS "    (WITH   H.  W.    SHIMER),    "GUIDES 
TO    THE   GEOLOGY    AND    PALEONTOLOGY    OF    NIAGARA 
FALLS,  —  OF  EIGHTEEN   MILE  CREEK,  —  AND  OF 
THE    SCHOHARIE    REGION,"    ETC.,    ETC. 


PART   I 
GENERAL   GEOLOGY 


D.   C    HEATH   &   CO.,    PUBLISHERS 

BOSTON          NEW  YORK         CHICAGO 


COPYRIGHT,  1920, 

BY  D.  C.  HEATH  &  Co. 

2iO 


DEDICATED 

TO 
MY    FORMER    STUDENTS 

WHO 

IN   THE   NEW   WORLD    AND   THE   OLD 

ARE  TRANSMITTING,    AUGMENTING,    AND    APPLYING 

THE   KNOWLEDGE   OF   THE 

EARTH    SCIENCE 

IN   THE    ACQUIRING   OF   WHICH 

IT    HAS    BEEN  MY    PRIVILEGE 

TO    AID    THEM 


438937 


PREFACE 

IN  the  preparation  of  this  book,  I  have  departed  somewhat  widely 
from  the  prevailing  order  of  treatment  in  current  texts.  Instead  of 
beginning  with  the  destruction  of  rocks,  it  has  seemed  more  logical  to 
give  the  student  some  knowledge  of  the  rocks  to  be  destroyed,  and  of 
their  character  and  origin.  Instead  of  treating  clastic  rocks  first  and 
igneous  and  other  non-clastic  rocks  later,  it  has  seemed  more  desir- 
able to  begin  with  those  rocks  from  which  elastics  are  largely  derived, 
before  dealing  with  the  elastics  themselves.  Twenty  years  of  experi- 
ence as  a  teacher  have  convinced  me  that  the  average  student  admitted 
to  courses  in  geology  receives  too  little  instruction  in  minerals,  and  al- 
though we  generally  recommend  mineralogy  as  a  desirable  prerequisite, 
few  teachers  can  insist  upon  a  preparation  in  this  subject  on  the  part 
of  the  student.  Yet  without  a  knowledge  of  at  least  some  minerals  the 
study  of  rocks  is  impossible,  and  few  geological  phenomena  can  be  ade- 
quately understood  without  at  least  a  general  knowledge  of  the  rocks 
which  they  affect.  Students  who  are  preparing  to  make  geology  their 
life  work,  will  in  any  case  undertake  a  more  extended  study  of  minerals, 
and  they  will  turn  to  the  excellent  textbooks  in  that  science  now  avail- 
able, and  some  of  which  are  listed  on  page  51.  But  the  great,  majority 
of  students  of  geology  come  to  this  subject  only  with  the  desire  to  gain 
some  knowledge  of  the  world  they  live  in,  of  the  material  of  which  it  is 
composed,  of  the  forces  which  have  fashioned  it,  and  of  the  laws  which 
have  governed  its  development.  They  may  do  so  from  a  desire  to  master 
the  secrets  of  nature  for  the  material  benefits  to  be  derived  from  such  a 
mastery,  or  for  the  power  which  such  a  knowledge  will  confer  upon  them ; 
or  they  may  undertake  the  study  of  the  earth,  because  they  wish  to 
broaden  their  mental  horizon  and  subject  themselves  to  that  stimula- 
tion of  the  intellect,  that  deepening  of  spiritual  perceptions,  and  that 
awakening  of  dormant  faculties,  which  others  have  found  in  a  sym- 
pathetic understanding  of,  and  love  for,  the  out-door  world,  and  which, 
in  its  fullest  measure,  is  most  frequently  vouchsafed  to  the  student  of 
geology.  From  whatever  motive  the  student  approaches  the  subject, 
he  should  be  made  to  realize  that  his  desires  can  best  be  attained, 
if  he  keep  in  mind  the  maxim  of  La  Rochefoucauld,  "Pour  bien  savoir 
une  chose  il  faut  en  savoir  les  details."  Detail  does  net  appeal  to  the 


vi  Preface 

average  student,  but  a  knowledge  of  a  certain  amount  of  detail  is  neces- 
sary in  any  subject,  if  it  is  to  be  well  understood,  and  in  geology,  as  in 
other  sciences,  no  real  understanding  of  principles  and  of  phenomena 
is  possible  without  some  conscientious  devotion  to  detail.  The  book 
of  nature  is  unsealed  only  to  him  who  is  willing  to  learn  the  language 
in  which  it  is  written. 

I  believe  that  at  the  very  outset  the  student -of  the  earth  should  sub- 
ject himself  to  a  moderate  discipline  in  the  elements  of  chemistry  and 
mineralogy  at  least.  To  those  who  can  not  devote  the  time  to  sepa- 
rate courses  in  these  sciences,  Chapter  IV  may  serve  as  guide  for  a  series 
of  laboratory  exercises,  which  may  be  carried  on  simultaneously  with 
the  study  in  the  classroom  of  the  more  general  subjects  treated  in  the 
first  three  chapters.  In  all  such  studies  the  individual  teacher  must 
select  and  amplify  the  subject  matter;  the  text  of  Chapter  IV  is  in- 
tended primarily  as  a  guide,  while  to  the  student,  already  familiar  with 
minerals,  it  may  serve  as  a  summary  for  convenient  reference.  In 
its  preparation,  and  especially  in  the  selection  of  the  mineral  species  in- 
cluded in  the  tables,  I  have  had  the  generous  advice  of  the  late  lamented 
Professor  Alfred  Moses  of  Columbia  University,  and  that  of  Professor 
L.  Luquer  of  the  same  institution,  while  the  experience  gained  in  teach- 
ing mineralogy  for  a  number  of  years  at  the  Rensselaer  Polytechnic 
Institute,  at  Tufts  College  and  at  the  Museum  of  the  Buffalo  Society 
of  Natural  Sciences  has  been  drawn  upon.  For  reasons  already  set 
forth,  I  have  next  taken  up  the  treatment  of  the  igneous  rocks.  Many 
years  of  teaching  elementary  students  both  in  college  and  in  the  summer 
sessions  has  satisfied  me  that  while  the  study  of  rocks  is  carried  on  in 
the  laboratory  the  broader  relations  of  those  rocks  are  advantageously 
treated  at  the  same  time  in  the  lectures  or  classroom  exercises.  There- 
fore I  would  recommend  that  Chapter  VI  be  used  entirely  as  a  labora- 
tory text,  while  Chapters  VII  to  IX  inclusive  serve  for  simultaneous 
classroom  exercises,  of  course  with  proper  material  for  illustration. 
Chapter  X  is  again  best  treated  as  a  laboratory  text  on  aqueous  (chem- 
ical) deposits,  and  here  again  each  teacher  will  make  his  selection  of 
material  as  time  and  equipment  permit.  Chapter  XI  serves  as  the 
accompanying  text  for  lectures  and  classroom  exercises,  amplified  and 
illustrated  according  to  the  teacher's  predilections  and  equipment. 

It  has  been  my  experience,  as  no  doubt  it  has  been  that  of  many  other 
teachers,  that  the  study  of  rocks  of  organic  origin  requires  some  under- 
standing on  the  part  of  the  student  of  the  types  of  organisms  which 
are  active  in  their  production,  and  that  the  study  of  organic  types 
should  not  be  relegated  exclusively  to  the  chapters  on  historical  geology. 
I  therefore  venture  to  hope  that  the  introduction  of  illustrations  of  the 
various  types  of  rock-forming  organisms  will  be  welcomed  by  the  teacher, 


Preface  vii 

and  be  of  value  to  the  student.  Wherever  possible  I  have  selected 
illustrations  of  material  easily  obtainable,  as  in  the  case  of  the  mol- 
luscan  shells  and  several  of  the  nullipores,  which  are  common  on  our 
Atlantic  coast,  and  in  our  rivers  and  lakes,  and  in  that  of  the  bryozoans 
and  echinoderms  (except  the  crinoids)  which  are  easily  obtained  on  our 
coasts.  Most  of  the  corals  too  are  the  common  species  found  in,  or 
easily  added  to,  any  collection.  It  is  recommended  that  Chapter  XII 
be  used  both  as  classroom  and  laboratory  text.  The  same  thing  ap- 
plies to  Chapter  XIII,  which  deals  chiefly  with  the  plants  that  enter 
into  the  construction  of  our  peat  and  coal  deposits,  and  specimens  of 
these  can  readily  be  obtained  by  any  one.  Here,  too,  laboratory  work 
with  specimens  and  classroom  exercises  may  go  hand  in  hand. 

The  study  of  clastic  rocks  per  se,  treated  in  Chapter  XVIII,  may 
again  be  carried  on  in  the  laboratory  during  the  simultaneous  treat- 
ment of  the  forces  which  produce  them  and  the  phenomena  of  weather- 
ing, erosion,  transportation,  and  deposition  (Chapters  XV-XVII)  in  the 
classroom  and  field.  Although  I  do  not  suggest  that  these  subjects  be 
entirely  ignored  during  the  earlier  part  of  the  course,  where  the  teacher 
may  point  out  phenomena  of  weathering,  stream  and  wave  erosion, 
etc.,  especially  during  field  exercises,  I  believe  that  systematic  study 
is  most  satisfactorily  deferred  until  non-clastic  rocks,  igneous  phenom- 
ena, and  other  forces  which  produce  material  from  which  clastic  rocks 
may  in  the  first  place  be  derived,  have  been  considered.  Other  teachers 
may  not  agree  with  me  and  they  may  find  it  desirable  to  transpose  the 
chapters.  I  have  endeavored  so  to  arrange  the  chapters  that  little 
if  anything  will  be  lost  in  effectiveness  by  their  transposition,  and  to 
this  end  I  have  given  repeated  cross  references.  Therefore  if  the  teacher 
so  desires,  he  may  pass  from  Chapter  III  directly  to  Chapter  XV,  taking 
up  the  intervening  chapters  in  any  order  he  deems  fit. 

I  believe,  however,  that  most  teachers  will  agree  with  me  that  the 
broader  subject  of  the  sculpturing  of  the  lands  (Chapters  XXII-XXIII, 
the  essentially  physiographic  chapters)  are  best  treated  after,  not  only 
the  formation  and  original  structural  characters  of  the  earth's  crust, 
but  also  its  deformations  have  been  studied.  Although  metamorphism 
by  igneous  contact  has  been  treated  in  connection  with  the  igneous 
phenomena,  where  it  belongs,  the  main  subject  of  metamorphism  and 
metamorphic  rocks  (Chapter  XX)  follows  the  treatment  of  deforma- 
tions (Chapter  XIX)  and  may  likewise  be  treated  largely  as  a  labo- 
ratory text. 

Wherever  possible,  I  have  adopted  the  method  of  treating  typical 
examples  of  geological  phenomena  in  some  detail,  with  an  abundance 
of  illustrations,  because  in  my  own  experience,  I  have  found  that  the 
student  grasps  the  details  of  isolated  examples  more  readily  and  more 


viii  Preface 

completely  assimilates  them  than  he  does  generalizations  with  illus- 
trations drawn  from  many  sources,  and  from  examples  which  are  to 
him  little  more  than  names.  On  this  account  I  have  devoted  more 
space  than  is  usually  given  in  textbooks  to  descriptions  of  well-known 
volcanoes,  such  as  Vesuvius,  Etna,  etc.,  to  a  few  typical  glaciers  such 
as  the  Aletsch,  the  Mer  de  Glace,  the  Malaspina,  etc.,  to  the  Florida 
reefs  and  the  Great  Barrier  reef,  and  to  such  special  examples  of  com- 
plex rivers  as  the  Niagara,  the  Genesee,  and  the  Colorado.  My  choice 
of  the  types  has  been  partly  influenced  by  their  accessibility,  and  it 
is  my  hope  that  the  study  of  such  accessible  examples  will  awaken  in 
the  student  the  determination  to  study  them  in  the  field,  a  possibility 
any  one  may  look  forward  to  in  these  days  of  comparative  lessening 
of  distances.  The  same  motive  of  treatment  of  types  rather  than  facts 
and  phenomena,  has  prompted  me  to  devote  what  some  may  consider 
an  undue  amount  of  space  to  the  details  of  the  great  historic  earth- 
quakes. I  believe,  however,  that  the  human  interest  which  these  phe- 
nomena have,  will  appeal  to  the  student,  and  he  will  duplicate  my  own 
experience,  that  such  a  narrative  will  give  him  a  deeper  knowledge  of 
seismic  phenomena  than  a  categorical  treatment  of  them  could  inculcate. 

After  due  consideration  and  conference  with  the  publishers,  I  have 
decided  to  use  the  old  style  of  endings  for  the  geological  systems  and 
periods,  i.e.,  Cambrian,  Ordovician,  Silurian,  etc.,  instead  of  the  more 
precise  and  uniform  ending  in  ic  (Cambric,  Ordovicic,  Siluric,  etc.) 
which  I  have  always  maintained,  and  still  believe,  to  be  superior,  not 
euphonically,  but  because  they  give  uniformity  to  the  terminology  which 
has  the  hit  and  miss  characteristics  of  a  language  grown  up  uncon- 
trolled, and  which  is,  therefore,  not  scientific.  I  am  confident  that  in  the 
future  we  shall  adopt  the  system  of  uniform  endings,  and  in  my  scientific 
writings  I  shall  continue  to  use  it  where  I  can  persuade  the  editor  to 
permit  it.  In  the  present  case,  however,  as  the  majority  of  teachers 
cling  to  the  old  terminology,  I  have,  though  with  some  reluctance, 
adopted  it,  more  especially  in  view  of  the  fact  that  it  would  make  the 
use  of  the  book  more  difficult  if  it  did  not  conform  to  the  language  em- 
ployed by  the  teacher.  I  desire,  however,  to  have  it  distinctly  under- 
stood that  I  have  not  surrendered  my  belief  in  the  superiority  of  the 
more  precise  terminology,  and  that  hereafter,  as  in  the  past,  I  shall 
continue  to  advocate  its  use. 

For  substantial  assistance  in  the  preparation  of  this  text  I  am  under 
obligation  to  a  number  of  friends  and  colleagues.  I  have  especially 
enjoyed  the  freely  given  advice  of  my  former  colleagues  at  Columbia 
University.  The  aid  in  the  mineralogical  chapter  has  already  been  re- 
ferred to.  Mr.  Frederick  K.  Morris  of  Columbia  read  the  proof  of 
Chapters  XV  to  XVII  inclusive  and  made  many  valuable  criticisms 


Preface  ix 

and  suggestions.    A  similar  valuable  office  was  performed  by  Professor 

C.  P.  Berkey  for  Chapters  XIX  to  XXI  inclusive,  and  by  Professor 

D.  W.  Johnson  for  Chapters  XXII  and  XXIII.     To  these  gentlemen 
my  best  thanks  are  tendered  herewith,  and  the  assurance  that  much 
that  may  be  valuable  in  these  chapters  is  due  to  their  advice,  while  for 
any  divergence  from  their  views,  especially  in  the  order  and  extent  of 
treatment,  I  take  full  responsibility.     The  entire  proof  was  read  by  Dr. 
Marjorie  O'Connell  of  the  American  Museum  of  Natural  History,  and 
again  by  my  friend  Mr.  Ernest  Welleck  of  the  editorial  staff  of  the 
Popular  Science  Monthly,  for  whose  gratuitously  given  and  extremely 
effective   service   I   gladly  render   full   acknowledgment.     My   former 
student  Miss  Mary  Welleck,  A.M.,  has  been  my  assistant  throughout 
the  arrangement  of  this  text  for  the  press,  and  has  been  of  the  greatest 
service  in  securing  illustrations.     She  has  a  number  of  block  diagrams 
entirely  to  her  credit  as  acknowledged  in  the  text,  and  also  a  number  of 
other  illustrations.     Mr.  Frederick  K.  Morris  has  also  contributed  sev- 
eral of  his  effective  drawings,  and  has  made  special  effort  to  secure 
photographs  as  elsewhere  acknowledged.     To  Miss  Florence  Holzwasser 
of  Barnard,  I  am  also  indebted  for  careful  work  in  comparing  manu- 
script and  copy,  and  to  Messrs.  C.  J.  Connelly  and  F.  K.  Morris  and 
to  Dr.  J.  J.  Galloway  for  reading  part  of  the  text  and  making  suggestions. 

To  Miss  Amy  Hepburn,  Librarian  of  Natural  Science  in  Columbia, 
and  to  the  geological,  botanical,  and  zoological  libraries  of  that  Uni- 
versity I  am  under  special  obligation  for  freely  furnished  material  for 
illustrations.  Prof.  J.  F.  Kemp  has  generously  loaned  his  portrait  of 
Werner.  The  United  States  Geological  Survey  has  furnished  a  large 
number  of  original  photographs  from  which  illustrations  were  made  for 
this  book,  and  Dr.  D.  W.  Johnson  of  Columbia  has  placed  his  entire 
collection  of  photographs  at  my  disposal  for  selection  of  illustrative 
material,  and  I  hereby  record  my  great  indebtedness  to  Dr.  Johnson 
and  to  Dr.  George  Otis  Smith,  Director  of  the  Federal  Survey,  for  their 
courtesy.  To  Dr.  John  M.  Clarke  and  the  State  Museum  at  Albany 
I  am  likewise  under  obligation  for  several  illustrations  in  this  part  of 
the  text  and  for  many  more  in  Part  II..  The  geological  department  of 
Harvard  University  has  generously  loaned  me  a  number  of  photographs 
from  the  Gardner  Collection  through  the  chairman,  Prof.  R.  A.  Daly. 
To  Prof.  J.  B.  Woodworth  of  that  University  I  am  also  indebted  for 
several  original  photographs.  The  Alaska  Engineering  Commission 
has  also  generously  furnished  a  number  of  photographs  of  glaciers 
through  Mr.  W.  A.  Ryan.  Prof.  Elizabeth  Fisher  of  Wellesley  College 
has  furnished  several  very  effective  photographs  taken  by  herself  and 
others  as  acknowledged  in  the  text.  From  Prof.  W.  H.  Sherzer  I  have 
received  prints  of  his  photographs  of  sand  grains  reproduced  in  Grabau 


x  Preface 

and  Sherzer's  Monroe  Formation  of  Michigan.  A  number  of  photo- 
graphs were  taken  by  Dr.  Marcus  I.  Goldman  (U.  S.  G.  S.)  during  a 
trip  with  the  author  in  England  and  Scotland  under  the  guidance  of 
Dr.  Benjamin  Peach.  Several  were  taken  by  Mr.  G.  W.  Stose  (U.  S.  G.  S.) 
during  an  early  trip  in  company  with  the  author,  in  Nova  Scotia.  The 
Philippine  Bureau  of  Science,  at  Manila,  Dr.  Elmer  D.  Merrill,  Di- 
rector, has  generously  furnished  the  fine  photograph  of  Mayon  Volcano 
reproduced  in  the  frontispiece.  Prof.  W.  O.  Crosby  has  furnished  a 
number  of  illustrations  used  by  him  in  his  illustrated  Museum  Guide 
(Dynamical  and  Structural  Geology,  Boston  Society  of  Natural  History, 
1892),  and  a  number  of  photographs  of  geological  features  in  Utah 
have  been  received  from  Prof.  F.  J.  Pack  of  the  University  of  Utah. 
To  all  these  contributors  my  best  thanks  are  given.  To  The  American 
Museum  of  Natural  History  my  thanks  are  gladly  given  for  the  fine 
photograph  of  the  eel-grass  in  the  Annulate  group,  constructed  by 
Dr.  Roy  Miner  and  reproduced  in  Fig.  274,  and  for  the  photographs  of 
the  Spine  of  Pelee  (Figs.  106,  107)  taken  by  Dr.  E.  O.  Hovey.  The 
American  Geographical  Society,  through  its  director  Dr.  Bowman,  also 
generously  loaned  a  number  of  cuts  as  elsewhere  acknowledged,  and  the 
Popular  Science  Monthly  has  furnished  a  number  of  photographs  for 
reproduction.  Others  to  whom  I  am  indebted  for  furnishing  original 
photographs  are  Dr.  C.  P.  Berkey,  Miss  A.  D.  Savage,  Dr.  M.  O'Con- 
nell,  Dr.  Elsworth  Huntington,  the  late  Prof.  C.  S.  Prosser,  Dr.  C.  C. 
Mook,  and  my  brother  Mr.  P.  L.  Grabau.  My  former  student  Dr. 
Bela  Hubbard  has  prepared  a  number  of  photographs  of  rocks  and  rock 
structures  from  original  specimens  and  thereby  put  me  under  great 
obligation.  To  Messrs.  Dodd  Mead  and  Co.,  Ginn  and  Co.,  Henry  Holt 
and  Co.,  The  Macmillan  Co.,  John  Wiley  and  Sons,  and  to  Yale  Uni- 
versity Press,  publishers  of  Military  Geology,  I  am  indebted  for  per- 
mission to  reproduce  illustrations  from  books  published  by  them.  These 
are  acknowledged  in  the  text,  as  are  also  the  sources  of  other  illustra- 
tions,—  especially  Kayser's  Lehrbuch;  Lake  and  Rastall,  Textbook 
of  Geology;  Le  Conte,  Elements  of  Geology;  De  Martonne,  Geographic 
Physique;  Rosenbusch,  Elemente  der  Gesteinslehre ;  LyelTs  Principles; 
Ratzel,  Die  Erde;  Gray's  Botany;  Davis,  Erklarende  Beschreibung 
der  Landformen;  Haug's  Traite;  Merrill's  Contributions  to  the  History  of 
American  Geology;  Verrill  and  Smith,  Invertebrates  of  Vineyard  Sound; 
Binney  and  Gould,  Invertebrates  of  Massachusetts;  and  books  by  Wal- 
ther,  Haas,  Bowman,  Geikie,  Zittel,  Steinmann,  Krummel,  Murray, 
Heim,  Suess,  J.  M.  Arms-Sheldon,  and  others.  To  the  publisher  of  my 
Principles  of  Stratigraphy,  and  of  North  American  Index  Fossils,  Mr. 
A.  G.  Seiler,  I  am  indebted  for  permission  to  reproduce  a  number  of 
illustrations  from  these  works.  Prof.  Moses  generously  permitted  the 


Preface  xi 

reproduction  of  a  number  of  illustrations,  especially  of  crystal  outlines, 
from  his  Elements  of  Mineralogy,  published  by  D.  Van  Nostrand  Co. 
Finally,  my  sincere  thanks  are  due  to  my  publishers,  Messrs.  D.  C. 
Heath  and  Co.,  for  their  generosity  in  giving  me  a  free  hand  in  the 
selection  of  illustrations,  in  placing  no  limit  upon  their  number,  and  in 
furnishing  a  considerable  proportion  of  them. 

NEW  YORK,  June  30,  1920. 


CONTENTS 

CHAPTER  PAGE 

I.    INTRODUCTION         ....  i 

The  Science  of  Geology.    The  Earth  Viewed  as  a  Whole. 

II.     SUBDIVISIONS  OF  THE  SCIENCE  OF  GEOLOGY         .      13 

Subdivisions  of  the  Science  in  its  Comprehensive  Sense. 
Subdivisions  of  the  Science  of  Geology  in  its  More 
Limited  Sense.  Lithology  or  the  Study  of  the  Litho- 
sphere. 

III.  METHODS  OF  APPROACH  IN  THE  STUDY  OF  THE 

EARTH 24 

The  Rise  of  Geological  Observation  and  Interpretation. 
The  Field  of  Geological  Observations.  The  Impor- 
tance of  Geological  Literature. 

IV.  MATERIAL  OF  THE  EARTH'S  CRUST       ...      38 

The  Chemical  Elements  and  Their  Primary  Combinations. 
Minerals. 

V.    ROCKS,    THEIR    CLASSIFICATION   AND    PRINCIPAL 

TYPES 64 

Definitions.  Age  Relations  of  Rocks.  Bed-Rock  and 
Mantle-Rock.  Classifications  of  Rocks.  The  Unaltered 
or  Little  Altered  Rocks. 

VI.    THE  PRINCIPAL  TYPES  OF  IGNEOUS  OR  PYROGENIC 

ROCKS 84 

The  Igneous  Magma.  Formation  of  Igneous  Rocks  by 
Cooling  of  Magma.  Types  of  Igneous  Rocks  Based  on 
Composition  and  Texture. 

A  VII.     MODERN  VOLCANIC  PHENOMENA    .        .        .        .109 

Distribution,  Classification,  and  Development  of  Vol- 
canoes. Characteristic  Forms  and  Activities  of  Typical 
Modern  Volcanoes.  Classification  of  Volcanoes  Ac- 
cording to  Type  of  Eruption  and  Form.  Geological 
Age  of  Volcanoes  and  Lava  Flows, 
xiii 


xiv  Contents 

CHAPTER  PAGE 

VIII.    STRUCTURAL    CHARACTERS    OF   VOLCANOES,    AND 

OTHER  IGNEOUS  PHENOMENA  .        .        .        .144 

Extinct  Volcanoes.  Extinct  Calderas  and  Sinks.  Volcanic 
Funnels  and  Pipes,  Spines,  Plugs,  and  Necks.  Sheet  La- 
vas Formed  by  Fissure  Eruption.  Minor  Phenomena 
Generally  Associated  with  Closing  Stages  of  Volcanicity. 

IX.  FORM  AND  STRUCTURE  OF  OLDER  IGNEOUS  MASSES  188 

Types  of  Older  Igneous  Masses.  Contact  of  Igneous 
Masses  with  Other  Rocks. 

X.  THE  AQUEOUS  OR  HYDROGENIC  ROCKS    .    .214 

General  Character  and  Varieties.  The  Textures  of  Aqueous 
Deposits.  The  Principal  Types  of  Aqueous  or  Hydro- 
genie  Deposits. 

XI.    MODE    OF    OCCURRENCE    AND    ORIGIN    OF    THE 

AQUEOUS  OR  HYDROGENIC  ROCKS  .        .        .227 

Types  of  Deposits.  Sea-Water  and  the  Evaporation  Prod- 
ucts and  Chemical  Precipitates  Formed  from  It.  Spe- 
cial Conditions  Favoring  Deposition  of  Sea-Salts.  An 
Ancient  Rock  Salt  Deposit  Formed  by  Evaporation  of 
Sea- Water.  Deposits  of  Salt  by  Concentration  in  La- 
goons. Bar  Theory  of  Ochsenius.  Deposition  of  Salt 
in  Inland  Desert  Basins.  Carbonate  of  Lime  Deposits. 
Other  Chemical  Deposits  in  the  Sea.  Chemical  Depos- 
its and  Evaporation  Products  of  Lakes.  Chemical  De- 
posits and  Evaporation  Products  of  Rivers.  Deposits 
by  Springs  and  Underground  Waters.  Mineral  Veins. 

XII.    THE  ORGANIC  OR  BIOGENIC  ROCKS       .  .    269 

Bioliths.  Types  of  Organic  Rocks  or  Bioliths.  Deposits 
of  Carbonate  of  Lime  by  Plants.  Foraminifera  and 
Foraminiferal  Oozes  and  Limestones.  Corals  and 
Related  Reef-Building  Animals.  Characters  and 
Types  of  Modern  Coral  Reefs.  Ancient  Coral  and 
Coralline  Reefs.  Other  Lime-Depositing  Organisms. 
Organic  Deposits  of  Phosphate  of  Lime.  Organic 
Deposits  of  Silica. 

•XIII.  THE  ORGANIC  OR  BIOGENIC  ROCKS:  DEPOSITS 
FORMED  FROM  THE  ORGANIC  TISSUES  OF 
PLANTS  AND  ANIMALS  .  •  328 

Deposits  Formed  from  Vascular  Plants.  Altered  De- 
posits of  Older  Vegetal  Material.  Accumulation  of 


Contents  xv 

CHAPTER  PAGE 

Decaying  Organic  Matter  from  Animal  Tissues,  and 
from  Non- Vascular  Plants.  Alteration  Products  from 
Organic  Slime  Produced  by  Non- Vascular  Plants  and  by 
Animal  Tissues. 

XIV.    ATMOSPHERIC  PRECIPITATES  AND  THEIR  DERIVA- 
TIVES       .  -354 
Types   of   Atmospheric   Precipitates.     Compacting   and 
Modification   of    Snow.     Glaciers.     Ice-Caps.     Conti- 
nental Glaciers.    Icebergs.    Causes  of  Ice  Movement. 

XV.    DESTRUCTION  OF  ROCKS  AND  THE  FORMATION  OF 

CLASTIC  MATERIAL 390 

Agents  Active  in  the  Formation  of  Fragmental  or  Clastic 
Material.  Processes  of  Erosion.  Destructive  Work 
of  the  Atmosphere.  Destructive  Work  of  the  Hydro- 
sphere. Destructive  Work  of  the  Pyrosphere.  De- 
struction of  Rocks  by  Movements  of  the  Earth's  Crust. 
Rock  Destruction  by  Glaciers.  Rock  Destruction  by 
Organisms. 

XVI.    TRANSPORTATION,  SORTING,  AND  DEPOSITION  OF 

CLASTIC  ROCK  MATERIAL        .        .        .        .438 

Agents  of  Transportation  and  Sorting  and  Regions  of 
Deposition.  Transporting  and  Sorting  by  Winds. 
Deposition  of  Wind-Blown  Sands.  Dust  Deposits. 
Transporting  and  Sorting  by  Streams.  River  Deposits. 
Glacial  Transportation  and  Deposition. 

XVII.    TRANSPORTATION,  SORTING,  AND  DEPOSITION  OF 

CLASTIC  MATERIAL  IN  THE  SEA      .        .        .    509 

The  Geographical  Subdivisions  of  the  Sea.  Bathymetric 
Districts  and  Zones.  Waves  and  Currents  of  the  Sea. 
Sources  of  Clastic  Sediments  Deposited  in  the  Sea. 
Transportation  and  Sorting  of  Clastic  Material  in  the 
Sea.  Types  of  Clastic  Deposits  in  the  Sea.  Summary 
of  Structures  of  Marine  Clastics.  Lateral  Changes  in 
Facies  and  Overlap  Relations  of  Marine  Clastics. 

XVIII.    CONSOLIDATION  OF  CLASTIC  MATERIAL;  TYPES  OF 

CLASTIC  ROCKS 563 

Consolidation  of  Clastics.  Classification  of  Clastic  Rocks. 
Structural  and  Other  Characters  Used  in  Defining 
Clastic  Rock  Types.  Varieties  of  Clastic  Rocks. 


xvi  Contents 

CHAPTER  PAGE 

XIX.    DEFORMATION  OF  THE  ROCKS    OF    THE  EARTH'S 

CRUST 582 

Effects  of  Deformation.  Types  of  Deformation  Struc- 
tures. Deformation  by  Folding.  Structures  Due  to 
Folding,  Erosion  and  Renewal  of  Deposition.  The 
Causes  of  Folding.  Deformation  by  Faulting.  Topo- 
graphic Features  Due  to  Faulting.  Minor  Features 
Accompanying  Faulting.  Other  Structures  Produced 
by  Deformation. 

XX.    METAMORPHISM  AND  METAMORPHIC  ROCKS  .        .    642 

Definition  and  Classification  of  Metamorphism.  Activi- 
ties of  the  Agencies  Producing  Metamorphism.  Meta- 
morphic  Structures  and  Textures.  Occurrence  and 
Age  of  Metamorphic  Rocks.  Types  of  Metamorphic 
Rocks. 

XXI.    MOVEMENTS    OF    THE    EARTH'S    SURFACE     AND 

THEIR  GEOLOGICAL  EFFECTS          .        .        .    655 

Sudden  Crustal  Movements  —  Earthquakes.  Great 
Earthquakes  of  Modern  Times.  Summary  of  Phenom- 
ena Due  to  and  Accompanying  Earthquakes.  Slow 
Changes  in  Levels,  Bradyseisms. 

XXII.    THE  SCULPTURING  OF  THE  EARTH'S  SURFACE     .    697 

Initial  Character  of  the  Land  Surface.  The  Erosion 
Cycle  in  the  Sculpturing  of  the  Land  Surface.  The 
Erosion-Cycle  on  a  Simple  Coastal  Plain.  The  Erosion 
Cycle  on  Domes  and  Basins.  The  Erosion  Cycle  on 
Anticlines  and  Synclines. 

XXIII.    THE    SCULPTURING    OF    THE    EARTH'S    SURFACE 

(Continued)       ....  -747 

The  Erosion  Cycle  in  a  Faulted  Region.  Some  Illustra- 
tions of  Complicated  River  Erosion.  Land  Forms 
Due  to  Glacial  Sculpture.  The  Sculpturing  of  the 
Edge  of  the  Land.  Erosion  Forms  Produced  by  At- 
mospheric Agencies. 

INDEX 825 


PART   I 
GENERAL   GEOLOGY 


TABLE  OF  THE  ERAS  AND  PERIODS  OF  GEOLOGICAL  TIME 

PSYCHOZOIC  OR  QUATERNARY  ERA 
Recent  or  Holocene  Period 
Pleistocene  Period 

CENOZOIC  OR  TERTIARY  ERA 
Pliocene  Period 
Miocene  Period 
Oligocene  Period 
Eocene  Period 
Palaeocene  Period 

MESOZOIC  (SECONDARY)  ERA 
Cretaceous  Period 
Comanchean  Period 
Jurassic  Period 
Triassic  Period 

PALAEOZOIC  ERA 
Permian  Period 

Carbonic  or  Pennsylvanian  Period 
Mississippian  Period 
Devonian  Period 
Silurian  Period 
Ordovician  Period 
Cambrian  Period 

PROTEROZOIC  AND  ARCHEOZOIC  ERAS 
Pre-Cambrian  Periods 
(Algonkian  and  Archaean) 


XV  111 


TEXTBOOK  OF  GEOLOGY 

PART   I  — GENERAL   GEOLOGY 

CHAPTER  I 
INTRODUCTION 

THE  SCIENCE  OF  GEOLOGY 

Definition.  —  Geology  is  the  science  of  the  earth  in  all  its  aspects, 
except  those  which  deal  with  the  relationship  of  the  earth  to  other 
planets  and  to  the  sun  of  our  solar  system.  That  aspect  of  the 
earth  properly  belongs  to  the  science  of  Astronomy,  for  our  earth 
is  one  of  the  heavenly  bodies  with  which  that  science  is  concerned. 
It  is  true  that  this  phase  of  the  subject  is  often  treated  by  the  geol- 
ogist under  the  name  of  astronomical  geology,  but  in  this  book  we 
shall  consider  it  as  belonging  primarily  to  the  field  of  astronomy. 
At  the  same  time  we  shall  recognize  the  importance  to  the  student 
of  at  least  a  general  understanding  of  these  astronomical  relations 
of  the  earth,  and  consider  such  an  understanding  a  desirable  pre- 
liminary preparation. 

Derivation  of  Terms.  —  The  term  Geology  is  derived  from  the 
Greek  words  ge  (yrj),  earth,  and  logia  (Aoyox),  science,  or  logical 
discourse.  The  science  of  geology  was  preceded  by  and  in  a  meas- 
ure grew  out  of  the  subject  of  geography,  which  literally  is  the 
description  of  the  earth  (graphein,  ypdfaw,  to  write)  and  was,  of 
course,  chiefly  confined  to  a  description  of  the  earth's  surface  fea- 
tures, and  their  significance  in  terms  of  human  existence.  But 
since  surface  features  are  generally  the  expression  of  underlying 
structure,  and  of  the  geological  history  of  the  region,  it  is  evident 
that  geography  cannot  be  divorced  from  geology,  and  that,  to  no 
inconsiderable  extent,  the  student  of  the  geography  of  a  region  must 
take  account  of  its  geological  structure  and  history. 

Scope  of  Geology.  —  In  its  broadest  sense,  then,  geology  is  the 
science  of  the  earth  and  all  that  pertains  to  it.  It  is  the  study  in 


2  rVn '«  l  Introduction 

detail  of  one  of  the  planets  in  one  solar  system,  by  the  inhabitants 
of  that  planet,  who  are  themselves  a  part  of  it.  If  other  planets 
of  our  solar  system  or  those  of  other  solar  systems  are  inhabited, 
the  study  of  those  planets  by  their  inhabitants  would  correspond 
to  our  geology  —  it  would  be  the  science  of  a  particular  planet. 
By  us  little  can  be  ascertained  regarding  the  structure  and  develop- 
ment of  other  planets,  except  in  an  indirect  way,  —  and  all  investi- 
gations along  such  lines  fall  properly  into  the  domain  of  the  astron- 
omer, though  he  concerns  himself  primarily  with  interrelations  of 
the  heavenly  bodies.  And  thus  we  may  consider  that  the  study 
of  the  physical  universe,  i.e.,  the  cosmos,  which  may  be  called 
the  science  of  cosmology,  has  only  the  two  primary  divisions, 
astronomy  and  geology.  This  may  be  summarized  as  follows : 


COSMOLOGY 

The  science  of  the 

universe 


1.  Astronomy. — The  science  of   all  the  heavenly  bodies, 
including  the  earth,  their  character,  distribution,  interrela- 
tions, movements,  etc.,  and  the  laws  which  govern  them. 

2.  Geology.  —  The  science  which  deals  with  the  material, 
structure,  history,  etc.,  of  one  of  those  bodies,  i.e.,  the  earth. 


Such  a  view  of  the  science  of  geology  gives  it  an  extremely 
comprehensive  scope,  and  we  must  recognize  that  in  ordinary 
parlance  the  term  is  used  in  a  much  more  restricted  sense 
Nevertheless  it  is  desirable  to  take  this  comprehensive  view  at 
the  outset,  and  to  note  the  several  subdivisions  into  which  such  a 
broad  science  naturally  falls,  and  with  all  of  which  the  student 
of  any  one  division  should  have  at  least  a  general  acquaintance. 
We  shall  best  get  the  proper  view-point  by  first  considering  the 
earth  in  its  entirety. 

THE  EARTH  VIEWED  AS  A  WHOLE 

Could  we  view  the  earth  in  its  entirety  from  some  extraterrestrial 
vantage  point,  we  would  recognize  it  as  an  oblate  spheroid,  that  is, 
a  body  approaching  that  of  a  sphere,  but  with  its  polar  diameter 
flattened  and  its  equatorial  belt  somewhat  inflated.  By  measure- 
ment the  polar  diameter  of  the  earth  is  found  to  be  7899.7  miles, 
while  the  equatorial  diameter  is  7926.5  miles.  If  the  observer  in 
extraterrestrial  space  is  sufficiently  removed  from  the  earth's  sur- 
face, that  surface  would  appear  essentially  smooth,  as  does  the 
surface  of  the  moon  to  our  unaided  eyes ;  the  irregularities  which 
the  dweller  on  the  earth  recognizes  as  mountains  and  valleys  would 


The  Earth  Viewed  as  a  Whole  3 

become  of  insignificant  proportions.  Were  we  to  represent  the 
earth  by  an  accurately  scaled  model  10  feet  in  polar  diameter, 
the  equatorial  diameter  would  exceed  the  polar  by  only  a  trifle 
over  ^  of  an  inch,  while  the  highest  mountains  on  the  earth's 
surface  would  form  elevations  on  the  model  less  than  -fa  of  an 
inch  in  height.  Thus  viewed,  the  prominences  which  appear  to 
us  formidable  are  after  all  of  minor  significance,  and  bearing  this 
in  mind,  we  can  understand  that  relatively  slight  bulgings  or  crum- 
plings  of  the  earth's  surface  may  produce  what  to  us  appear  as  great 
elevations.  The  wrinklings  on  the  skin  of  a  drying  apple  consti- 
tute far  more  prominent  elevations  in  proportion  to  the  size  of  the 
apple  than  do  the  highest  mountain  chains  on  the  earth's  surface, 
when  compared  with  the  size  of  the  earth,  while  the  scratches  and 
notchings  formed  upon  the  surface  of  a  glass  marble,  after  a  brief 
play,  form  vastly  greater  depressions,  in  proportion,  than  do  the 
largest  river  canons,  such  as  that  of  the  Colorado,  or  the  deepest 
ocean  depressions.  Thus  when  the  geologist  finds  evidence  that 
the  summit  of  the  high  peaks  of  the  Himalaya  Mountains  or  the 
top  of  the  Apennine  chain  were  once  a  part  of  the  sea-bottom,  he 
no  longer  concludes  that  the  sea  once  covered  these  mountains,  as 
was  formerly  supposed,  but  he  infers  rightly  that  these  mountain 
chains  were  raised  by  a  wrinkling  of  the  earth's  crust  or  by  an 
upward  warping  which  occurred  at  a  time  subsequent  to  that  in 
which  this  region  was  beneath  the  sea.  The  evidence  from  the 
material  of  the  mountain  which  leads  the  geologist  to  such  a  con- 
clusion will  be  discussed  in  subsequent  chapters,  and  the  corrobo- 
rative evidence  from  the  structure  of  the  mountains  will  also  be 
given.  The  chief  lesson  which  it  is  intended  to  impress  upon  the 
student  at  present  is  that  surface  features  are  relatively  unimpor- 
tant when  we  consider  the  earth  as  a  whole,  and  that  elevations  and 
depressions  —  except  those  which  we  recognize  as  continental 
masses  and  oceanic  depressions  —  are  of  local  significance  chiefly, 
and  may  change  from  one  to  the  other  not  once  but  many  times. 

There  rolls  the  deep  where  grew  the  tree. 

O  earth,  what  changes  hast  thou  seen ! 

There  where  the  long  street  roars  hath  been 
The  stillness  of  the  central  sea. 

This  is  more  than  the  expression  of  the  poet's  fancy ;  it  is  a  truth 
which  the  observation  of  the  earth  and  logical  deductions  from 
these  observations  force  upon  the  geologist's  attention  at  every  step. 


4  Introduction 

The  Successive  Inorganic  Shells  or  Spheres  of  the  Earth 

The  Spheres  Open  to  Direct  Study. —  The  observer  from  his 
extraterrestrial  view-point  will,  however,  note  clearly  that  a  large 
part  of  the  earth's  surface  —  to  be  precise,  about  70.8%  of  it  —  is 
covered  with  water.  This  is  the  sea  which  is  divided  into  a  number 
of  oceans  and  surrounds  all  the  lands,  and,  moreover,  forms  a 
continuous  body,  the  surface  of  which  comprises  approximately 
137,070,000  square  miles  or  361.1  million  square  kilometers.  The 
land  has  a  surface  area  of  about  59,870,000  square  miles  or  148.8 
million  square  kilometers,  giving  a  total  surface  area  for  the  earth 
of  196,940,000  square  miles  or  509.9  million  square  kilometers. 
Certain  portions  of  the  land  surfaces  are  also  covered  by  water 
bodies,  the  lakes,  which  are  not  in  direct  connection  with  the  sea. 
Among  these  the  Great  Fresh- water  Lakes  of  North  America,  and 
the  Caspian,  a  great  salt-water  lake,  are  the  most  prominent  ex- 
amples. These  are  to  be  classed  as  a  part  of  the  land,  as  conti- 
nental water  bodies,  distinct  from  the  sea.  Moreover,  the  upper 
layers  of  the  land  in  nearly  all  parts  are  water-bearing,  this  water 
filling  the  empty  spaces  of  the  soil,  and  occupying  the  pore-spaces 
within  the  solid  rock.  This  is  the  ground  water,  which  is  tapped  by 
wells,  mines,  or  borings,  or  issues  on  the  surface  in  springs  which 
feed  the  brooks  or  rivers,  or  form  swamps,  ponds,  and  lakes.  It  is 
thus  possible  to  speak  of  a  nearly  or  quite  continuous  mantle  of 
water  which  completely  covers  some  parts  of  the  solid  surface  of 
the  earth  and  more  or  less  completely  saturates  the  exposed  parts. 
This  mantle  of  water  may  be  viewed  as  an  aqueous  sphere  envelop- 
ing the  rock  sphere  of  the  earth  and  it  is  spoken  of  as  the  hydro- 
sphere. It  in  turn  is  enveloped  by  the  sphere  of  gas  and  vapor,  the 
well-known  atmosphere. 

These  two  spheres  or  shells  constitute  the  outer  layers  of  the 
earth  thus  viewed  in  its  entirety,  and  surround  the  more  rigid  mass 
of  the  earth,  with  which  we  are  most  familiar.  This  consists  of 
solid  rock,  and  of  uncpnsolidated  soil  and  other  loose  material,  all 
of  which,  except  the  surface  film  of  organic  matter,  we  may  recog- 
nize, on  examination  with  a  magnifier  or  by  other  means,  as  broken- 
down  rock  material  in  a  fine  state  of  division.  From  observation 
of  soundings,  dredgings,  etc.,  and  by  inference,  we  know  that  sim-  • 
ilar  material  forms  the  ocean  floor,  and  thus  we  recognize  that  be- 
neath the  hydrosphere  is  a  sphere  or  a  shell  of  rock  material.  This 


The  Earth  Viewed  as  a  Whole 


is  called  the  rock  sphere,  or  lithosphere,  and  in  it  are  included  all 
of  the  soil  and  other  unconsolidated  rock  material  of  the  earth's 
surface.  How  thick  this  shell  of  rock  is,  we  have  no  means  of  know- 
ing from  observation.  The  deepest  borings  into  the  earth's  crust 
have  penetrated  only  a  little  over  a  mile  in  depth,  while  the  deeper 
mines  go  less  than  two  miles  beneath  the  surface.  But  from  logical 
deductions  of  many  observed  physical  facts,  we  conclude  that  this 
shell  has  a  thickness  of  at  least  75  miles,  and  perhaps  much  more. 
It  is  indeed  generally  held  that  the  earth  is  solid  rock  to  the  core, 
but  there  are  those  who  have  held,  and  some  who  still  hold,  that 
the  central  portion  of  the  earth  consists  of  fluid  or  perhaps  even  of 
gaseous  matter. 

There  are  thus  three  inorganic  spheres,  the  atmosphere,  the  hy- 
drosphere and  the  lithosphere,  open  to  partial  observation.  The  re- 
lationships and  relative 
magnitudes  are  shown 
in  the  following  diagrams 
(Figs,  i  a  and  b). 

The  Inner  Spheres.  — 
From  the  observation 
of  many  phenomena, 
geologists  and  physicists 
have  reasoned  that  be- 
neath the  lithosphere, 
other  spheres  character- 
ized by  certain  peculiari- 
ties may  be  recognized, 
though  their  boundaries 
are  variable  and  probably 
not  very  definite.  One  of  these  is  the  sphere  or  zone  in  which  the 
temperature  of  the  earth  is  sufficiently  high  to  permit  the  fusion 
of  rock  if  the  pressure,  which  ordinarily  keeps  it  solid  by  raising 
the  fusing  point,  were  removed  or  lowered  by  some  structural  change 
in  the  earth's  crust.  There  may  be  regions  in  which  essentially 
permanent  pools  of  molten  rock  exist  within  the  crust,  forming 
feeders  of  volcanoes,  but  the  majority  of  such  feeding  areas  are 
more  probably  temporary.  It  is  convenient  to  speak  of  this  more 
or  less  concentric  zone  as  a  distinct  sphere  and  the  name  pyrosphere 
has  been  applied  to  it. 

Another  equally  indefinite  sphere  or  zone,  variously  regarded  as 


FIG.  i  a.  —  Diagram  of  the  successive  in- 
organic spheres  of  the  earth.  The  heavy 
black  line  represents  the  lithosphere,  including 
the  hydrosphere,  taken  as  75  miles.  The 
white  semicircle  above  it  indicates  the  atmos- 
phere. C,  center  of  the  earth.  The  part  in- 
cluded between  the  two  radii,  2  degrees  apart, 
is  enlarged  in  Fig.  i  b. 


Introduction 


lying  from  30  to  75  miles  beneath  the  surface  (according  to  our 
estimate  of  the  thickness  of  the  lithosphere) ,  is  one  of  relative  weak- 
ness, between  a  strong  external  crust  and  a  rigid  central  mass.  Here 
the  rock  yields  most  readily  under  long- 
enduring  strains  of  limited  magnitude, 
and  earth-movements,  resulting  in  the 
deformation  of  the  rocks,  occur.  This 
zone  or  sphere,  which  has  been  called  the 
tectosphere  (Murray)  or  the  astheno sphere 
(Barrell),  may  in  part  include  the  pyro- 
sphere;  nevertheless,  it  is  convenient  to 
speak  of  it  as  distinct. 

Finally  there  remains  the  central  portion 
of  the  earth,  the  centrosphere,  which  is  by 
far  the  largest  part  of  it,  and  about  which 
we  know  nothing  from  observation  and  but 
little  from  inference.  From  the  known 
rate  of  temperature  increase  in  borings 
and  mines,  it  is  inferred  that  the  temper- 
ature near  the  center  of  the  earth  may 
be  between  200,000  and  350,000  degrees 


FIG.  i  b.  —  Enlargement 
of  a  part  of  the  sector 
shown  in  Fig.  i  a,  to  show 
the  relative  thickness  of 
the  crust  of  the  earth,  the  Fahrenheit,  a  temperature  so  high  that 

mean  and  greater  depths  were  it  not  for  the  enormous  pressure, 
all  the  rocks  there  would  probably  be 
vaporized.  From  the  fact  that  the  aver- 

inch=no  miles.    On  this    age  specific  gravity  of  the  rocks  which 
scale  the  two  radii  meet  at     constitute  the  earth's  crust  is  only  about 
2.6  while  that  of  the  earth  as  a  whole  is 
hydrosphere    (black)   this     5.6,  it  is  further  inferred  that  the  material 
point  then  representing  the    of  tne  centrosphere  is  very  heavy  and  it 
has  been  calculated  that  if  there  were  a 

steady  increase  in  the  specific  gravity  of  the  material  from  the 
surface  toward  the  center,  this  would,  at  the  latter  point,  become 
1 1. 2,  which  suggests  the  possibility  of  a  central  core  of  iron,  if  not 
of  gold  and  platinum  or  some  other  heavy  metal.  Because  of 
this  greater  weight,  the  centrosphere  is  also  spoken  of  as  the  bary- 
sphere  or  heavy  sphere. 


of  the  sea  and  the  mean 
and  great  heights  of  the 
land.  Approximate  scale  i 


a  distance  of  about  3  feet 
from   the    surface  of   the 


The  Earth  Viewed  as  a  Whole  7 

The  Organic  Sphere  or  Biosphere 

To  these  inorganic  or  lifeless  spheres  there  is  added  the  organic 
sphere,  or  sphere  of  life,  the  biosphere  (bios,  /ftbs,  life).  It  would 
surprise  the  average  student  could  he  see  the  universality  of  this 
organic  shell  of  the  earth.  In  and  out  among  the  mass  of  the  water 
and  of  the  air  and  upon  and  through  the  upper  layers  of  the  rocky 
surface,  the  thread  of  living  matter  is  woven,  now  constituting  an 
almost  solid  mass  of  tissue,  as  in  the  bodies  of  peat  or  in  the  dense 
vegetation  of  a  forest,  or  the  almost  continuous  tangle  of  seaweed 
or  layer  of  floating  organisms  in  the  sea ;  again  forming  a  network, 
the  meshes  of  which  vary  greatly  in  size,  but  are  on  the  whole  con- 
tinuous, except  where  momentarily  broken  by  the  hand  of  man, 
or  by  some  abrupt,  not  to  say  cataclysmic  disturbance,-  such  as  an 
avalanche  or  outpouring  of  a  mass  of  lava.  But  left- for  even  a 
short  time,  the  steepest  quarry  wall,  or  the  most  precipitous  cliff 
formed  by  the  sudden  dislodgment  of  a  mountain  side  or  by  the 
sinking  of  a  portion  of  the  earth's  crust,  such  as  may  take  place 
during  an  earthquake-producing  disturbance,  will  be  again  taken 
possession  of  by  some  form  of  plant,  if  not  of  animal  life.  Even 
the  stony  lava  field  will  be  covered  in  time  by  a  succession  of  organic 
forms  of  increasing  complexity.  The  apparently  barren  desert,  too, 
has  its  wide-meshed  net  of  living  beings,  except  perhaps  where  the 
sands  are  constantly  in  motion ;  and  the  presence  of  these  organisms 
is  often  shown  by  the  countless  tracks  and  trails  which  appear  upon 
the  surface  of  the  sand  on  a  dewy  morning,  or  the  sudden  springing 
up  of  vegetation  from  hidden  seeds  when  moisture  furnishes  the 
condition  for  expansive  growth.  And  in  the  sea,  life  of  some  form 
is  never  wanting  long,  —  here  covering  the  bottom  with  a  continu- 
ous living  carpet,  there  forming  an  ever  changing  web  of  floating 
animal  and  plant  life,  through  which  the  swimming  world  of  animals 
weaves  an  intricate  pattern  with  the  threads  of  its  never  ceasing 
wanderings. 

Thus  it  is  perfectly  in  accord  with  the  facts,  when  we  speak  of 
a  shell  or  sphere  of  living  matter,  the  biosphere,  which  surrounds 
the  lithosphere  and  penetrates  its  upper  layers  as  well  as  the  hydro- 
sphere and  the  atmosphere,  but  is  distinct  from  all.  That  such  a 
biosphere  has  existed  throughout  most  of  the  past  ages  of  the  earth's 
history  is  clearly  shown  by  the  countless  remains  of  the  hard  parts 
of  animals,  such  as  the  shells  of  mollusks,  the  bones  of  vertebrates 


8 


Introduction 


and  the  like,  which  fill  many  of  the  rocky  layers  of  the  litho- 
sphere,   and   indeed   sometimes   actually   compose   these   layers. 

For    example,    the     white 

C     ^^f^S^^i^^^  chalk  of  the   English   and 

French  coastal  regions  is 
literally  made  up  of  the 
shells,  and  fragments  of 
shells,  of  organisms  (Fora- 
minifera,  coccoliths,  etc.), 
as  shown  in  the  following 
illustration  (Fig.  2),  and 
many  of  our  great  lime- 
stones are  consolidated  re- 
mains of  shells  of  animals 
which  formerly  inhabited 
the  seas,  or  fresh-water 
lakes.  In  the  following 


FIG.  2.  — Thin  sections  of  chalk,  as  they 
appear  under  the  microscope,  showing  the 
shells  etc.,  of  minute  marine  organisms 
(chiefly  Foraminifera  and  coccoliths)  of  figure  (Fig.  3)  IS  shown  an 
which  the  rock  is  composed.  A,  Chalk  enlargement  of  a  group  of 
from  Sussex,  England,  enlarged  60  times. 

B,  Chalk   from  Farafrah,  Libyan  desert, 
enlarged  60  times ;  the  most  characteristic 
shells  are  Textularia  (a)  and  Rotalia  (ft). 

C,  Dried  residue  of  milky  chalk  water  with 
coccoliths    enlarged     700    times.      (After 
Zittel.) 


minute  needle-like  shells, 
each  one  of  which  was  built 
by  a  separate  marine  animal 
and  of  which  a  limestone, 
traceable  over  wide  areas 
in  the  state  of  New  York, 

is  almost  entirely  composed.  From  careful  study,  it  has  been  de- 
termined that  a  cubic  inch  of  this  rock,  when  pure,  contains  about 
40,000  individual  shells  (J.  M.  Clarke). 
Whole  mountains  are  sometimes 
made  up  of  rock,  which  is  largely,  if 
not  entirely,  the  product  of  accumu- 
lation of  the  hard  structures  of  former 
organisms.  An  example  of  this  is 
found  in  the  famous  Dolomites  of  the 
Tyrol  (Fig.  4),  the  chief  rock  masses 
of  which  are  made  up  of  calcareous 
matter  separated  from  a  sea  by  marine 
plants  (Fig.  5),  and  the  many  centuries 
of  time  during  which  the  present  forms 
of  these  peaks  were  carved  from  this 


FIG.  3.  —  Styliolina  fissu- 
rella,  a  minute  molluscan. 
(pteropod)  shell.  Fragment 
with  numerous  individuals 
enlarged  three  times;  and  a 
specimen  much  enlarged. 
(After  Hall,) 


The  Earth  Viewed  as  a  Whole 


10 


Introduction 


rock  have  witnessed  only  a  partial  obliteration  of  the  organic 
structures,  though  no  inconsiderable  portion  of  the  mass  has  been 
removed. 

The  remains  of  animals  and  plants  which 
compose,  or  are  included  in,  the  rocks  of  the 
earth's  crust,  are  known  to  the  geologist  as 
fossils  (from  the  Latin  fossilis,  something  that 
is  dug  up)  and  their  study  forms  an  important 
and  integral  part  of  the  science  of  geology. 
It  is  indeed  the  study  of  the  constantly 
changing  elements  of  the  biosphere,  of  which 
the  existing  animals  and  plants  form  only  the 

most  modern  phase,  that  has  made  possible 
,        «     .   »      •  <•     T      i  •  r     i 

the  deciphering  of  the  history  of  the  earth. 


FIG.  5.  —  Diplo- 
pora  porosa  Schafh. 
A  calcareous  alga  or 
marine  plant  which 


was  chiefly    respon-    This  subject  will  be  more  fully  considered  in 

sible  for  the  making     later  chapters  of  this  book. 

of   the  rock  out   of          ««    «        «  •    ,          .*      t 

Beds  of  coal>  too>  Pomt  to  the  former  ex~ 
tensive  accumulation  of  vegetable  material, 
ancj  the  oj}  of  our  o[\  shales  and  pools  owes  its 

.  .     ,         ,  j         i  i,        Al  ,1 

ongm  ^IS&*  and  Perhaps  altogether,  to  the 

distillation,  during  many  centuries  of  time,  of 


which  the  Dolomites 
are  carved.  A,  Nat- 
ural  size.  B,  en- 
larged  3  times,  a. 
canals  which  appear 
upon  the  surface  as 


pores,  ,p-,r,  constrict-  buried   organic    matter   which   constituted   a 
ing  rings   character-  .          r    ,       ,  .       ,  .   ,  i     •     i 

istic  of  this  genus,  portion  of  the  biosphere  of  former  geological 

(After  Steinmann.)  periods. 


Contributions  to  the  Lithosphere  from  Other  Spheres 

We  now  see  that  the  biosphere  contributes  no  inconsiderable 
portion  to  the  growing  lithosphere,  and  this  is  true  to  a  certain 
extent  also  of  several  of  the  other  spheres.  For  not  only  was  the 
carbon  of  ancient  plants,  which  is  now  locked  up  in  the  earth's, 
crust  as  coal  and  oil,  contributed  by  the  atmosphere  of  the  past, 
but  the  water  vapor  of  the  air,  freezing  and  descending  as  snow  or 
hail,  or  crystallizing  directly  upon  cold  surfaces,  forms  a  not  unim- 
portant, though  in  most  regions  very  transitory,  addition  to  the 
solid  rock  of  the  earth.  So,  too,  the  conversion  of  water  to  ice  forms 
rock  at  low  temperature,  for  ice  is  a  rock.  More  permanent  con- 
tributions are  made  by  the  crystallizing  of  various  salts,  chief  among 
them  the  common  salt  (sodium  chloride),  which,  as  we  shall  see 
later,  may  become  buried  by  other  rock  material,  and  be  preserved 


The  Earth  Viewed  as  a  Whole  n 

for  millions  of  years  as  an  essential  part  of  the  earth's  crust.  Thus 
the  salt  now  mined  in  New  York  State  and  in  southeastern  Mich- 
igan was  derived  from  the  waters  of  an  ancient  sea,  the  deriva- 
tion being  by  a  roundabout  process,  which  will  be  detailed  later. 
That  this  occurred  in  a  period  of  time  many  millions  of  years  ago 
is  amply  shown  by  the1  ascertainable  facts.  So,  too,  the  great  salt 
deposits  of  North  Germany,  which  have  furnished  the  world  in  the 
past  with  most  of  its  potash,  are  the  product  of  the  evaporation  of 
cut-off  portions  of  an  ancient  sea  which  covered  much  of  Europe, 
though  the  period  of  its  production  is  not  quite  so  remote  as  that 
of  the  best-known  American  salts.  (See  Chapters  XXXIV  and 
XXXVIII.) 

That  the  pyrosphere  also  contributes  its  quota  to  the  rock  mass 
of  the  earth  is  abundantly  shown  by  the  vast  extent  of  ancient 
lavas  and  other  igneous  masses  which  now  form  a  large  proportion 
of  the  solid  crust  of  the  earth  and  are  visible  to  us  as  the  result  of 
surface  cooling  in  comparatively  modern  times,  as  in  the  case  of 
the  great  Columbia  lava  fields  of  Northwestern  America  and  the 
preserved  fragments  of  similar  flows  on  the  British  coast  (Staff a, 
Giants'  Causeway,  etc.),  or  which  were  uncovered  by  the  removal, 
during  long  geological  epochs,  of  the  overlying  portions  of  the 
earth's  crust  beneath  which  the  ancient  igneous  masses  assumed 
their  solid  form. 

Throughout  the  entire  history  of  the  earth,  the  lithosphere  has 
received  additions  by  the  cooling  of  molten  rock  material  injected 
into  it  or  poured  out  on  its  surface,  and  this  addition  is  in  progress 
even  to-day. 

Contributions  to  the  Lithosphere  from  Outer  Space 

Finally  it  may  be  added  that  the  earth's  crust  receives  to-day, 
and  undoubtedly  has  received  in  the  past,  additions  from  the  cosmic 
spaces  which  surround  our  atmosphere.  When  a  meteor  reaches 
the  earth  it  forms  an  integral  part  of  the  unconsolidated  or  discrete 
portion  of  the  earth's  crust,  at  least  until  it  is  gathered  in  by  the 
discerning  collector  of  these  messengers  from  the  extraterrestrial 
spaces. 

Summary  of  Spheres 

We  may  now  summarize  in  tabular  form  the  several  spheres  or 
shells  into  which  the  earth  as  a  whole  is  divisible,  and  add  to  this 
summary  the  derivation  of  their  names. 


12  Introduction 


The  Inorganic  Spheres,  (o-^atpa)  =  sphere. 

1.  Atmosphere,  Gr.  atmos  (aT/xd?)  =  vapor  or  gas. 

2.  Hydrosphere,  Gr.hydor  (vSup)  =  water. 

3.  Lithosphere,  Gr.  lithos  (\i'0os)  =  stone. 

4.  Pyrosphere,  Gr.  pyr  (irvp)  =fire. 

5.  Asthenosphere,  Gr.  asthenos  (do-tfev^s)  =  feeble. 

6.  Centrosphere,  Gr.  centron  (KeVrpov)  =  center, 
or  Barysphere,  Gr.  &arys  (/Sa/ws)  =  heavy. 

The  Organic  Sphere. 

7.  Biosphere,  Gr.  6^5  (/3tos)  =  life. 


CHAPTER  II 

SUBDIVISIONS  OF  THE  SCIENCE  OF  GEOLOGY 

IT  is  but  natural  that  the  mind  of  man,  always  eager  to  enter 
into  details,  should  have  developed  distinct  branches  of  inquiry 
into  the  character  and  history  of  the  several  parts  of  the  earth 
which  is  his  home.  In  the  beginning  of  such  inquiry,  a  single  mas- 
ter mind  may  have  encompassed  the  entire  field ;  but  with  the  in- 
creasing mass  of  detail,  the  individual  range  became  limited  to 
special  portions  of  the  field,  and  in  the  course  of  time  the  several 
lines  of  inquiry  into  the  nature  and  history  of  our  earth  developed 
into  distinct  sciences,  each  with  its  host  of  devoted  searchers  after 
truth. 

SUBDIVISIONS  OF  THE  SCIENCE  IN  ITS  COMPREHENSIVE  SENSE 

We  may,  then,  at  this  point  of  our  study,  endeavor  to  ascertain 
the  natural  subdivisions  of  the  Science  of  the  Earth,  or  Geology 
in  its  broadest  sense,  and  after  that  confine  ourselves  to  those 
branches  which  by  common  consent  are  reserved  as  the  special 
field  of  the  geologist,  as  it  is  understood  at  the  present  time. 

Atmology  or  Meteorology.  —  From  an  inspection  of  the  table  of 
spheres  into  which  the  earth  may  be  divided,  we  form  the  natural 
conclusion  that  the  first  line  of  cleavage  would  be  in  accord  with 
these  natural  subdivisions.  The  study  of  the  atmosphere  has  now 
developed  into  a  separate  science,  which  is  familiarly  known  as 
Meteorology  because  the  meteors,  or  extraterrestrial  bodies,  the 
familiar  "  shooting  stars,"  which  on  entering  our  atmosphere  be- 
come luminous  through  friction  and  frequently  reach  the  earth  as 
meteorites,  have  from  the  remotest  days  formed  one  of  the  chief 
attractions  for  those  whose  gaze  was  turned  away  from  the  solid 
earth.  Thus  atmospheric  phenomena  came  to  be  designated  as 
meteoric  phenomena  and  the  science  of  these  phenomena  became 
meteorology.  But  since  the  sphere  with  which  this  science  is  con- 
cerned is  the  atmosphere,  a  more  appropriate  name  for  this  science 
itself  is  Atmology. 

13 


14  Subdivisions  of  the  Science  of  Geology 

Hydrology.  —  That  the  hydrosphere,  that  is,  the  oceans  and 
other  water  bodies,  early  attracted  the  serious-minded  student 
was  but  natural,  since  these  water  bodies  form  the  natural  high- 
ways of  commerce,  and  since  they  furnish  so  large  a  part  of  the 
earth's  population  with  the  means  of  a  livelihood.  Thus  the  science 
of  the  water,  or  Hydrology,  was  developed,  which  is  variously  sub- 
divided again  into  the  sciences  of  the  oceans,  or  Oceanography,  and 
into  the  sciences  of  the  lakes,  the  ponds,  and  the  rivers.  That  some 
of  these,  for  example  oceanography,  have  been  developed  to  a 
remarkable  extent,  is  shown  by  the  great  oceanographic  institutes 
of  the  world,  of  which  that  of  Berlin  under  Director  Penck,  and  that 
at  Paris,  under  the  direction  of  the  Prince  of  Monaco,  are  the  best 
known.  In  America,  oceanographic  studies  have  chiefly  been  car- 
ried on  by  the  federal  government  and  by  private  individuals, 
among  these  the  late  Alexander  Agassiz ;  and  there  is  a  hydrographic 
department  of  our  government,  the  chief  business  of  which  is  the 
charting  of  our  sea-coasts,  lakes  and  rivers. 

In  England,  too,  oceanographic  studies  have  been  largely  fostered 
by  the  government,  under  whose  auspices  the  famous  Challenger 
Expedition,  for  the  exploration  of  the  oceans,  under  the  leadership 
of  Sir  John  Murray,  was  carried  to  successful  completion.  The 
governments  of  other  countries,  too,  have  fostered  exploring  ex- 
peditions for  oceanographic  research,  these  being  generally  known 
by  the  name  of  the  vessel  employed  in  the  expedition. 

The  student  of  geology,  even  in  its  narrower  sense,  must  keep 
abreast  of  these  inquiries,  at  least  to  a  moderate  extent,  for  without 
them  many  of  his  discoveries  lack  the  clarifying  light  by  the  aid  of 
which  he  must  interpret  them ;  and  in  proportion  as  the  researches 
of  the  oceanographers  and  the  other  hydrologists  become  available, 
the  analysis  of  the  structure  and  composition  of  the  earth's  crust, 
the  special  field  of  the  geologists  in  the  modern  sense,  and  the 
interpretation  of  these  facts  in  terms  of  earth  history,  become 
more  precise. 

Lifhology  or  Geology  Proper. — As  has  just  been  said,  the  special 
field  of  inquiry  of  the  modern  geologist  is  the  crust  of  the  earth,  or 
the  lithosphere.  He  is  thus  primarily  a  student  of  Lithology  — 
though  that  term  has  also  been  used  in  the  past  in  a  narrower 
sense,  the  study  of  rocks  per  se.  It  will  be  well,  however,  for  the 
sake  of  unity  of  terminology  —  a  great  desideratum  in  every  science 
—  to  adopt  this  term  in  its  wider  sense,  namely,  that  which  encom- 


The  Science  in  its  Comprehensive  Sense          15 

passes  all  the  fields  of  inquiry  which  are  concerned  with  the  com- 
position, structure,  and  history  of  the  earth's  rocky  shell,  the  litho- 
sphere,  generally  assigned  as  the  only  legitimate  field  of  the 
modern  geologist.  Still,  to-day  more  than  ever,  the  geologist  can- 
not limit  his  inquiries  to  the  facts  disclosed  by  the  rocks  alone, 
but  must  draw  his  conclusions  with  the  aid  of  a  wide  knowledge 
of  the  researches  of  the  atmologist,  the  hydrologist,  and  even  the 
biologist. 

Biology  ;  Palaeontology.  —  Biology  is  the  science  of  living 
things.  It  is  the  study  of  the  biosphere,  and  has  perhaps  been  more 
sedulously  developed,  and  has  a  larger  body  of  votaries,  than  any 
other  branch  of  the  earth,  science.  Biology,  as  currently  under- 
stood, limits  itself  to  the  study  of  modern  living  beings,  the  plants 
and  animals  of  to-day,  and  naturally  falls  into  the  two  divisions,  or 
separate  sciences,  of  Botany  (Phy  tology  *)  ,  the  study  of  plant  life, 
and  Zoology,2  the  study  of  animal  life.  Biologists  are,  however, 
aware  of  the  fact  that  the  modern  world  of  plants  and  animals  is 
only  a  fragment  of  the  great  host  of  living  beings  which  has  in- 
habited the  earth  from  the  remotest  ages  ;  it  is  the  life  record  of 
the  youngest  and  shortest  chapter  in  the  history  of  the  earth. 
Throughout  all  the  past  ages,  living  forms  existed  upon  the  earth, 
and  their  remains,  as  we  have  already  seen,  are  embedded  in  the 
rocks  of  the  earth's  crust  as  fossils.  The  study  of  these  was  at 
first  left  to  the  geologist,  but  has  now  developed  into  a  separate 
science,  that  of  Paleontology  (from  the  Greek  palaios  [TraXtuos], 
ancient,  onto,  [ovra],  living  beings,  and  logos  or  logia  [Aoyia],  science). 

Other  Divisions.  —  When  we  come  to  the  consideration  of  the 
inner  spheres  of  the  earth,  we  must  admit  that,  owing  to  our  in- 
ability to  make  direct  observations,  no  separate  sciences  worthy 
of  that  designation  have  yet  been  developed.  It  is  true  that  a 
science  of  earth  disturbances  of  Seismology  exists,  and  that  this 
properly  belongs  to  the  study  of  the  asthenosphere  or  tectosphere. 
But  the  observations  on  which  this  science  is  based  are  chiefly 
possible  upon  surface  manifestations  and  the  effects  which  these 
produce,  though  inferences  can  also  be  drawn  from  the  study 
of  disturbances  which  have  occurred  in  the  past,  the  effects  of 
which  are  recorded  in  the  rocks.  The  same  is  true  of  the  study  of 
the  pyrosphere  —  which  can  only  be  approached  through  observa- 


Greek  phylon  [0vr6i>],  plant.  2  Greek  zoon  [fv°*']>  animal. 


1 6  Subdivisions  of  the  Science  of  Geology 

tions  of  modern  volcanoes  (Vulcanology)  and  of  the  temperatures 
in  deep  borings  and  mines.  Of  the  interior  of  the  earth  we  may 
perhaps  never  know  much  through  observation,  and  so  deduc- 
tions from  physical  phenomena  alone  must  guide  us.  No  separate 
science  of  the  centro-  or  barysphere  has  thus  been  developed. 

From  the  foregoing,  the  student  is  led  to  the  realization  of  the 
truth  that  all  true  sciences  are  based  on  two  great  fundamentals, 
observation  and  deduction.  To  observe  and  record  facts  alone  does 
not  constitute  the  whole  of  the  work  of  the  man  of  science.  Such 
used  to  be  the  sole  aim  of  the  older  naturalists,  so  called,  who 
dissented  from  the  attitude  of  the  ancient  philosophers,  because 
these  based  their  speculations  on  mental  processes  rather  than  ob- 
servation of  facts.  To-day,  however,  we  know  that  the  inter- 
pretation of  facts,  in  the  light  of  our  growing  knowledge  of  causes, 
is  as  legitimate,  indeed  as  important,  a  function  of  the  student  of 
the  earth  as  the  observation  of  facts,  and  moreover  a  certain  amount 
of  speculation  —  always  held  in  check  by  the  appeal  to  facts  —  is 
not  only  desirable  but  necessary.  The  student  of  nature  must  be 
a  philosopher  in  the  true  sense  of  the  word,  and  so  long  as  he  does 
not  lose  sight  of  his  fundamental  base  —  the  observation  of  facts  — 
his  work  will  gain  in  value  by  allowing  his  reasoning  faculties  their 
fullest  play. 

SUBDIVISIONS  OF  THE  SCIENCE  OF  GEOLOGY  IN  ITS  MORE  LIM- 
ITED SENSE.     LITHOLOGY  OR  THE  STUDY  OF  THE  LITHOSPHERE 

Restricting  our  attention  now  to  the  more  limited  field  to  which 
the  modern  geologist  applies  himself,  namely  the  study  of  the  ma- 
terial, structure,  and  history  of  the  earth's  crust,  or  the  lithosphere, 
we  may  note  at  the  outset  that  there  are  two  phases  or  aspects  of 
this  field,  one  or  the  other  of  which  is  cultivated  more  sedulously 
by  the  geologist  according  to  his  predilections  or  circumstances, 
but  neither  of  which  can  be  wholly  neglected  by  the  worker  in 
either  field.  These  two  aspects  are  the  purely  scientific,  and  the 
practical  or  applied.  The  geologist  who  works  in  the  pure  science 
field  of  his  domain  does  so  primarily  for  the  intellectual  satisfac- 
tion derived  from  the  discovery  of  facts  and  principles.  His  aim 
is  chiefly  to  search  for  nature's  truths  irrespective  of  their  bearing 
on  human  welfare,  and  his  principal  endeavor  is  directed  toward 
widening  the  boundaries  of  human  knowledge  and  pushing  forward 


The  Study  of  the  Lithosphere  17 

the  frontiers  of  discovery.  The  chief  aim  of  the  practical  geologist, 
on  the  other  hand,  is  directed  toward  making  the  forces  and  ma- 
terials of  the  world  available  to  man,  to  augment  the  welfare  of 
the  human  race,  and  to  push  forward  the  boundaries  of  civiliza- 
tion. But  this  work,  important  and  noble  as  it  is,  can  only  be 
carried  on  successfully  in  proportion  as  the  facts  and  laws  of  the 
science  are  discovered  and  demonstrated,  and  the  practical  geolo- 
gist must  forever  depend  on  the  worker  in  pure  science,  whose  re- 
ward too  often  is  little  more  than  the  satisfaction  which  is  to  be 
derived  from  a  devotion  to  the  search  for'truth.  We  shall  for  the 
sake  of  convenience  speak  of  the  pure-science  phase  of  our  subject  as 
the  scientific  aspect  of  geology,  and  of  the  applied  phase  of  geology  as 
geology  in  its  relation  to  man.  Each  phase  has  its  special  S4ubdi- 
visions,  to  which  attention  may  now  be  drawn. 


I.    The  Scientific  Aspect  of  Geology 

Structural  Geology.  —  A  logical  analysis  of  this  field  of  investi- 
gation reveals  a  threefold  aspect  and  three  corresponding  methods 
of  approach  in  study.  In  the  first  place  the  material  of  the  earth's 
crust,  and  the  composition  and  the  structure  of  this  material,  in- 
vite attention.  This  portion  of  the  subject  is  generally  treated 
under  the  caption  Structural  Geology.  It  takes  account  of  the  com- 
position, form,  and  architecture  of  the  earth's  crust,  and  is  primarily 
an  analytic  branch  of  the  science.  From  this  point  of  view  the 
earth's  crust  may  be  considered  under  the  following  subdivisions, 
given  in  the  order  of  their  magnitude. 

Subdivisions  of  Structural  Geology.  —  These  include  the  following 
fields : 

1.  Chemical  elements  and  ions. 

2.  The  combination  of  these  elements  and  ions  into  salts  and 
other  compounds  which  either  take  on  crystalline  form  or  remain  in 
an  uncrystalline  (amorphous)  condition.     These  compounds  as  they 
occur  in  nature  are  designated  minerals. 

3.  The  combination  of  crystals  or  fragments  of  the  same  or 
different  minerals  into  large  masses,  either  bound  together  into  a 
more  or  less  solid  mass,  or  remaining  in  an  unconsolidated  condi- 
tion.    These  are  the  rocks  of  the  earth's  crust,  and  for  their  study 
a  recognition  of  at  least   the  principal   component   minerals   is 
essential. 


i8  Subdivisions  of  the  Science  of  Geology 

4.  The  association  of  rock  masses  into  original  or  primary  struc- 
tural units,  which  constitutes  the  primary  architecture  of  the  earth's 
crust ;  or  the  impression  upon  it  of  secondary  structures,  through 
deformation  or  other  influences  of  an  outside  nature.     This  is  struc- 
tural geology  in  the  narrower  sense  —  and  has  also  been  called 
Geotectology.     The   original   structure   corresponds   to   the   initial 
architecture  of  a  building,  the  stones  or  bricks  of  which  correspond 
to  the  rock  masses  of  the  earth's  crust.     The  secondary  structures 
correspond  to  the  changes  subsequently  made  by  additions,  removal 
or  modification  resulting  from  sagging  with  age,  etc. 

5.  The  surface  forms  resulting  through  the  activities  of  destruc- 
tive as  well  as  reconstructive  forces,  and  recognizable  in  the  moun- 
tains and  valleys,  the  plains  and  plateaus,  and  other  physical 
features,  generally  considered  under  the  subject  of  Physical  Geog- 
raphy.    The  term  Geomorphology  —  or  the  study  of  the  details 
of  the  earth's  surface  forms  —  has  been  commonly  applied  to  this 
branch  of  inquiry,' when  it  is  not  merely  descriptive  or  analytic, 
but  takes  account  of  the  history  or  genesis  of  the  form  as  such.     It 
corresponds  to  the  study  of  the  form  of  a  building  as  a  whole,  either 
in  its  completeness  or  when  changed  to  a  ruin  by  destructive  in- 
fluences. 

The  study  of  most  of  these  divisions  of  structural  geology  has 
been  carried  to  such  detail  that  separate  sciences  have  been  devel- 
oped. Thus  the  study  of  the  natural  substances,  or  minerals,  has 
become  the  science  of  Mineralogy,  and  that  of  the  rocks  the  science 
of  Petrology  (also  sometimes  designated  as  lithology  in  its  narrower 
sense),  while  the  science  of  rock  structures  is  Structural  Geology  in 
the  narrower  sense,  and  that  of  the  surface  forms  is  Physiography, 
and  their  relation  to  man  is  Geography. 

In  all  such  studies,  while  emphasis  is  laid  on  the  analytical  and 
descriptive  sides,  the  causal  or  dynamic  side  and  the  historical  or 
developmental  side  are  given  their  due  attention.  This  is  expressed 
by  the  preference  of  terms  ending  in  ology  (mineralogy,  petrology, 
geotectology,  geomorphology)  over  those  ending  in  graphy  (petrog- 
raphy, physical  geography,  etc.),  which  emphasize  mainly  the 
descriptive  and  analytical  side. 

Dynamical  Geology.  — -A  second  mode  of  approach  is  that  which 
lays  the  emphasis  upon  the  forces  that  work  upon  and  within  the 
earth  to  produce  results,  which,  in  this  view,  take  a  secondary  rank. 
This  is  Dynamical  Geology  and  it  naturally  falls  into  the  two  great 


The  Study  of  the  Lithosphere  19 

divisions,  the  physical  and  chemical.  The  sciences  of  physics  and 
chemistry,  in  so  far  as  they  lay  the  stress  upon  the  forces  at  work, 
are  the  specialized  development  of  these  aspects.  That  chemistry, 
in  its  analytical  as  well  as  its  dynamic  aspects,  is  of  fundamental 
importance  to  the  geologist  has  generally  been  recognized,-  but  the 
importance  of  physics  to  the  student  of  earth  science  has  only  re- 
cently received  proper  recognition  by  the  establishment  of  geo- 
physical laboratories. 

Both  chemical  and  physical  forces  may  be  viewed  as  constructional, 
or  those  building  up  rock  masses  and  rock  structures,  and  as  de- 
structional,  or  those  destroying  them.  A  third  view  of  these  forces 
is  that  which  deals  with  their  effects  in  modifying  or  deforming 
the  materials  and  structures,  and  this  may  be  termed  the  reconstruc- 
tional  or  deformational  aspect. 

Historical  Geology  or  Geogenesis.  —  The  third  method  of  ap- 
proach is  the  historical  or  evolutionary  method,  in  which  emphasis 
is  laid  upon  development  and  the  causes  which  underlie  this  de- 
velopment. It  is  apparent  that  this  phase  of  geology  is  the  latest 
and  most  specialized  aspect,  and  that  for  its  proper  prosecution  a 
thorough  preparation  in  the  other  two  phases  is  needed.  More- 
over, since  the  history  of  life  upon  the  earth  is  intimately  bound 
up  with  the  history  of  the  lithosphere,  a  knowledge  of  biology,  and 
especially  of  palaeontology,  is  indispensable  for  the  prosecution 
of  any  but  the  most  general  studies  in  earth  history. 

Several  special  branches  have  been  developed  within  this  field. 
One  of  these  deals  with  the  origin  or  genesis  of  the  rocks  and  their 
structures,  or  the  origin  of  the  lithosphere.  To  this  branch  the 
name  Lithogenesis  is  commonly  applied.  Another  branch  considers 
the  character,  arrangement,  succession,  order  of  formation,  and 
age  relations  of  the  stratified  rocks.  This  is  the  science  of  Stratig- 
raphy, primarily  a  descriptive  one.  A  third  branch  deals  with  the 
succession  and  distribution  of  the  organic  remains,  the  petrifactions 
or  fossils  in  the  rocks  in  so  far  as  they  have  a  bearing  on  the  geolog- 
ical history  of  the  earth,  i.e.,  the  index  fossils.  This  is  the  special 
geological  phase  of  Paleontology,  which  has  also  been  designated 
by  the  name  Petrifactology  (Haeckel).  Again,  there  is  the  science 
which  deals  with  the  development  or  genesis  of  the  surface  forms 
of  the  earth  not  merely  in  the  descriptive  manner,  but  from  the 
view-point  of  origin.  This,  as  already  noted,  is  Geomorphology  in 
its  proper  sense,  though  it  is  more  commonly  known  by  the  name 


2o  Subdivisions  of  the  Science  of  Geology 

of  Physiography,  which  formerly  had  a  very  different  meaning. 
Finally  there  is  the  study  of  the  changes  in  the  earth's  surface,  or 
its  geography  through  the  successive  ages  of  the  earth's  history, 
-  together  with  the  changes  in  climate  and  the  dispersions  and 
migrations  of  the  organisms  and  the  causes  which  effected  these. 
This  is  the  latest  of  the  several  aspects  of  Historical  Geology  and 
is  now  termed  Palao  geography  or  the  geography  of  the  past.  In 
this  field  the  palaeogeography  of  the  Pleistocene  period  has  been 
most  extensively  studied,  and  a  separate  branch,  that'  of  glacial 
geology,  has  been  developed.  Geography,  in  the  usual  sense  of  the 
term,  is  the  geography  of  the  modern  or  Holocene  period  of  the 
earth's  history. 


II.    Geology  in  its  Relation  to  the  Welfare  of  Man 

So  far  we  have  been  considering  geology  in  its  pure  science  aspect, 
that  which  appeals  to  the  inquiring  mind  of  man  in  search  after 
truth  and  knowledge,  without  ulterior  motives  of  usefulness.  There 
is,  however,  another  aspect  of  our  science,  and  one  which  in  recent 
times  has  come  strongly  to  the  front.  This  is  applied  geology,  in 
which  geological  facts  and  forces  are  viewed  in  their  relation  to 
the  needs  and  requirements  of  man.  As  has  already  been  intimated, 
the  application  of  any  science  for  any  purpose  whatsoever  is  success- 
ful in  direct  proportion  to  the  profundity  of  knowledge  possessed 
by  the  applier.  No  successful  exploration  of  geological  products  or 
application  of  forces  is  possible  without  a  thorough  understanding 
of  the  facts  and  principles  of  the  science,  and  the  student  who 
wishes  to  follow  the  applied  side  of  his  science  should  not  fail  to 
make  his  preparation  in  the  pure  science  side  as  broad  and  as  pro- 
found as  circumstances  will  permit. 

Among  the  earliest  problems,  to  the  solution  of  which  geological 
knowledge  has  been  applied,  are  those  of  mining.  Indeed,  the 
science  of  geology,  in  a  measure,  developed  in  response  to  the  needs 
of  the  miner  for  accurate  knowledge  of  the  conditions  of  occurrence, 
distribution,  and  mode  of  origin  of  the  valuable  mineral  deposits. 
To  such  an  extent  has  this  been  carried,  that  a  separate  branch  of 
mining  geology  has  come  into  existence.  Moreover,  as  our  knowledge 
increased  and  the  possibility  of  more  detailed  application  of  our 
science  became  apparent,  special  subdivisions  of  mining  geology 
have  been  developed,  and  it  is  found  that  individuals  can  profitably 


The  Study  of  the  Lithosphere  21 

devote  themselves  to  the  cultivation  of  a  narrow  field,  to  the  prac- 
tical exclusion  of  the  others.  Thus  there  have  been  developed  the 
branches  of  coal  geology,  of  petroleum  and  gas  geology,  and  of  salt 
geology,  including  the  geology  of  potash,  phosphates,  nitrates,  borax, 
and  other  salts,  and  of  bauxites  and  other  aluminum  ores.  In 
these  branches  the  investigator  confines  himself  to  the  problems 
involved  in  the  occurrence  of  these  substances,  which  are  chiefly  re- 
stricted to  the  stratified  rocks.  It  is  now  well  recognized  that  suc- 
cessful search  for  such  deposits  can  only  be  undertaken  by  one  well 
versed  in  the  science  of  stratigraphy  (including  index  fossils)  and 
structural  geology,  while  a  thorough  understanding  of  the  prin- 
ciples of  physiography  and  palaeogeography  is  almost  indispensable. 
The  mining  geologist  who  devotes  himself  primarily  to  the  problems 
of  the  metallic  deposits  must  have  not  only  a  thorough  knowledge 
of  mineralogy,  petrology,  and  structural  geology,  but  of  dynamic 
geology  as  well,  and  especially  of  the  chemical  and  physical  principles 
involved  in  ore  deposition.  As  many  ores  are  also  found  in  strat- 
ified deposits,  a  knowledge  of  stratigraphy  and  of  index  fossils 
is  necessary,  while  an  understanding  of  the  principles  of  physiog- 
raphy and  palaeogeography  will  also  be  found  of  value  in  many 
cases. 

Geology,  too,  plays  an  indispensable  role  in  the  solution  of  many 
important  engineering  problems.  In  the  construction  of  the  Cats- 
kill  aqueduct  for  New  York  City,  a  force  of  competent  geologists 
was  constantly  employed,  and  specialists  were  frequently  called 
upon  for  consultation.  This  same  need  was  felt  after  the  con- 
struction of  the  Panama  Canal  had  been  undertaken,  and  a  resident 
geologist  was  appointed  to  supervise  the  later  phases  of  construc- 
tion. That  many  difficulties  might  have  been  avoided  had  such 
supervision  existed  from  the  outset  of  the  undertaking,  is  now 
generally  conceded. 

Geological  advice  has  always  been  employed  in  the  construction 
of  great  tunnels  such  as  those  piercing  mountains  or  passing  under 
rivers  and  other  water  bodies.  In  many  cases,  too,  the  selection 
of  sites  for  bridges,  dams,  and  other  great  engineering  works  has 
been  based  on  geological  advice,  while  in  other  cases,  where  such 
advice  has  not  been  sought  or  has  been  disregarded,  disastrous 
consequences  have  resulted.  In  consequence  of  the  growing  recog- 
nition of  the  need  of  geological  study  in  the  undertaking  of  engi- 
neering problems,  the  special  branch  of  engineering  geology  has  been 


22  Subdivisions  of  the  Science  of  Geology 

developed.  The  geological  engineer  must  be  primarily  a  struc- 
tural geologist  and  one  who  has  a  thorough  grasp  of  the  princi- 
ples of  dynamic  geology,  including  hydrology  as  well,  while  physi- 
ography too  is  of  great  importance  to  him.  To  a  lesser  degree  a 
knowledge  of  rock  types,  of  stratigraphic  principles,  and  of  index 
fossils  will  be  needed  by  him,  and  not  infrequently  a  knowledge 
of  palaeogeography,  especially  that  phase  which  deals  with  the 
Pleistocene,  or  the  problems  of  the  glacial  period,  will  be  found  of 
the  greatest  value. 

Finally  there  has  been  developed  in  recent  years  the  special 
branch  of  geology  which  deals  with  the  problems  involved  in  mili- 
tary campaigns,  and  to  this  the  name  war  or  military  geology  has 
been  applied.  Some  of  these  problems  are  concerned  with  the 
proper  location  of  sites  for  camps,  and  trenches,  and  with  water 
supply  and  sanitation,  and  for  these  a  knowledge  of  structural 
geology,  of  rock  types,  of  stratigraphy,  and  of  glacial  geology  has 
been  found  necessary.  Other  problems  are  of  an  engineering  type 
and  require  the  preparation  of  the  geological  engineer.  Again,  the 
problems  involved  in  military  operations  need  for  their  solution 
a  well-trained  physiographer  and  a  competent  meteorologist  as 
well.  Problems  concerned  with  naval  warfare  require  the  atten- 
tion of  one  well  versed  in  hydrology,  especially  that  phase  of  it 
.which  deals  with  the  oceans,  or  oceanography. 

There  are  other  ways  in  which  a  knowledge  of  geology  has  be- 
come useful  to  man,  and  as  the  science  itself  is  developed  new 
channels  of  application  into  which  it  may  be  directed  will  no  doubt 
be  discovered. 

III.    The  History  of  Geology 

The  student  should  further  realize  that  the  development  of  his 
science,  the  history  of  geologic  thought,  cannot  be  neglected  by 
him.  We  profit  by  the  mistakes  of  our  predecessors  as  much  as 
we  do  by  their  achievements,  and  the  history  of  the  discovery  of 
facts  and  of  the  development  of  geological  opinion  since  the  days 
of  the  Greek  philosophers  is  fraught  with  lessons  equal  in  import 
to  those  gained  from  the  pursuit  of  the  history  of  any  other  depart- 
ment of  human  thought  and  endeavor.  At  this  point  it  will  be 
desirable  for  the  student  to  read  the  masterly  sketch  of  this  history 
from  the  pen  of  Sir  Archibald  Geikie,  the  book  entitled  Found- 
ers of  Geology,  and  if  possible  follow  this  by  a  perusal  of  the  older 


The  Study  of  the  Lithosphere  23 

historical  sketch  by  Sir  Charles  Lyell  in  volume  I  of  his  Principles 
of  Geology.  For  greater  details  the  student  is  finally  referred  to 
the  History  of  Geology  and  Paleontology  by  Carl  von  Zittel, 
translated  into  English  by  Maria  Ogilvie  Gordon.  The  history  of 
geology  in  America  is  adequately  and  fully  treated  by  Dr.  "George 
P.  Merrill  in  his  book,  Contributions  to  the  History  of  American 
Geology. 


CHAPTER  III 


METHODS   OF  APPROACH   IN   THE   STUDY   OF 
THE   EARTH 

THE  RISE  OF  GEOLOGICAL  OBSERVATION  AND  INTERPRETATION 

THE  geologist  is,  above  all  things,  an  observer  in  the  great  out- 
of-door  world.     The  man  whose  horizon  is  bounded  by  the  walls 

of  a  city  can  never  be  a  geologist, 
though  he  may  gain  much  scien- 
tific knowledge  from  books  and 
from  an  inspection  of  collections 
in  museums  and  laboratories. 
The  true  geologist,  however,  goes 
directly  to  tlie  earth  and  there 
begins  his  inquiries.  Not  until 
observations  of  natural  facts  and 
phenomena  were  made  in  extenso 
was  the  inquiry  of  the  philoso- 
phers regarding  the  earth  and 
its  history  placed  on  a  scientific 
basis.  Scattered  observations 
and  more  or  less  accurate  de- 
ductions were  made  even  in 
antiquity.  Thus  Aristotle,  in 
the  third  century  B.C.,  had  a 

very  considerable  understanding  of  the  work  of  rivers  and  reasoned 
correctly  regarding  the  changes  in  the  land  and  sea  at  former  times. 
The  painter  Leonardo  da  Vinci  (1452-1519)  correctly  reasoned,  from 
an  observation  of  the  fossils  found  in  the  foothills  of  the  Apennine 
Mountains,  that  they  were  the  shells  of  once  living  animals,  though 
they  were  generally  regarded  either  as  freaks  of  nature  (lusus  nature) 
or  as  modern  shells  dropped  by  the  pilgrims  in  their  voyages  across 
these  mountains..  It  is  true  that  the  significance  of  fossil  sea  shells 
was  recognized  by  the  Greek  philosophers  but  their  explanations 

24 


FIG.  6.  —  Georges  Leopold  Chretien 
Frederic  Dagobert  Cuvier. 


The  Rise  of  Geological  Observation 


FIG.  7.  —  Jean  Baptiste  Pierre  Antoine 
de  Monet  de  Lamarck. 


were  generally  ridiculed  during  the  Middle  Ages.  It  was,  however, 
not  until  the  latter  part  of  the  eighteenth  and  the  early  part  of  the 
nineteenth  century  that  sys- 
tematic investigations  of  the 
rocks  of  the  earth  and  their  con- 
tained fossils  began,  and  from 
this  period  we  date  the  birth  of 
geology  as  a  science.  Scientific 
geology  arose  more  or  less  sim- 
ultaneously in  the  different 
countries  of  Europe.  In  France 
Etienne  Guettard  (1715-1786) 
and  Nicholas  Desmarest  (1725- 
1815)  were  among  the  first  to 
bring  observation  of  facts  to  the 
fore,  while  Buff  on  (1707-1788) 
indulged  in  brilliant  speculations 
on  the  origin  of  the  earth.  Later 
Alexander  Brongniart  (1770- 
1847)  investigated  the  rocks 

around  Paris,  and  Georges  Cuvier  (1769-1832,  portrait,  Fig.  6)  and 
the  Chevalier  de  Lamarck  (1744-1829,  portrait,  Fig.  7)  described 

their  fossils.  In  Germany  Abraham 
Gottlob  Werner  (1750-1817,  portrait, 
Fig.  8),  who  is  often  called  the  founder 
of  German  geology  and  who  was  Pro- 
fessor at  the  Mining  School  at  Frei- 
berg i.  S.,  exerted  a  profound  influence 
on  geology  especially  by  his  teachings, 
to  which  men  flocked  from  all  coun- 
tries. Being  mostly  an  observer  of 
the  details  of  specimens  and  rarely 
venturing  beyond  his  immediate  sur- 
roundings for  field  observations,  many 
of  his  geological  deductions  have 
proved  erroneous  —  though  his  pupils 
and  followers,  notably  Leopold  von 
Buch,  extended  their  observations 

over  wide  areas  and  added  much  to  the  store  of  geological  facts 
as  well  as  to  its  philosophy.     Switzerland  had  its  enthusiastic 


FIG.  8.  —  Abraham  Gottlob 
Werner. 


26- 


Methods  of  Approach 


FIG.  9.  —  James  Hutton,  M.D. 


student  of  the  structure  and  history  of  the  Alps  in  the  person  of 
H.  B.  de  Saussure  (1740-1799),  and  Russia  had  in  Pierre  Simon 

Pallas  (1741-1811)  its  careful 
student  of  the  Ural  Mountains, 
and  the  rocks  outcropping  there 
and  elsewhere  in  the  empire. 
In  Great  Britain  many  men  con- 
tributed to  the  discovery  of  facts 
and  the  interpretation  of  these. 
Among  them  the  first  rank  is 
given  to  James  Hutton  (1726- 
1797,  Fig.  9),  whose  work  marks 
a  turning  point  in  the  history  of 
geology,  for  he  insisted  that 
"  the  past  history  of  the  globe 
must  be  explained  by  what  can 
be  seen  happening  now,  or  to 
have  happened  only  recently,"1 
a  dictum  which  has  since  become  the  very  cornerstone  of  geology. 
Mutton's  great  work,  Theory  of  the  Earth  with  Proofs  and  Illustra- 
tions, is  better  known  through  the  classic  volume,  Illustrations  of  the 
Huttonian  Theory,  by  his  friend  John  Playfair  (1802,  Fig.  10), 
which  no  student  of  geology  should 
neglect  to  read.  In  this  work  are 
contained  many  of  the  fundamental 
principles  with  which  geologists 
are  concerned  to-day,  and  they  are 
illustrated  by  a  wealth  of  facts 
gleaned  by  Hutton  from  his  ram- 
bles through  Scotland  and  other 
countries. 

Another  of  the  Scottish  founders 
of  geology  was  Sir  James  Hall,  to 
whom  we  owe  the  origination  of 
experimental  geology.  The  best 
known,  however,  among  the  early 
English  geologists  was  William 

Smith  (1769-1839,  portrait,  Fig.  n),  who  is  generally  called  the 
"  Father  of  English  Geology."    He  determined  not  only  the  correct 

1  Geikie,  Founders  of  Geology,  p.  299. 


FIG.  10.  —  John  Playfair. 


The  Rise  of  Geological  Observation 


27 


FIG.  ii.  —  William  Smith. 


successions  of  the  English  rock  formations,  and  made  the  first 
geological  map  of  England,  but  gave  to  many  of  the  formations 
the  names  which  they  bear 
to-day. 

Finally,  the  student  should 
remember  among  English  geolo- 
gists the  name  of  Sir  Charles 
Lyell  (1797-1875,  portrait',  Fig. 
12),  as  that  of  a  man  who  has 
had  the  most  profound  influence 
on  geological  thought.  His 
great  work,  the  Principles  of 
Geology,  has  become  a  classic  of 
geological  literature. 

Among  the  men  who  exerted  a 
profound  influence  on  American 
geology  in  the  early  days  of  its 
development,  the  names  of  William  McClure  (portrait,  Fig.  13) 
and  Amos  Eaton  (portrait,  Fig.  14)  stand  out  prominently.  Mc- 
Clure, born  in  Scotland  in  1763,  became  an  American  citizen  near 
the  close  of  the  century.  In  1809  he  published  the  first  important 

work  on  American  Geology,  in 
which  appeared  the  first  geolog- 
ical map  of  the  Eastern  United 
States,  and  one  of  the  first 
geological  maps  of  the  country. 
Amos  Eaton  (1776-1842,  Fig. 
14),  born  in  New  York  state,  is 
known  especially  for  his  Index 
to  the  Geology  of  the  Northern 
States  (1818),  which  was  the 
first  geological  textbook  pub- 
lished in  America,  and  in  this 
and  subsequent  works  he  laid 
the  foundation  for  the  New 
York  geological  system.  Many 

FIG.  12.  -  Sir  Charles  Lyell.  of  the  names  of  American  for- 

mations still  current  were  first 

applied  by  him.     He  also  made  the  first  geological  map  of  New 
York.    The  important  influence  which  McClure  and  Eaton  had 


28 


Methods  of  Approach 


FIG.  13.  —William  McClure. 


on  the  development  of  American  geology  has  been  recognized  by 
the  designation  of  the  first  two  eras  in  the  history  of  this  science 

in  America  as  the  Maclurean 
(1785-1819)  and  the  Eatonian 
(1820-1829)  (Merrill).  Other 
early  American  geologists  will 
be  referred  to  in  later  chapters. 

THE  FIELD  OF  GEOLOGICAL 

OBSERVATIONS 
"  Where,  then,"  the  student 
will  ask,  "can  the  facts  of 
geology  be  observed,  and  how 
have  they  become  available  ?  " 
To  get  at  the  facts  of  structural 
geology  the  student  must  go 
to  the  rocks.  True,  the  rocks 
and  the  minerals  and  the  fossils  are  brought  to  him  in  museums 
and  laboratories,  and  he  will  dp  well  to  begin  his  studies  of  selected 
examples  thus  brought  together  and  capable  of  being  examined 
under  the  most  favorable  conditions.  But  the  knowledge  thus 
gained  must  be  amplified  and  correlated  by  repeated  visits  to  the 
home  of  the  rocks,  where  alone  their  larger  relations  and  their 
true  significance  in  the  history 
of  the  earth  can  be  ascertained. 

The    Field  for    the    Study    of 
Rocks  and  Rock  Structures 

Rock  Exposures  in  Flat 
Countries.  —  The  dwellers  in 
the  interior  of  our  country,  or 
the  traveler  on  the  broad  plains 
of  northern  Germany,  of  Rus- 
sia, of  Hungary,  or  of  China, 
will  find  little  opportunity  to 
get  a  view  of  the  rocks  which 
underlie  these  regions,  for  an 
almost  continuous  mantle  of 
soil  and  drift  covers  the  solid  rock.  Only  where  rivers  have  cut 
channels  through  the  surface  layer  of  loose  material,  the  mantle- 


FIG.  14.  —  Amos  Eaton. 


The  Field  of  Geological  Observations  29 

rock,  or  where  quarries  have  been  opened,  or  construction  opera- 
tions have  necessitated  excavation  down  to  and  into  the  solid  bed 
rock,  is  there  an  opportunity  for  observation  of  the  rocks  beneath 
the  mantle-rock.  River  valleys  and  gorges  are  therefore  the 
favorite  resorts  of  the  geologist  of  the  plains,  while  quarries, 
railroad  cuts,  and  other  excavations  which  expose  the  rock,  like- 
wise receive  his  attention.  Borings  and  drillings  for  oil,  gas,  or 
water  often  prove  of  use  to  him,  though  generally,  except  where 
the  diamond  drill  is  used,  and  the  core  preserved,  the  record  of 
such  borings  is  only  of  doubtful  and  minor  value.  Salt  shafts, 
such  as  those  in  central  New  York  and  that  near  Detroit,  Mich., 
furnish  excellent  sections  at  the  time  of  excavation.  Fortunately 
for  the  geologist  of  the  plains,  however,  few  regions  of  any  great 
extent  are  wholly  covered  by  mantle-rock ;  more  commonly  low- 
lying  ridges  of  rock,  formed  by  resistant  layers,  are  exposed  above 
the  general  surface  of  the  soil  and  drift  mantle.  Such  "  ledges  " 
generally  mark  the  edges  of  beds  of  hard  rock  which  have  a  gentle 
slope  away  from  the  edge  of  the  ledge.  When  the  ledge-forming 
layer  is  very  thick,  a  cliff  of  some  height  may  result,  and  this 
generally  furnishes  abundant  opportunity  for  observation  and 
deduction. 

All  natural  exposures  of  the  rock  are  called  outcrops,  and  the 
outcropping  ledges  together  with  the  exposures  in  the  stream  chan- 
nels, especially  those  which  cut  across  the  cliffs,  furnish  to  the  geolo- 
gist of  the  flat  countries  his  most  satisfactory  data.  We  may  note 
a  few  examples. 

Illustration  from  New  York  State.  —  The  state  of  New  York 
furnishes  an  illustration  of  the  types  of  rock  outcrop  in  ledges  re- 
ferred to  in  the  preceding  paragraphs.  Over  the  greater  part  of 
the  state  the  rocks  are  so  gently  inclined  that  they  appear  horizontal 
to  the  eye,  and  it  is  only  when  they  are  seen  in  the  cliffs  of  Niagara 
gorge  and  along  Lake  Erie  that  their  gentle  southward  descent 
becomes  noticeable.  Such  a  section,  considerably  generalized,  is 
shown  in  the  following  diagram  (Fig.  I5).1  Where  the  beds  end 
in  the  air  upon  the  north,  the  harder  ones,  such  as  limestones  and 
sandstones,  form  a  series  of  low  cliffs,  while  the  ends  of  the  softer 
shale  beds  are  usually  marked  by  broad  flat-bottomed  valleys.  The 

1  The  usual  method  of  drawing  sections  is  to  place  the  north  end  upon  the  right, 
but  this  is  here  reversed,  because  the  observer  along  Lake  Erie  views  the  cliffs  from 
the  west,  and  therefore  south  is  on  his  right. 


Methods  of  Approach 


C    «J 
o  O  * 


6 


bol-a.3  §p 

Iflil 

•S  S-oio 


largest  and  deepest  of  these  valleys  is  occupied  by  Lake  Ontario, 
while  others  are  filled  by  soil  which  conceals  the  rock.  These  cliffs 
or  escarpments  generally  have  an  abrupt  northern  face  across  the 
edge  of  the  hard  rock,  and  in  these  faces 
&  E  fi  i  quarries  are  generally  opened. 

These  cliffs  can  be  traced  with  more 
or  less  interruption  across  New  York 
state  from  the  Niagara  River  and  Lake 
Erie  (Fig.  16)  to  the  Hudson,  although 
they  become  modified  because  some  of 
the  beds  seen  in  the  western  part  of  the 
state  die  out,  or  change  in  character  and 
new  ones  appear  (Fig.  17).  This  is 
shown  on  the  geological  map  of  the  state 
of  New  York,  where  a  series  of  broad 
color  bands  extends  east  and  west  across 
the  state.  Each  color  band  in  general 
represents  one  rock  layer  or  group  of 
layers,  and  the  width  of  the  color  band 
indicates  the  amount  of  exposure  of  each 
E  ^  which  would  appear  were  all  the  cover- 
•  tJ*  ^  ing  soil  and  vegetation  removed ;  or  in 
.-•§  other  words,  the  amount  by  which  each 
|  g  lower  bed  projects  beyond  the  next  higher 
S  2  one  which  covers  it. 
T5  of  Wherever  streams  have  cut  across  this 
series  of  beds,  gorges  are  formed,  in  the 
walls  of  which  the  cut  edges  of  the  rocks, 
the  soft  layers  as  well  as  the  hard  ones, 
are  shown.  The  most  striking  examples 
of  such  gorges  are  shown  .along  the 
Genesee  River,  which  crosses  the  state 
from  south  to  north.  In  the  section 
from  Rochester  northward  it  cuts  the 
lower  beds,  the  harder  strata  producing 
waterfalls.  In  the  section  between  Portage  and  Mt.  Morris,  it 
cuts  the  higher  strata,  and  here  too  several  waterfalls  are  formed  by 
hard  layers.  Between  these  two  points  many  smaller  tributary 
streams  have  cut  into  the  sides  of  the  valley  and  exposed 
the  rocks.  Here,  too,  are  situated  several  deep  shafts  which  go 


P"      * 

§  3 


4)  «-*3 

+* 


The  Field  of  Geological  Observations  31 

down  vertically  to  the  Salina  salt  beds  and  during  the  cutting  of 
which  the  succession  of  the  rocky  beds  was  ascertained. 

A  good  understanding  of  the  succession  of  this  series  of  rocky 
formations  is  obtained  by  the  traveler  who  passes  from  the  Adi- 
rondack Mountains  southwestward  across  the  state  to  Elmira, 
especially  if  he  take  advantage  of  the  various  sections  exposed 
in  the  gorges  of  the  streams  and  the  banks  of  the  Finger  Lakes. 


4»        •** 


FIG.  1 6.  —  Cliff  of  Devonian  rocks  exposed  on  the  shores  of  Lake  Erie,  south 
of  Buffalo.  Typical  of  rock  exposures  shown  on  the  lake  shore  from  Buffalo 
to  Cleveland.  (The  section  shows  Hamilton  (Wanakah)  shales  at  the  base, 
the  projecting  Morse  Creek  limestone,  above  which  lie  the  Windom  shales  and 
higher  Devonian  shales.) 

It  was  by  the  study  of  these  natural  outcrops  of  the  state,  sup- 
plemented by  those  made  on  the  sections  exposed  during  the  cutting 
of  the  Erie  Canal  in  1817-1825,  that  the  foundation  of  American 
geology  was  laid  by  such  men  as  Amos  Eaton,  and  by  James  Hall 
and  others  associated  with  him  on  the  geological  survey  of  New 
York  state. 

Other  Exposure  of  this  Type.  —  The  type  of  outcrop  just  described 
is  found  in  most  regions  of  flat-lying  rocks  in  our  own  country  as 


32  Methods  of  Approach 

well  as  abroad.  Thus  the  geologist  who  starts  from  Baraboo,  Wis- 
consin, and  proceeds  southeastward  to  Lake  Michigan  at  Mil- 
waukee, will  cross  such  a  series  of  rocky  ridges  separated  by  soil- 
filled  valleys,  and  a  similar  experience  awaits  the  traveler  across 
Kansas  from  southeast  to  northwest,  or  the  one  who  proceeds  south- 
ward across  Oklahoma,  or  journeys  from  central  Texas  either  north- 
westward or  southeastward.  The  traveler  across  England  from 
Liverpool  to  London  also  crosses  such  a  series  of  rocky  ridges,  which 


FIG.  17.  —  View  of  West  Hill,  Schoharie,  a  terraced  edge  of  the  Helderberg 
escarpment  or  cuesta,  in  eastern  New  York.  Note  the  prominent  cliffs  formed 
by  the  limestone  members  and  the  intermediate  slopes  formed  by  softer  beds. 
(Courtesy  of  N.  Y.  State  Geological  Survey.) 

in  general  extend  in  a  northeast-southwest  direction,  and  are  formed 
by  a  succession  of  nearly  horizontal  rock-layers  with  westward 
facing  cliffs,  though  these  are  not  generally  visible  from  the  train. 
Similar  outcrops  of  nearly  horizontal  layers  of  rock  surround  Paris 
in  constantly  widening  circles  on  three  sides.  The  edges  of  some 
of  these  rocks  form  cliffs  or  escarpments  facing  outward.  If  one 
were  to  represent  the  rock  formations  which  underlie  Paris,  and 
which  have  a  shallow  basin-like  structure,  by  a  nest  of  plates,  the 
smallest  at  the  top,  the  successive  rims  of  these  plates  would  repre- 
sent the  encircling  cliffs,  while  the  location  of  Paris  would  be  in 
the  center  of  the  uppermost  and  smallest  plate.  Many  rivers  have 
cut  channels  through  the  edges  of  these  rock  plates,  while  others 


The  Field  of  Geological  Observations  33 

flow  in  the  depressions  between  successive  rims.  Thus  numerous 
and  excellent  exposures  of  the  rocks  are  found,  even  though  the 
marginal  portions  between  the  edges  of  successive  rock  layers  are 
frequently  covered  by  soil  and  vegetation.  These  numerous  rock 
exposures  made  possible  the  observations  which  gave  the  French 
founders  of  our  science  the  data  on  which  to  build  their  deductions, 
and  the  cliffs  which  they  form  around  Paris  were  of  the  utmost  im- 
portance in  the  conduct  of  the  Great  War  so  recently  closed. 

Outcrops  in  Mountainous  Countries.  —  It  is,  however,  in  the 
elevated  portions  of  the  earth  —  the  mountains  and  chains  of  up- 
lands —  that  rock  outcrops  are  most  frequent  on  the  surface  of  the 
land.  Here  the  soil  and  rock  debris  lodges  only  in  the  depressions, 
while  between  these  the  ledges  protrude  and  give  opportunities 
for  observation.  Here,  too,  deep  canons  are  cut  by  the  rivers  and 
glaciers  and  thus  additional  rock- walls  are  opened  for  observation. 
German,  Austrian,  and  Swiss  geologists  have  for  the  most  part 
been  limited  to  such  regions  for  their  observations,  and  the  wonder- 
ful rock  exposures  of  the  Alps  and  other  mountains  have  made  it 
possible  for  them  to  carry  their  studies  along  certain  lines  to  great 
lengths.  Werner,  the  father  of  German  geology,  made  most  of 
his  observations  in  the  subdued *  mountain  district  of  Saxony,  es- 
pecially the  Erzgebirge,  which  has  long  been  famous  for  its  old 
and  extensive  mining  operations. 

Since  France  and  Italy  also  border  on  the  Alps,  and  have  mountain 
ranges  of  their  own,  the  geologists  of  these  countries  were  enabled 
to  avail  themselves  of  the  rocks  and  rock  structures  thus  revealed. 
British  geologists,  too,  have  been  able  to  some  extent  to  resort  to 
this  type  of  exposure,  though  in  these  moisture-enveloped  islands, 
as  over  parts  of  Scandinavia  as  well,  the  dense  though  low  cover  of 
vegetation  and  the  peat  accumulations  obstruct  much  of  the  un- 
derlying rock,  as  all  students  of  Irish  geology  know  only  too  well. 
The  Highlands  of  Scotland,  however,  furnish  many  good  opportu- 
nities for  observation,  as  do  also  many  of  the  higher  English  dis- 
tricts and  especially  the  mountain  region  of  Wales.  Swedish  geolo- 
gists have  frequently  had  to  resort  to  digging  through  the  surface 
layers  to  get  at  the  underlying  rock,  and  it  is  not  an  uncommon 
sight  to  see  the  Swedish  geologist  in  the  southern  interior 
accompanied  by  a  factotum,  whose  duty  it  is  to  wield  pick  and 
shovel. 

1  This  term  implies  that  the  old  mountains  have  been  much  worn  down. 


34  Methods  of  Approach 

In  European  Russia  and  adjoining  districts  there  are  vast  areas 
of  flat-lying  rocks  covered  by  soil  and  drift,  so  that,  except  along 
the  coast  and  in  the  river  channels,  outcrops  of  the  bedrock  are 
difficult  to  find.  But  where  these  rocks  are  uplifted  —  in  the  Ural 
Mountains  on  the  east,  the  Caucasus  on  the  south,  and  the  Carpa- 
thians on  the  west  —  outcrops  abound,  and  here  the  true  relation- 
ships of  the  rock  formations  may  be  ascertained. 

In  America,  the  New  England  uplands,  the  White,  Green,  and 
Adirondack  Mountains,  and  the  Appalachian  Chain  furnish  an 


FIG.  1 8.  —  Marsten  Rock :  View  on  the  North  Sea  coast  of  England  (Dur- 
ham), showing  characteristic  erosion  features  in  a  detached  mass  of  Magnesian 
Limestone  of  Permian  age,  which  was  formerly  united  with  the  cliff  on  the  left. 
This  is  typical  of  the  rocky  character  of  much  of  the  British  coast,  though  the 
kind  of  rock,  the  structures,  and  the  erosion  forms  vary  from  point  to  point. 

abundant  series  of  rock  outcrops  for  the  eastern  geologist.  The 
old  and  generally  much-subdued  mountain  system,  the  rocks  of 
which  may  be  traced  by  frequent  outcrops  from  New  York  City 
to  the  Highlands  of  the  Hudson,  and  which  can  be  followed  south- 
westward  through  New  Jersey,  Pennsylvania,  and  Maryland,  and 
indeed  all  the  way  down  to  the  Carolinas,  where  it  constitutes  the 
older  Appalachian  Chain,  is  a  typical  example  of  an  elevated,  though 
for  the  most  part  not  very  mountainous,  country,  and  here  out- 
crops abound.  Many  of  our  western  mountains  are  especially  well 
adapted  for  geological  observation,  for  here  the  aridity  of  the  climate 
prevents  the  growth  of  much  vegetation,  and  the  rock  structures, 


The  Field  of  Geological  Observations 


35 


frequently  developed  on  a  gigantic  scale,  are  visible  for  many  miles. 
This  makes  the  Rocky  Mountains  a  veritable  paradise  for  the 
American  geologist. 

Outcrops  upon  the  Coast.  —  Of  all  the  natural  rock  exposures, 
however,  those  of  the  coast-line  are  the  most  attractive,  and  in 
many  respects  the  most  satisfying.  Wherever  the  sea-coast  or  the 
shore  of  a  large  lake  is  formed  by  rocks  which  rise  above  the  sur- 
face of  the  water,  the  cutting  work  of  the  waves  keeps  the  exposure 
fresh.  A  tramp  along  a  rocky  sea-coast  is  replete  with  interest  to 
the  geologist,  and  many  of  the  choicest  bits  of  geological  observa- 
tion have  been  made  on  such  sea-cliffs.  Great  Britain,  with  its 
wonderful  rocky  sea-coast,  probably  leads  the  world  in  the  variety 
and  significance  of  rocks  and  rock  structures  there  exposed  (Fig. 
iS).1  No  student  of  geology 
can  afford  to  neglect  the  won- 
derful English  and  Scottish 
coast,  which  has  furnished  the 
British  geologists  so  many  op- 
portunities for  the  observation 
of  facts  that  there,  more  than 
elsewhere,  geological  science 
has  advanced,  since  the  days 
of  William  Smith,  with  phe- 
nomenal strides.  This  is 
probably  the  reason  why  Eng- 
lish geology  quickly  became 
the  standard  of  comparison 
for  other  nations,  in  whose 
home  countries  observation 
was  a  more  arduous  task,  be- 
cause they  did  not  include 
such  marvelous  coast  ex- 
posures. 


FIG.  19.— Pulpit  Rock,  Nahant  (Mass.) . 
One  of  the  most  picturesque  and  in- 
structive rock  sections  upon  the  Atlantic 
coast  of  New  England.  The  rocks  are 
metamorphosed  Cambrian  shales  and 
limestones  with  a  great  diabase  sheet 
(sill)  intruded  between  the  strata,  and 
the  whole  eroded  by  the  waves  working 
chiefly  upon  the  softer  strata.  (Photo 
by  A.  W.  G.) 


Northern  France,  too,  has  a  coast-line  of  great  interest  to  the 
geologist,  and  so  has  Norway.  The  coast-line  of  Germany,  on  the 
other  hand,  is  mostly  sandy,  and  there  is  little  diversity  in  the  types 
of  the  facts  which  it  discloses. 

The  Atlantic  coast-line  of  North  America  is  for  the  most  part  a 
sandy  one.  Only  in  New  England,  in  the  Canadian  coastal  prov- 

1  See  also  Figs.  113,  114,  120,  121,  203,  498,  510,  511,  530-532,  719-721,  723  a,  b. 


36  Methods  of  Approach 

inces,  and  in  Newfoundland  can  be  seen  coastal  sections  compa- 
rable to  some  extent  to  those  of  Great  Britain.  The  northern  re- 
gions, however,  are  accessible  with  difficulty,  and  have  only  recently 
been  investigated.  But  the  New  England  coast,  and  especially  that 
of  Massachusetts  (Fig.  19),  is  a  Mecca  for  American  geologists, 
and  many  of  the  workers  in  American  geology  have  had  their  pre- 
liminary training  through  a  study  of  that  interesting  region. 

Regions  for  the  Study  of  Dynamic  Geology 

Although  the  principles  of  dynamic  geology,  the  workings  of  the 
chemical  and  physical  forces,  may  be  studied  to  much  advantage 
in  the  laboratory  —  such  study,  too,  is  incomplete  without  recourse 
to  the  outdoor  field.  It  is  upon  the  sea-shore  that  some  of  the  pro- 
foundest  lessons  of  erosion  by  waves,  of  transportation  by  currents, 
and  of  deposition  in  quieter  waters  can  be  learned.  Here,  too, 
the  method  of  entombment  of  fossils  and  the  formation  of  many 
original  structures,  such  as  ripple-marks  and  the  like,  can  be  ob- 
served. Rocky  as  well  as  sandy  and  muddy  shores  should  be  visited. 
Shores  of  large  lakes  may  serve  as  a  substitute  in  inland  regions, 
but  lakes  have  in  addition  many  characters  of  their  own.  Ponds 
and  temporary  pools  also  teach  their  lessons.  River  valleys  and 
gorges,  rapids  and  waterfalls,  brooks,  and  even  the  roadside  gutter, 
furnish  lessons  in  dynamic  geology,  as  do  also  the  hillside,  the 
mountain  slopes,  and  the  elevated  peaks,  where  rocks  are  shattered 
by  frost,  and  decay  under  atmospheric  influence.  Glaciers  present 
many  illustrations  of  dynamic  geology,  while  caverns  and  under- 
ground channels  have  special  lessons  to  teach.  The  deserts  and  all 
regions  where  wind  is  at  work  furnish  illustrations  of  the  mechan- 
ical activities  of  the  wind,  while  pools  and  salt  pans  in  arid  regions 
furnish  illustrations  of  chemical  activities  and  of  precipitation  of 
salts  through  condensation  of  the  water  under  evaporation.  Springs, 
too,  illustrate  dynamical  activities,  both  physical  and  chemical, 
and  artesian  wells,  oil  wells,  geysers,  and  similar  phenomena  are 
replete  with  them.  Finally,  volcanoes  and  other  such  phenomena 
furnish  the  means  for  the  study  of  igneous  activities. 

The  great  English  geologist,  Sir  Charles  Lyell,  whom  we  some- 
times call  the  "  Father  of  Modern  Geology,"  has  said  that  the 
geologist  must  be  primarily  a  traveler  —  he  must  go  to  other  lands 
than  his  own  and  so  widen  the  scope  of  his  experience.  Werner, 


Geological  Literature  37 

the  founder  of  German  geology,  confined  his  observations  mainly 
to  his  limited  Saxon  district,  and  attempted  to  formulate  from 
these  observations  laws  which  should  govern  the  rest  of  the  world. 
Naturally  he  fell  into  many  and  profound  errors,  so  that  to-day 
scarcely  one  of  his  theories  is  held.  Since  his  day  German  geologists 
have,  however,  become  great  travelers,  not  only  in  their  own  but 
in  most  other  lands.  As  a -result,  their  observations  have  become  of 
wide  scope,  and  they  have  added  much  to  geological  knowledge. 
British  and  American  geologists  have  only  recently  begun  to 
follow  the  advice  of  Lyell,  but  already  their  efforts  have  been 
crowned  with  considerable  success. 

Let  the  student  of  geology,  then,  come  to  realize  that  the  value 
of  his  deductions  increases  in  proportion  to  the  range  of  his  obser- 
vations, and  that  no  single  country  or  region  of  the  world  will  give 
him  all  he  needs.  The  American  student,  owing  to  the  wide  ex- 
tent and  diversity  of  his  country,  is  perhaps  more  favored  in  this 
respect  than  is  the  geologist  of  any  other  nationality,  but  at  present 
only  a  limited  portion  of  our  country  has  become  sufficiently  ac- 
cessible to  make  extended  observations  possible. 

THE  IMPORTANCE  OF  GEOLOGICAL  LITERATURE 

Finally,  it  must  not  be  overlooked  that  the  observations  of  our 
predecessors  are  recorded  in  the  literature  of  the  science,  and  that 
here  we  find  a  mine  of  information,  the  value  of  which  cannot  be 
overestimated.  No  one  can  repeat  all  of  the  observations  which 
have  been  made  in  the  past,  even  were  such  repetition  desirable. 
In  addition  to  the  laboratory  and  field,  then,  the  student  of  geology 
must  go  to  the  library,  and  a  thorough  understanding  of  the  liter- 
ature on  his  special  field  is  of  fundamental  importance  to  the  worker. 
Besides  special  books  on  different  aspects  of  the  science,  the  student 
should  gain  familiarity  in  the  use  of  the  official  publications  issued 
by  the  governments  of  the  various  countries,  the  proceedings  of 
scientific  societies,  and  the  special  journals  devoted  to  geology  and 
kindred  sciences. 


CHAPTER  IV 

MATERIAL   OF  THE   EARTH'S   CRUST 

THE  material  of  which  the  crust  of  the  earth  consists  is  spoken 
of  as  rock,  a  term  which  we  shall  presently  define  more  precisely. 
Rocks  are  in  turn  combinations  of  minerals  or  large  aggregates  of  a 
single  material,  and  these  are  formed  by  the  combination  of  chemical 
elements,  or  by  the  union  of  those  elementary  combinations  of 
elements  which  are  called  ions.  The  study  of  chemical  elements 
and  of  their  combination  into  ions  and  the  union  of  these  to  form 
other  substances  (salts,  etc.),  belongs  in  the  domain  of  chemistry. 
The  study  of  minerals,  their  properties  and  occurrence,  belongs 
to  the  special  branch  of  the  earth  science  called  mineralogy.  An 
elementary  preparation  in  chemistry  and  mineralogy  is  necessary 
to  the  student,  and  should  be  obtained  by  him  if  possible  before 
undertaking  the  study  of  geology.  In  this  book  we  can  give 
only  a  brief  summary  of  the  more  important  elements  and  minerals 
with  which  the  student  should  have  some  acquaintance.  The 
important  minerals  which  enter  into  the  composition  of  the  rocks, 
or  which  themselves  occur  in  rock-like  masses,  will  be.  dealt  with 
somewhat  more  fully  in  the  discussion  of  these  rocks. 

THE  CHEMICAL  ELEMENTS  AND  THEIR  PRIMARY  COMBINATIONS 

Of  all  the  chemical  elements  which  enter  into  the  composition 
of  the  earth's  crust,  only  a  comparatively  small  number  are  of 
importance  in  combining  to  form  the  more  common  minerals  and 
rocks.  The  principal  ones  are  given  in  the  following  list  from 
F.  W.  Clarke,  in  which  their  relative  importance  is  also  indicated. 

Some  of  these  elements  occur  pure  in  nature  and  are  then  called 
native  elements.  Among  these  are  oxygen,  nitrogen,  sulphur, 
carbon,  and  the  metals  gold,  silver,  copper,  platinum,  etc.  The 
majority  of  elements,  however,  form  combinations  among  them- 
selves, with  the  result  that  more  or  less  stable  compounds  are 

produced. 

38 


Chemical  Elements  and  Their  Combinations      39 


The  More  Important  Elements  and  Their  Distribution 


NAME  OF  ELEMENT 

SYMBOL 

LlTHOSPHERE 

93  PER  CENT 
or  WHOLE 

HYDROSPHERE 
7  PER  CENT 
OF  WHOLE 

AVERAGE 
INCLUDING 
ATMOSPHERE 

Oxygen     ...          ... 
Silicon 

0 

Si 

47-33 

27  74. 

85-79 

50.02 
25  80 

Aluminum 

Al 

7  8s 



7  3O 

Iron     

Fe 

4-  SO 



4.  l8 

Calcium 

Ca 

•j  An 

OS 

322 

Magnesium 

Mg 

2  24. 

14. 

2  08 

Sodium     .                        . 

Na 

2  4.6 

I  14. 

2  "?6 

Potassium     

K 

2  4.6 

O4. 

2  28 

Hydrogen     
Titanium 

H 
Ti 

.22 
4.6 

10.67 

•95 

A-l 

Carbon 

c 

10 

OO2 

18 

Chlorine  
Bromine  ...          ... 

Cl 
Br 

.06 

2.07 
OO8 

.20 

Phosphorus  
Sulphur 

P 

s 

.12 
I  2 

OQ 

.11 
1  1 

Barium     
Manganese        .          ... 

Ba 
Mn 

.08 
08 

.08 
08 

Strontium     ...... 
Nitrogen 

Sr 

N 

.02 

— 

.02 

Q-l 

Fluorine 

Fl 

IO 



IO 

All  other  Elements  including 
Gold,    Silver,    Platinum, 
Arsenic,    Copper,    Lead, 
Mercury,     Nickel,     Tin, 
Zinc,  Radium,  etc.  .     .     . 

•50 

•47 

Total     

TOO  OO 

IOO  OO 

IOO  OO 

Chemical  Combinations 

The  following  types  of  chemical  combinations  exist  in  nature 
or  are  produced  in  the  laboratory. 

Oxides.  —  Combination  of  an  element  with  oxygen.  Examples : 
Silica  (SiO2) ;  Carbon  dioxide  (COg) ;  Iron  oxide  (Fe2O3) ;  Water 
(H20). 

In  the  first  of  these,  two  parts  of  oxygen  unite  with  one  of  silicon 
to  form  silica  or  quartz ;  in  the  second,  two  parts  of  oxygen  in  like 
manner  unite  with  one  of  carbon  to  form  the  gas  carbon  dioxide. 
In  the  third  example,  three  parts  of  oxygen  unite  with  two  of  iron 
to  form  the  sesquioxide  of  iron ;  and  in  the  fourth  example  one  part 
of  oxygen  unites  with  two  of  hydrogen  to  form  water. 


40  Material  of  the  Earth's  Crust 

Hydroxides.  —  These  are  combinations  of  an  element  (metal) 
or  a  group  of  elements  with  oxygen  and  hydrogen,  the  last  two 
in  equal  parts.  Examples:  Sodium  hydroxide  or  caustic  soda 
(NaOH) ;  Potassium  hydroxide  or  caustic  potash  (KOH) ;  Alumi- 
num hydroxide  (the  mineral  Gibbsite,  A1(OH)3).  In  this  last 
example  it  requires  three  parts  of  the  (OH)  group  to  satisfy  the 
combining  power  of  one  part  of  aluminum. 

Oxyhydroxides.  —  Like  the  preceding,  but  with  an  additional 
molecule  of  oxygen.1  Examples:  Aluminum  oxyhydroxide,  the 
mineral  diaspore  (AIO(OH)) ;  Iron  oxyhydroxide  or  goethite 
(FeO(OH)).  Both  hydroxides  and  oxyhydroxides  may  also  be 
expressed  as  combinations  of  oxides  and  water  (H2O) ;  thus : 

(Gibbsite) 


(Diaspore) 
(Goethite) 


The  hydroxides  and  oxyhydroxides  also  form  bases  with  which 
acids  combine  to  form  salts. 

Acids.  —  These  are  combinations  of  certain  elements  such  as 
chlorine,  carbon,  sulphur,  silicon,  etc.,  which  are  called  negative 
elements,  or  their  oxides  (negative  ions),  with  hydrogen  or  with 
the  oxyhydrogen  (OH)  combination  or  radical.  Examples: 
Hydrochloric  acid  HC1,  (Hydrogen  chloride  +  water) ;  Carbonic 
acid,  H2CO3  =  CO+2(OH) ;  Sulphuric  acid,  H2SO4  =  SO2+2(OH). 

Salts.  —  A  compound  formed  by  the  reaction  between  an  acid  and 
a  base  (hydroxide  or  oxyhydroxide)  with  the  simultaneous  forma- 
tion of  water,  is  called  a  salt. 

Thus: 
Na(OH)  ]  f      HC1      ]       [        NaCl        ]       [  H2O 

Sodium  +     {  Hydrochloric  }  =  {  Sodium  [chloride  }  +  {  Water, 

hydroxide  J  I         acid          J         I  or  common  salt  J         I 

Ca(OH)2|  f     H2S04    1       f        CaS04       ]       f  2H2O 

Calcium      f      +      {    Sulphuric      )  =  j          Calcium          |  +  {  Water 
hydroxide  J  I         acid         J         (         sulphate         J         I 

1  More  correctly  derived  from  the  hydroxide  by  the  abstraction  of  water,  as  shown 
on  comparison  of  the  formulas  of  Diaspore  and  Gibbsite,  the  former  having  two  mole- 
cules of  water  less. 


f    A1(OH)3 

2  {     Aluminum 
[     hydroxide 

1       i    A12O3    | 

i   ==   i  Aluminum  ? 
J          1     oxide        J 

f  H20 

+3  \  Water 

j  AIO(OH) 

1       f    A1203    ] 

i   H2O 

2  \      Aluminum 
I  oxyhydroxide 

}  =  {  Aluminum  } 
J          1       oxide       J 

+     i  Water 

FeO(OH) 

1       }    Fe203    1 

f  H20 

Iron  oxy- 

Iron 

+    i  Water 

hydroxide 

J         1      oxide      J 

I 

Chemical  Elements  and  Their  Combinations      41 

New  salts  may  also  be  formed  by  the  reaction,  in  solution,  of  a 
strong  acid  upon  a  salt  with  a  weak  acid,  when  the  weaker  acid  is 
set  free. 
Thus: 

SrCl2    ]  f      H2SO4    ]       f     SrSO4     1       f          2HC1 

Strontium  f       +      {      Sulphuric      }  =  {      Strontium     }  +  {         Hydrochloric 
chloride    j  [         acid          J         [      sulphate      J         [  acid 

Or  they  may  be  formed  by  the  interaction  of  two  salts  in  solution 
to  form  a  less  soluble  salt. 
Thus  :  . 

BaCL,  |  I   Na2SO4     1       f     BaSO4     ]       f       2NaCl 

Barium    f        ~{~  Sodium         f  =  \        Barium        f  ~^|  Sodium 

chloride  J  1      sulphate       J         (       sulphate      J         I         chloride 

The  barium  sulphate  is  insoluble  and  will  be  precipitated  out. 

Ions.  —  When  certain  chemical  compounds  such  as  acids,  salts 
and  the  bases,  are  dissolved  in  water,  they  are  believed  to  be  dis- 
sociated into  two  or  more  parts  which  are  either  the  elements  or 
their  simple  combinations,  and  which  are  called  ions.  They 
exhibit  a  marked  behavior  towards  the  passage  of  an  electric 
current  through  the  solution;  some,  regarded  ?as  charged  with 
positive  electricity,  being  attracted  by  the  negative  electrode,  and 
others,  regarded  as  negatively  charged,  being  attracted  to  the 
positive  electrode.  Examples  are  : 


Acids 


=H  Positive  and       Cl     .     .     .     .     negative  ions 

+ 
H2SO4  =H2  positive  and      SO41      .     .     .     negative  ions 

+ 
Base        KOH    =  K  positive  and      (OH)     .     .     .    negative  ions 

+ 

[  NaCl    =  Na  positive  and      Cl     .     .     .     .    negative  ions 
Salts  + 

I  CaSO4  =  Ca  positive  and      SO4        .     .     .     negative  ions 

1  In  dibasic  acids  the  dissociation  takes  place  in  two  stages.     In  fairly  concentrated 
solutions  sulphuric  acid  dissociates  wholly  or  in  part  as  a.  monobasic  acid.     Thus: 

I  _ 

H2S04  =  H+HSO4.    The  second  stage  takes  place  when  the  solution  is  more  dilute. 
Thus:  HSO4  =  H+SO4.     (Jones,  H.  C.,  The  Nature  of  Solutions.) 


42  Material  of  the  Earth's  Crust 

MINERALS 

All  native  elements,  oxides,  hydroxides,  oxy hydroxides,  acids, 
and  salts  which  occur  in  nature  in  a  solid  state,  are  called  min- 
erals. They  occur  either  in  crystalline  or  uncrystalline  (amor- 
phous) form  or  both.  Acids  are  rare  as  minerals,  but  native 
elements  and  oxides  are  common,  while  hydroxides  and  oxyhy- 
droxides  are  not  infrequently  met  with.  By  far  the  larger  number 
of  minerals,  however,  belong  to  the  category  of  salts,  among  which 
the  dominant  ones  are  the  silicates  formed  by  the  combinations 
of  metals,  etc.,  with  silicic  acid.  The  determination  of  minerals 
depends  upon  the  recognition  of  their  physical  characters  as  well 
as  of  their  chemical  composition.  There  are  many  physical  char- 
acters of  which  the  more  important  ones  will  be  briefly  summarized. 

Crystalline  Form 

Most  minerals  assume  definite  forms  in  which  certain  planes 
appear,  which  are  found  to  have  a  definite  relation  to  certain 
imaginary  lines  or  crystallographic  axes  (coordinate  axes)  about 
which  the  crystal  may  be  supposed  to  be  built  up.  There  are 
six  systems  recognized,  based  on  the  relative  length  and  relation- 
ship of  the  axes.  In  the  systems  with  three  axes,  these  axes 
may  differ  in  length,  when  they  are  designated  by  the  letters  a,  &, 
and  c,  respectively,  the  c  axis  being  the  vertical  one.  If  a  and  b 
are  equal  both  are  designated  by  the  letter  a ;  if  all  three  are  equal 
they  are  all  called  a.  The  various  faces  of  the  crystal  are  read 
with  reference  to  the  points  at  which  they  intersect  the  axis  or 
would  do  so  if  both  were  extended. 

If,  in  a  simple  crystal  of  one  set  of  planes,  a  plane  intersects  all 
three  axes  (unit  length),  these  axes  being  unequal,  this  plane  is 
given  the  symbol  a:b:c  (pyramid).  If  the  two  horizontal  axes 
are  equal,  it  is  designated  a:a:c  (tetragonal  pyramid) ;  if  all 
three  axes  are  equal,  it  is  designated  a:  a:  a  (octahedron) .  If 
the  plane  cuts  two  axes  and  is  parallel  to  the  third,  this  parallelism 
is  indicated  by  the  infinity  sign  (<*>)  and  the  formula  becomes 
a :  b :  <*>  c  (prism) ;  a :  a :  °o  c  (prism)  or  a  :  a :  <*>  a  (dodecahedron) 
as  the  case  may  be ;  if  it  cuts  only  one  axis  and  is  parallel  to  the 
other  two  the  designation  is  a:  aob:  <&c  or  <xa:b:  we  (pina- 
coids) ;  oo  a :  °o  b :  c  (basal  pinacoids) ;  a :  °o  a :  °o  c  (second  order 


Minerals  43 

prism) ;  °o  a :  <*>  a :  c  (basal  plane)  or  a :  <*>a:  <*>  0  (cube)  according 
to  the  relative  lengths  of  the  axes.  Planes  cutting  all  three  axes  at 
the  unit  length  are  called  pyramid  planes;  those  that  cut  the  two 
horizontal  axes  at  the  unit  length  and  are  parallel  to  the  vertical 
one,  are  called  prism  planes,  while  those  that  cut  only  one  of  the 
horizontal  axes,  being  parallel  to  the  other  and  to  the  vertical  one, 
are  called  pinacoidal  planes  except  in  the  case  where  the  two  horizon- 
tal axes  are  of  equal  length  (tetragonal  system),  when  they  are 
called  prism  planes  of  the  second  order.  Those  which  cut  the  c 
axis  and  are  parallel  to  the  others  are  called  basal  pinacoids. 

Finally,  planes  parallel  to  one  horizontal  axis  and  cutting  the 
other  and  the  vertical  one  are  called  dome  planes,  except  in  the  case 
where  the  two  horizontal  axes  are  equal  (tetragonal),  when  they 
are  called  pyramid  planes  of  the  second  order.  Other  planes  may 
occur  which  do  not  cut  the  axes  at  the  unit  length.  These  are 
designated'  by  the  coefficient  m  for  the  first  variation  from  the 
unit  length  and  n  for  the  second.  Thus  with  three  equal  axes  we 
may  have  planes  with  the  formula  a:  a:  ma  (trigonal  trisocta- 
hedron),  or  a:  ma:  ma  (tetragonal  trisoctahedron) ;  or  finally, 
a:na:  ma  (hexoctahedron) .  The  system  with  four  axes  has  the 
three  horizontal  ones  equal  and  at  angles  of  60°  with  one  another, 
while  the  vertical  one  is  at  right  angles  to  the  others. 

The  Six  Systems  of  Crystallization 

I.  Isometric.  —  Three  axes  of  equal  length  or  interchangeable 
and  at  right  angles  to  one  another.     Fundamental  forms :    cube 

(a:  oo a :  QO a) ;  octahedron  (a :  a :  a) ;  etc.  (Fig.  20). 

II.  Tetragonal.  —  Two  horizontal  axes  equal  and  interchange- 
able, the  vertical  one  (c)  of  different  length.     All  at  right  angles 
to    one     another.       Fundamental    forms:      tetragonal     prism1 
(a  :a:  QO  c) ;   tetragonal  pyramid  (a :  a:c);  etc.  (Fig.  21). 

III.  Hexagonal.  —  Three  equal  horizontal  or  interchangeable 
axes,  forming  angles  of  60  degrees;    a  vertical  axis  of  different 
length  at  right  angles  to  the  horizontal  ones.     Fundamental  forms  : 
hexagonal    prism    (a :  a :  <*>a:  <*>  c) ;     hexagonal    pyramid    (a:  a: 
oo  a:  c) ;  etc.  (Fig.  22). 

1  All  the  prisms  require,  of  course,  basal  planes  or  pyramids  to  complete  the  solid. 


44 


Material  of  the  Earth's  Crust 


FIG.  20.  —  Isometric  System.  Principal  Forms.  The  general  symbols  and  the 
values  of  the  coefficients  for  the  figures  given  are  added.  (After  Moses  and 
Parsons.) 


Holohedral.     (All  planes  developed.) 

A.  Octahedron.  —  a:  a:  a. 

B.  Trigonal    Trisoctahedron.  —  a  :  a  :  ma 

(«-a). 

C.  Tetragonal    Trisoctahedron  or  Trape- 

zohedron.  —  a  :  ma  :  ma.     (m—  2.) 

D.  Hexoctahedron.  —  a:na:  ma. 

E.  Dodecahedron.  —  a:  a:  oo a. 

F.  Tetrahexahedron. —  a  :na:  <x>a.(n—2.} 

G.  Cube  or  Hexahedron.  —  a  :  oo  a  :  co  a. 


HemihedraJ. 

(In  this  division  only  every  alternate  plane 
is  developed,  thus  giving  only  half  the  num- 
ber of  planes  found  in  the  corresponding 
holohedral  form.  This  is  indicated  by  pre- 
fixing a/2  to  the  symbol.) 

H.  Tetrahedron.  —  %(a:a:  a). 
I.   Deltohedron.  —  \(a:a:ma). 
J .    Tr is tetrahedron .  —  \  (a  :  ma :  ma) . 
K.  Hextetrahedron.  —  |(a  :  na  :  ma). 
L.  Pyritohedron.  —  \(a:na:  oo  a). 


Minerals 


45 


.p 


A 


FIG.  21.  —  Tetragonal  System.    Principal  Forms. 
(After  Moses  and  Parsons.) 

A.  Tetragonal  Pyramid,  ist  order.  —  a :  a :  c.1 

B.  Tetragonal  Pyramid,  2d  order.  —  a  :  oo  a  :  c.1 

C.  Ditetragonal  Pyramid.  —  a :  na:c.1 

D.  Ditetragonal  Prism.  —  a:na  :<&  c. 

E.  Tetragonal  Prism,  id  order.  —  a:<x>a:  ooc  (with  pyramid  of   ist  order 
(p) . —  na:na:  me} . 

F.  Tetragonal  Prism,  ist  order.  —  a :  a  :  oo  c. 
D,  E,  F,  show  Basal  Pinacoids  —  oo  a :  oo  a  :  c. 


1  When  occurring  in  combination  a  unit  length  for  c  is  selected  and  the  formula  be- 
comes a:  a:  me. 


(<--•>—-. 

:c     } 

I    \ 

V"- 

•4,4-- 

,      t 
1       ; 

!     i 

~«* 

»*cl^ 

i 

FIG.  22. — Hexagonal  System.     Principal  Forms. 

(After  Moses  and  Parsons.) 
Holohedral :  (All  faces  developed.) 

A.  Hexagonal  Pyramid,  ist  order.  —  a  :  a  :oo  a  :  c.1 

B.  Hexagonal  Pyramid,  zd  order.  —  a :  na :  na :  c.1 


C.   Dihexagonal  Pyramid.  —  a  :  na  :  pa :  c.1 

D.  Dihexagonal  Prism.  —  a  :  na  :  pa  :  <x>c. 

E.  Hexagonal  Prism,  2d  order.  —  a :  na :  na  :  oo  c.     (n=  2.) 

F.  Hexagonal  Prism,  ist  order.  —  a :  a  :  oo  a  :  oo  c. 
D,  E,  F,  show  Basal  Pinacoids.  — oo  a :  oo  a :  oo  a  :  c. 

Hemihedral.     (Half  the  number  of  faces  developed.) 

G.  Rhombohedron,  ist  order.  —  ^(a  :  a  :  oo  a :  c).1 
H.   Scalenohedron.  —  ^(a  :  na:  pa:  c).1 

I.     Trigonal  Prism,  ist  order.  —  \(a  :  a  :  oo  a  :  oo  c). 

J.     Ditrigonal  Prism.  —  ^(a :  na  :  pa :  oo  c). 

I  and  J  show  Basal  Pinacoids.  —  oo  a  :  oo  a :  oo  a :  c. 

1  me  in  combination ;  p  greater  than  n. 
46 


Minerals 


47 


IV.  Orthorhombic.  —  Three  axes,  all  of  unequal  length,  but  all 
forming  right  angles  with  one  another.  Fundamental  forms: 
orthorhombic  prism  (a :  b :  oo c) ;  orthorhombic  pyramid  (aibic); 
etc.  (Fig.  23). 


<4s\.     /  N 


FIG.   23.  —  Orthorhombic  System.     Principal  Forms  in  combination. 
(After  Moses  and  Parsons.) 

A.  Orthorhombic  (unit)  Pyramid  (p.).  —  a:b:c  (or,  na:b:mc). 
Orthorhombic  (unit)  Prism  (m).  —  a:b:co c  (or,  na:b:<x>c). 
Brachy-prism  (/).  —  na\b:<x>  c.     (n  =  2.) 

Brachy-dome   (/). — <x>a:b:c   (oroo  a  :  b  :  2  c). 

B.  Same  forms  as  in  A  with  addition  of  Basal  Pinacoid  (c). — ooa:oo6:c. 

C.  Same  forms  as  in  B  with  addition  of  two  other  pyramids  (i).  —  a  :  b  :  I  c, 
and  (q). — a  :b  :  2  c;  and  two  macro-domes  (h). — a  :oo  b  :  f  c,  and  (k).  —  a:<xb: 
2  c,  and  a  macro-pinacoid  (a).  —  a  :oo  b  :  oo  c. 

V.   Monoclinic.  —  Three  axes,  all  of  unequal  length,  the  hori- 
zontal ones  at  right  angles  to  each  other,  the  vertical  one  (c) 


J 


FIG.   24.  —  Monoclinic  System.     Principal  Forms  in  combination. 
(After  Moses  and  Parsons.) 

A.  Monoclinic  (unit)  Prism  (m).  —  a:b:v$c  (or,  na:b:<x>c). 
Hemi-pyramid  (negative  p,  positive  '»).  —  a:b:c  (or,  na :  b  :  me). 
Ortho  Pinacoid  (a).  —  a  : oo  b  : oo  c. 

Clino  Pinacoid  (b).  —  oo  a  :  b  :  oo  c. 
Basal  Pinacoid  (c).  — oo  a  :  oo  b  :  c. 

B.  Same  planes   as   in    A,   except   positive  hemi-pyramid   (v)    and   basal 
pinacoids  (c). 

C.  Same  planes  as  in  A  except  positive  hemi-pyramid  (v). 

D.  Unit  prism;  basal  pinacoid;  two  positive  hemi- pyramids  v.  and  w. 
(a :  b  :  3  c )  and  a  dino-dome  z  =  (ooa:6:  2  c). 


Material  of  the  Earth's  Crust 


inclined  with  reference  to  a,  but  forming  a  right  angle  with  b. 
Fundamental  forms :  monoclinic  prism  (a :  b  :  °o  c) ;  monoclinic 
pyramid  (a:b:c)  (really  2  hemi-pyramids,  a  positive  and  a  negative 
one) ;  etc.  (Fig.  24). 

VI.  Triclinic.  —  Three  unequal  axes  all  inclined  with  reference 
to  one  another.  Fundamental  forms:  triclinic  prism  (hemi- 
prisms)  (a :b:  oo c) ;  triclinic  pyramid  (a :  b  :  c)  (Fig.  25). 


FIG.  25.  —  Triclinic  System.     Principal  Forms.     (After  Moses  and  Parsons.) 

A.  Triclinic  Pyramid.  —  a:b:c,  consists  of  4  sets  of  2  parallel  planes  each. 

B.  Hemi-br achy-dome  (e).  — oo  a  :  b  :  c.1 
Macro-pinacoid  (a).  —  a  :  oo  b  :  oo  c. 
Brachy-pinacoid  (b).  —  <x>a:b:°oc. 
Basal- pinacoid  (c).  —  oo  a  :  oo  b  :  c. 

C.  Hemi-maoro-dome  (d).  —  a  :  <x  b  :  c.1 
pinacoids  (c). 

D.  Triclinic  Hemi-prism  (m}.  —  a  :  b  :  oo  c. 
Basal-pinacoid  (c). 

E.  Combination  of  Macro-  (a),  Brachy-  (b),  and  Basal- pinacoids  (c). 


Macro-  (a),  Brachy-  (b),  and  Basal- 
Macro-  (a),  Brachy- -(6),  and 


Other  Physical  Characters 

Cleavage. —  The  ability  of  a  mineral  to  split  along  one  or  more 
planes  parallel  to  actual  or  possible  crystal  planes  is  called  cleavage, 
and  is  an  important  aid  in  identifying  mineral  species. 

Fracture.  —  The  mode  of  breaking  in  directions  other  than  those 
of  cleavage  is  the  type  of  fracture  of  the  mineral.  It  is  conchoidal 
(Fig.  41)  when  it  has  rounded  surfaces  suggestive  of  a  shell ;  even 

1  me  in  combination. 


Minerals  49 

or  uneven,  when  nearly  plain,  or  rough  and  irregular;  hackly  or 
splintery,  when  it  has  ragged  sharp  points  and  depressions,  or 
separates  in  a  fiber-  or  splinter-like  manner. 

Tenacity.  —  A  mineral  is  brittle  when  it  breaks  into  powder ; 
sectile,  when  small  slices  can  be  shaved  off  which  crumble  under  a 
hammer ;  malleable  when  slices  from  it  will  flatten  under  a  hammer ; 
tough,  when  great  resistance  to  tearing  apart  under  strain  or  a 
blow  is  shown ;  ductile,  when  it  can  be  drawn  into  wire. 

Hardness.  — The  resistance  of  a  smooth  plane,  whether  crystal, 
cleavage,  or  fracture,  to  abrasion  is  called  the  hardness,  and  is 
commonly  determined  by  scratching  the  surface.  It  is  expressed 
in  terms  of  a  scale  of  ten  common  minerals  (Mohs  scale).  Each 
mineral  will  scratch  all  those  softer  than  itself. 

Scale  of  Hardness 

1.  Talc  6.  Orthoclase 

2.  Gypsum  (Selenite)  7.  Quartz 

3.  Calcite  8.  Topaz 

4.  Fluorite  9.  Sapphire 

5.  Apatite  10.  Diamond 

Minerals  below  2.5  in  hardness  can  usually  be  scratched  with  a 
finger  nail ;  those  below  6  by  a  pocket  knife.  Any  mineral  above 
5.5  will  scratch  window  glass.  By  these  simple  tests  hardness 
can  be  determined  approximately. 

Luster.  —  The  brilliancy  or  shine  of  a  mineral  is  called  its  luster. 
It  is  dependent  upon  the  refractive  power,  transparency,  and 
structure  of  the  mineral.  The  following  types  are  recognized : 

a.  Metallic :  luster  of  metals,  gold,  silver,  copper,  etc. 

b.  Non-metallic  luster  comprising  : 

Vitreous  —  the  luster  of  a  fractured  surface  of  glass ;  example, 
quartz. 

Adamantine  —  the  luster  of  uncut  diamond,  zircon,  etc.,  due 
to  hj.gh  index  of  refraction. 

Resinous  —  the  luster  of  resin ;  example,  sphalerite. 

Greasy  —  the  luster  of  oiled  glass ;  example,  elaeolite. 

Pearly  —  the  luster  of  mother  of  pearl ;   example,  foliated  talc. 

Silky  —  the  luster  of  silk ;  example,  satin  spar. 

Dull  —  without  luster  or  shine  of  any  kind;  examples,  chalk, 
kaolin. 

The  prefix  sub-  is  used  to  express  a  lesser  degree  of  the  partic- 
ular luster ;  e.g.  sub-metallic,  sub-vitreous,  etc. 


50  Material  of  the  Earth's  Crust 

Color.  —  This  depends  on  chemical  composition  and  is  variable ; 
or  on  physical  constitution,  when  a  variety  of  color-changes  with 
the  changes  in  the  direction  of  light  is  produced.  These  are: 
Play  of  color  (opal,  labradorite) ;  Iridescence,  bands  of  prismatic 
color;  Tarnish,  surface  discoloration;  Opalescence,  milky  or 
pearly  reflection;  Asterism,  showing  a  star  by  reflected  or  trans- 
mitted light,  as  in  ruby,  etc.,  or  in  some  micas. 

Streak.  —  The  color  of  the  fine  powder  of  a  mineral  is  its  streak. 
It  is  obtained  by  scratching  the  mineral  or  rubbing  it  upon  a  smooth, 
white,  and  hard  surface.  (Arkansas  stone ;  streak  stone.) 

Translucency.  —  The  capacity  for  transmitting  light  is  the 
translucency  of  a  mineral.  A  mineral  is  transparent  when  objects 
can  be  seen  through  it  with  clearness ;  translucent,  when  it  transmits 
light,  but  objects  cannot  be  seen-;  opaque  when  no  light  passes 
through  even  the  thin  edges.  Sub-transparent  and  sub-translucent 
are  also  used. 

Specific  gravity.  —  The  weight  of  a  substance  divided  by  the 
weight  of  an  equal  volume  of  distilled  water  (at  4°  C.)  is  its  specific 
gravity.  Exact  determinations  are  made  by  fine  balances,  but 
rough  determinations  can  be  made  by  weighing  in  the  hand  and 
comparing  with  a  mineral  of  equal  size  and  known  specific  gravity. 

Taste.  —  Some  minerals  have  a  taste,  such  as  astringent  (alum) ; 
salty  (common  salt) ;  bitter  (epsom  salts) ;  alkaline  (soda) ;  acid 
(sassolite) ;  cooling  (niter) ;  pungent  (sal-ammoniac) , 

Odor.  —  On  heating  or  burning,  some  minerals  give  off  odors,  of 
which  those  of  garlic  (arsenic  minerals),  horseradish  (selenium 
minerals),  or  sulphur  are  examples.  Fetid,  bituminous,  and 
argillaceous  (clay)  odors  also  occur,  the  latter  noticeable  on  breath- 
ing upon  the  substance. 

Feel.  —  The  response  of  a  mineral  to  the  sense  of  touch  may 
be  smooth,  soapy  (talc),  harsh,  meager  (aluminite),  or  cold,  the  latter 
distinguishing  gems  from  glass. 

Other  Characters.  —  A  few  minerals  are  magnetic,  and  there  is 
great  variation  in  transmission  of  heat-rays  and  of  conductivity. 
Various  electric  phenomena  also  exist. 

Classification  of  Minerals 

Minerals  are  classified  on  a  chemical  basis,  and  two  distinct 
methods  have  been  employed  which  may  in  general  be  considered 
as  classifications;  first,  according  to  the  acid  radical  (including 


Minerals  51 

oxides,  etc.)  and  second,  according  to  the  basic  radical.    In  the 
first  system  the  minerals  are  divided  into  the  following  classes : l 

1.  Native  elements. 

2.  Sulphides,  Selenides,  Tellurides,  Arsenides,  Antimonides. 

3.  Sulpho  salts. 

4.  Chlorides,  Bromides,  Iodides,  Fluorides. 

5.  Oxides  (Hydroxides,  Oxy hydroxides). 

6.  Carbonates. 

7.  Silicates. 

8.  Titano-Silicates,  Titanates. 

Q.  Niobates  or  Columbates,  Tantalates. 

10.  Phosphates,  Arsenates,  Vanadates,  Antimonates. 

11.  Nitrates. 

12.  Borates. 

13.  Uranates. 

14.  Sulphates,  Chromates,  Tellurates. 

15.  Tungstates,  Molybdates. 

16.  Oxalates,  Mellitates.     (Salts  of  organic  acids.) 

17.  Hydrocarbon  compounds. 

Tables  of  Important  Minerals 

In  the  following  tabular  list,  arranged  essentially  according  to 
the  basic  radical,  the  more  important  minerals  are  given,  with  a 
brief  characterization  of  their  essential  features.  For  more 
details  the  student  is  referred  to  the  textbooks  cited  below. 

1.  A.  J.  MOSES  AND  C.  L.  PARSONS.    Elements  of  Mineralogy,  Crystallog- 
raphy, and  Blowpipe  Analysis.    5th  edition,  1916.    N.  Y.,  D.  Van  Nostrand 
Company. 

2.  DANA-FORD.    Manual  of  Mineralogy,     isth  edition.    John  Wiley  and 
Sons,  N.  Y.     1912. 

3.  H.  A.  MIERS.    Mineralogy.    An  introduction  to  the  scientific  study  of 
minerals.    Macmillan  and  Co.,  London.     1902. 

4.  A.  H.  PHILLIPS.     Mineralogy.     The  Macmillan  Co.,  N.  Y.     1912. 

5.  A.  F.  ROGERS.     Introduction  to  the  Study  of  Minerals.    McGraw  Hill 
Book  Co.,  N.  Y.     1912. 

6.  DANA,  EDWARD  S.    A  Text  Book  of  Mineralogy,  etc.     John  Wiley  and 
Sons,  N.  Y. 

7.  J.  D.  DANA.     The  System  of  Mineralogy,  Descriptive  Mineralogy  by 
E.  S.  Dana.     John  Wiley  and  Sons,  N.  Y. 

8.  W.  O.  CROSBY.     Tables  for  the  Determination  of  Common  Minerals, 
Chiefly  by  their  Physical  Properties.     Boston.     Published  by  the  Author. 

1  Dana,  J.  D.  and  E.  S. :  The  System  of  Mineralogy.     6th  ed. 


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CHAPTER  V 
ROCKS,  THEIR  CLASSIFICATION  AND  PRINCIPAL  TYPES 

DEFINITIONS 

A  ROCK  may  be  defined  as  a  mineral  mass  or  an  association  of 
minerals,  which  in  their  natural  occurrence  form  an  essential 
part  of  the  earth's  crust.  This  distinction  is  not  a  very  precise 
one,  especially  when  the  material  of  the  rock  consists  of  .only  one 
mineral.  Thus  the  calcite  in  a  vein  would  be  considered  a  mineral, 
while  essentially  the  same  material  in  a  bed  of  marble  would  be 
considered  a  rock.  We  shall  see,  however,  as  we  proceed  with  the 
discussion  of  the  rocks,  that  in  practice  the  distinction  between 
rock  and  mineral  can  as  a  rule  be  readily  made. 

AGE  RELATIONS  OF  ROCKS 

It  will  be  useful  at  this  early  stage  of  our  study  to  recognize  the 
fact  that  the  rocks  of  the  earth's  crust  are  of  various  ages.  Some, 
like  the  rocks  which  make  up  the  Adirondack  Mountains,  Pikes 
Peak  in  Colorado,  the  Highlands  of  the  Hudson,  the  Scottish  High- 
lands, the  main  mass  of  Finland,  and  a  great  part  of  central  France, 
etc.,  are  very  old;  others,  like  those  of  the  "  puys  "  which  are 
scattered  over  the  central  French  region,  the  basalts  of  the  Columbia 
and  Snake  River  plateaus  of  the  northwestern  United  States,  the 
rocks  immediately  underlying  Paris,  London,  Vienna,  and  Berlin, 
and  the  rocks  of  southern  Florida  are  very  young,  though  not  all 
of.  the  same  age.  It  is  possible  to  divide  the  history  of  the  earth 
into  a  number  of  periods  and  eras,  just  as  human  history  can  be 
divided.  But  whereas  the  successive  periods  of  human  history  are 
measured  by  centuries  at  the  most,  those  of  the  pre-human  earth 
history  are  measured  by  hundreds  of  thousands  if  not  by  millions 
of  years.  And  as  we  can  refer  the  monuments  and  buildings  of 
human  origin  to  their  successive  periods  in  human  history,  often 
from  the  character  of  these  monuments  and  buildings  themselves, 

64 


Bed-Rock  and  Mantle-Rock  65 

so  we  can  refer  rrfost  of  the  different  rocks  and  rock  structures  of 
the  earth's  crust  to  their  respective  geological  periods,  or  to  the 
period  of  the  earth's  history  when  they  came  into  existence  as  rocks. 
It  is  desirable  that  the  student  should  begin  to  familiarize  himself 
at  this  point  with  the  names  of  the  different  periods  of  earth  history 


FIG.  26. — Ledge  of  glaciated  rock  overlain  by  glacial  drift,  showing  the 
sharp  contact  line  between  the  bed-rock  and  the  mantle-rock.  New  York  City. 
(F.  K.  Morris,  Photo.) 

as  given  in  the  table  in  Chapter  XXIV  of  this  book.  At  a  later 
stage  of  his  studies  he  will  learn  by  what  means  it  becomes  possible 
to  refer  rocks  and  rock  structures  to  their  proper  period.  When  in 
the  succeeding  pages  we  refer  to  the  geological  age  of  any  rock  mass 
the  student  should  consult  the  table  until  he  has  become  familiar 
with  the  succession  of  the  periods.  (See  p.  xviii.) 

BED-ROCK  AND  MANTLE-ROCK 

In  general  we  may  distinguish  between  the  solid  or  bed-rock  and 
the  unconsolidated  rock  or  rock  material,  which  latter  is  commonly 
called  the  mantle-rock  because  it  covers  or  mantles  the  bed-rock 
which  everywhere  underlies  it  and  projects  through  it  as  ledges. 
The  mantle-rock  is  of  course  much  younger  than  the  bed-rock 
upon  which  it  rests.  In  the  northern  United  States  and  Canada 
and  the  northwestern  part  of  Europe,  the  mantle  rock  generally 
rests  abruptly  upon  the  solid  or  bed-rock,  the  contact  line  between 
the  two  being  commonly  sharp  (Fig.  26).  In  many  other  por- 


66       Classification  and  Principal  Types  of  Rocks 

tions  of  the  earth,  however,  there  is  a  gradation  between  the  two, 
the  mantle-rock  becoming  more  stony  downwards,  and  passing  into 
rotten  rock  and  finally  into  fresh  bed-rock.  This  relationship 
will  be  discussed  more  fully  in  a  subsequent  chapter ;  the  present 
one  will  orily  take  account  of  the  solid  rock  which  forms  all  except 
the  surface  film  of  the  crust  of  the  earth. 

CLASSIFICATIONS  OF  ROCKS 

General  Principles  of  Classification.  —  Classification  of  natural 
objects  may  be  made  either  on  a  natural  or  on  an  artificial  basis. 
A  natural  classification  is  based  on  the  origin  and  development 
of  the  objects  classified  or  on  their  genesis.  Such  a  classification 
is  therefore  called  a  genetic  one,  and  it  alone  has  a  permanent  scien- 
tific value.  It  is  true  that  a  genetic  classification  can  be  made 
only  when  the  genesis  or  origin  of  the  objects  classified  has  been 
determined,  and  that  therefore  the  artificial  systems  of  classifica- 
tion will  always  precede  the  scientific  or  genetic  one.  The  artificial 
classification  is  generally  based  upon  the  possession  in  common,  by 
the  objects  classified,  of  a  single  character  or,  at  most,  a  few  char- 
acters. Thus  whales  were  formerly  classified  with  fish  because, 
like  these,  they  lived  in  the  water  and  had  a  fish-like  form.  Their 
true  relationship  is,  however,  more  closely  with  elephants,  both 
being  mammals.  The  eel-grass  of  our  coast  is  not  a  grass,  but 
belongs  to  the  family  of  water  lilies,  although  its  leaves  are  grass- 
like.  Rock-salt  and  sandstone  have  little  in  common,  though 
generally  classified  together  as  sedimentary  rocks ;  both  may  have 
been  deposited  in  lagoons  near  the  seashore,  and  therefore  in  a 
sense  are  sediments. 

In  general,  it  may  be  said  that  the  progress  of  any  science  is 
indicated  by  the  replacement  of  the  original  artificial  by  the 
natural  or  genetic  classification.  Thus  as  the  study  of  plants 
developed  into  the  science  of  botany,  the  original  artificial  classi- 
fication of  plants,  based  on  the  number  of  stamens  of  the  flower, 
and  on  other  superficial  characters,  became  superseded  by  the 
natural  classification,  which  is  based  upon  community  of  origin 
of  the  members  of  the  same  group ;  and  in  zoology,  we  find  that 
artificial  classifications,  based  on  superficial  resemblances  or  the 
possession  of  a  common  character,  are  constantly  discarded  as  the 
true  genetic  or  natural  relationships  of  animals  are  becoming  more 
fully  understood. 


Classifications  of  Rocks  67 

A  Convenient  Artificial  Classification  of  Rocks 

A  convenient  classification  of  rocks,  which  has  come  into  general 
use,  recognizes  three  fundamental  divisions,  as  follows : 

1.  Igneous  Rocks.     Rocks  which  result  from  the  cooling  of  a 
molten  mass,  or  magma,  either  upon  the  surface  of  the  earth,  or 
within  the  crust  at  greater  or  less  depths.    An  example  of  the 
first  is  basaltic  lava ;  of  the  second,  granite. 

2.  Sedimentary  Rocks.     Rocks  which  were  formed  (a)  as  me- 
chanical sediments  in  water  or  air,  (b)  as  chemical  precipitates  and 
evaporation  products  from  solution  in  water,  and  (c)  as  deposits 
formed  by  organisms  either  in  water  or  air.     Examples  of  these 
are :     (a)  sandstone,    shales,    etc. ;     (b)  rock-salt,    gypsum,    cave 
deposits  (stalactites),  etc. ;    (c)  coral  and  shell  limestones,  chalk, 
guano  beds,  coal.     The  three  groups  here  indicated  are  commonly 
made  subdivisions  of  the  sedimentary  rocks,  and  designated  re- 
spectively  as   follows:     (a)  mechanical   sediments,    (b)   chemical 
sediments,  and  (c)  organic  sediments. 

3.  Metamorphic  Rocks..    These  rocks  were  originally  members 
of  one  or  the  other  of  the  preceeding  divisions,  but  have  become 
sufficiently  altered  by  natural  agencies,  such  as  pressure,  heat, 
and  so  forth,  so  that  for  the  most  part  the  original  characters  are 
obliterated  or  lost,  and  new  and  special  characters  are  added. 
Examples   of   such   metamorphic   rocks  are :    (a)  gneiss  —  which 
may  be  derived  from  an  original  granite,  but  may  also  have  origi- 
nated from  another  rock,  even  from  a  sediment ;    (b)  mica  schist, 
which  may  have  been  derived  from  a  shale  or  an  impure  sandstone 
or  some  other  rock ;  (c)  marble,  which  is  derived  from  some  form  of 
limestone  that  may  have  been  of  organic,  of  chemical,  or  of  me- 
chanical origin;    (d)  graphite,  which  may  be  derived  from  coal 
beds  or  from  carbonaceous  shales,  etc. 

Such  a  classification  is  sufficiently  serviceable  for  all  practical 
purposes,  but  it  cannot  be  called  a  scientific  classification,  because 
in  the  second  and  third  group  are  included  types  of  very  diverse 
origin;  for  the  three  groups  mentioned  under  sedimentary  rocks 
have  little  or  nothing  in  common,  except  that  they  are  sediments 
in  a  very  broad  and  indefinite  comprehension  of  that  term.  More 
correctly  speaking,  they  are  not  igneous  rocks,  and  not  pro- 
nouncedly metamorphic  rocks,  and  this  is  almost  their  only  claim 
to  a  grouping  under  a  separate  division.  The  metamorphic  group, 


68       Classification  and  Principal  Types  of  Rocks 

too,  includes,  as  we  have  seen,  rocks  of  very  diverse  origin,  but 
since  it  is  not  as  a  rule  possible  to  determine,  except  perhaps  after 
prolonged  and  careful  study  and  analysis,  if  then,  what  the  char- 
acter of  the  original  rock  was,  it  will  probably  always  be  necessary 
to  retain  this  group  as  a  matter  of  convenience. 

Principles  of  a  Natural  or  Genetic  Classification  of  Rocks 

While  the  practical  worker  may  find  the  preceding  classification 
sufficient  for  all  needs,  and  while  in  the  succeeding  chapters  we 
may  frequently  refer  to  these  convenient  three  types,  the  student 
should  nevertheless  understand  the  principles  of  a  natural  classi- 
fication, and  so  far  as  it  is  practicable  we  shall  base  our  subsequent 
discussions  upon  such  a  natural  classification.  For  in  geology 
as  in  other  sciences,  logical  thinking  is  of  the  first  importance, 
and  logical  thought  is  best  fostered  by  the  most  rigid  adherence  to 
exact  methods  of  classification,  and  the  only  exact  classification 
of  natural  objects  is  one  based  upon  genetic  relationship,  that  is, 
upon  community  of  origin  of  the  members  of  each  group. 

At  the  outset  of  our  endeavor  to  understand  the  natural  relations 
of  rocks  we  must  clearly  comprehend  two  fundamental  principles. 
The  first  of  these  is,  that  in  this  natural  world  of  ours  all  things 
are  subject  to  continued  change,  even  though  that  change  may  be  a 
very  slow  one,  so  slow  that  the  years  of  a  man's  life  or  those  of  many 
successive  generations  are  of  insufficient  length  to  permit  the 
recognition  of  such  changes.  All  rocks  are  constantly  undergoing 
modification,  and  though  sometimes  these  changes  may  be  rapid, 
as  in  the  coking  of  coal  in  a  burning  mine,  or  the  change  of  clay  to 
brick  in  a  kiln,  the  more  usual  method  of  change  is  a  slow  and  grad- 
ual one.  In  a  certain  sense  all  rocks  may  be  considered  as  meta- 
morphic  or  altered  rocks,  a  view  strongly  held  by  some  geologists. 
Extreme  metamor]5hism,  such  as  produced  the  common  types  which 
are  usually  spoken  of  as  metamorphic  rocks,  is  only  a  phase  of  the 
general  alteration  or  metamorphism  which  all  rocks  undergo,  and 
this  phase  is  characterized  by  the  greater  or  complete  obliteration 
of  the  characters  which  the  rock  possessed  during  its  early  history. 
This  more  pronounced  change  may  have  been  brought  about  by 
the  greater  intensity  of  the  activities  responsible  for  it,  or  by  their 
longer  continuance  in  time,  or  by  both. 

The  second  principle,  a  knowledge  of  which  is  fundamental  to 
our  understanding  of  rocks  as  well  as  all  other  natural  objects, 


The  Unaltered  or  Little  Altered  Rocks  69 

is  that  related  types  are  not  separated  as  a  rule  by  sharp  lines  of 
demarcation,  but  that  always  and  everywhere  in  nature  gradation 
is  the  predominant  rule. 

Thus  a  sandstone  may  grade  into  a  shale,  on  the  one  hand,  and  into  a  lime- 
stone on  the  other,  and  shale  and  limestone  may  intergrade,  though  of  course 
there  are  always  types  which  retain  the  characters  of  one  or  the  other  with 
sufficient  distinctness  to  make  their  classification  possible.  A  granite  may 
grade  into  a  syenite,  and  this  into  a  diorite ;  and  again,  a  granite  and  a  diorite 
may  approach  each  other  so  closely  in  character  that  classification  becomes 
difficult.  In  like  manner  a  sandstone  may  have  so  many  characters  of  a  schist 
that  it  becomes  a  difficult  question  to  which  of  the  two  divisions  it  belongs. 
A  shale  may  grade  into  a  slate  and  a  granite  into  a  gneiss.  With  slightly  meta- 
morphosed rocks,  classification  is  often  very  difficult,  though  a  thoroughly  meta- 
morphosed rock  is  easily  recognized  as  such. 

Extreme  metamorphism  is  only  a  later  or  final  phase  of  the 
change  which  all  rocks  are  undergoing,  and  in  a  natural  classifica- 
tion such  end-products  are  to  be  placed  with  the  group  from  which 
they  are  derived.  Two  gneisses,  for  example,  one  of  which  is 
derived  from  a  granite  and  one  from  some  type  of  sandstone,  are 
not  related  to  each  other  any  more  than  two  human  beings  are 
related  because  they  speak  the  same  language,  or  obey  the  same 
laws  of  civilization.  As  before  said,  however,  the  practical  diffi- 
culty of  determining  the  original  character  of  a  metamorphic 
rock  must  be  reckoned  with,  just  as  in  the  study  of  thoroughly 
civilized  human  beings  of  different  but  entirely  disguised  nation- 
alities, the  practical  difficulty  of  ascertaining  the  nationality  of 
each  (assuming  that  the  individuals  refuse  or  are  unable  to  disclose 
it)  must  be  taken  into  account.  In  this  respect  thoroughly  meta- 
morphosed rocks  hold  the  same  relation  to  their  ancestral  type, 
as  thoroughly  Americanized  individuals  descended  from  different 
nationalities  hold  to  their  ancestors. 

It  thus  becomes  necessary  for  us  to  study  first  the  unaltered  or 
but  slightly  altered  rocks,  the  history  of  which  is  ascertainable 
from  the  characters  which  they  retain. 

THE  UNALTERED  OR  LITTLE  ALTERED  ROCKS 

When  we  study  the  unaltered  or  slightly  altered  rocks,  we  again 
note  that  they  fall  into  two  quite  readily  recognizable  divisions. 
The  first  of  these  comprises  rocks  which  have  been  produced  from 
material  not  originally  rock,  —  for  example,  by  cooling  from  a  lava, 


70       Classification  and  Principal  Types  of  Rocks 

crystallizing  from  a  solution  in  water,  and  the  like,  —  a  group 
which  in  a  broad  sense  may  be  spoken  of  as  produced  in  a  chemical 
manner.  The  other  division  comprises  rocks  which  are  made  up 
of  fragments  or  particles  of  other  rocks,  such  as  sandstone  made  of 
grains  of  sand,  a  conglomerate  made  of  pebbles  of  various  kinds 
and  of  sand,  and  others  like  these.  This  is  the  group  of  fragmental 
or  clastic  rocks  (Latin :  clastus,  broken)  which  are  always  made 
from  some  preexisting  rock  that  has  been  broken  down  into  discrete 
particles,  which  then  are  recombined  either  directly  or  after  more 
or  less  assorting.  The  first  group,  on  the  other  hand,  is  that  of  the 
non-fragmental  or  non-clastic  type,  which  is  not  formed  from  frag- 
ments of  other  rocks,  but  from  non-rock  material.  In  the  regular 
order  of  formation  this  type  would  appear  first,  though  it  is  per- 
fectly possible  that  non-fragmental  rocks  may  originate  from 
material  which  itself  was  derived  from  other  rock,  by  melting, 
solution,  or  vaporization  of  such  a  rock  and  subsequent  resolidifica- 
tion.  But  in  this  case  there  is  an  intermediate  non-rock  stage, 
while  the  material  of  the  fragmental  or  clastic  rocks  is  always  rock, 
though  it  may  be  broken  into  the  finest  particles.  We  shall  con- 
sider the  characteristics  of  each  group  briefly. 

The  Non-Fragmental  Rocks 

(Endogenetic  Rocks) 

The  non-fragmental  rocks  may  be  properly  regarded  as  contri- 
butions to  the  lithosphere  from  three  of  the  other  spheres ;  namely, 
the  hydrosphere,  the  pyrosphere,  and  the  atmosphere.  This 
contribution  may  be  direct,  as  in  the  case  of  the  hardening  of  a 
lava  from  the  pyrosphere,  the  separation  out  of  salt  from  the  water 
of  the  hydrosphere,  by  the  concentration  of  that  water  or  by 
chemical  reactions,  or  the  separation  of  snow  or  hail-stones  by 
the  solidification  of  the  water  vapor  from  the  atmosphere.  Each 
of  these  spheres  may  thus  in  a  measure  be  credited  with  the  genera- 
tion of  these  respective  rocks,  and  it  is  therefore  possible  to  speak 
of  these  three  types,  respectively,  as :  pyrogenic,  generated  by  the 
pyrosphere;  hydrogenic,  generated  by  the  hydrosphere;  and 
atmogenic,  or  generated  by  the  atmosphere.  Instead  of  these 
names  we  may  of  course  speak  of  these  deposits  as  of  igneous, 
aqueous,  and  atmospheric  origin,  respectively,  recognizing  the  fact 
that  the  rocks  contributed  by  the  atmosphere  direct,  that  is,  the 
snow  and  hail,  are  of  an  evanescent  character. 


The  Unaltered  or  Little  Altered  Rocks  71 

In  addition  to  these  three  types,  —  the  direct  contributions  of 
three  of  the  inorganic  spheres  to  the  lithosphere,  —  there  is  an 
indirect  contribution  of  material  from  two  of  these  spheres  by  the 
agency  of  organisms.  Both  animals  and  plants  take  lime  from  the 
sea-water  to  build  their  shells  and  other  calcareous  structures,  and 
other  organisms  take  silica  from  sea-water  to  build  their  hard  parts. 
Deposits  of  such  organically  secreted  lime  and  silica  are  very 
important  in  the  construction  of  the  lithosphere,  and  form,  as  a 
rule,  readily  recognizable  types  of  rocks. 

Again,  carbon  is  taken  by  plants  from  the  carbon  dioxide  of  the 
atmosphere,  and  the  plant  tissues  built  up  from  it  may  often  become 
compacted  into  beds  of  coal  which  form  important  members  of 
the  rock  series  of  the  lithosphere.  Here  again  is  an  organically 
formed  rock,  the  material  of  which  is  obtained  from  the  atmosphere. 

Such  organically  formed  rocks  are  the  contribution  of  the  bio- 
sphere to  the  lithosphere,  and  in  conformity  with  the  method  of 
designation  which  implies  such  a  relation  to  that  sphere,  they  may 
be  called  bio  genie  rocks. 

We  may  then  summarize  in  the  following  table  these  four  funda- 
mental types  of  non-clastic  rocks,  rocks  which  are  produced  essen- 
tially by  chemical  (including  physiological)  reactions  from  non- 
rock  material.  We  term  these  rocks  endo genetic  because  they 
are  formed  by  forces  in  a  measure  inherent  in  the  material  of  the 
spheres  which  produce  them. 

THE  FOUR  TYPES  OF  ENDOGENETIC  (NON-  CLASTIC)  ROCKS 

1.  Pyrogenic  or  Igneous ;  Produced  by  the  pyrosphere. 

2.  Hydrogenic  or  Aqueous  :  Produced  by  the  hydrosphere. 

3.  Atmogenic  or  Atmospheric  :  Produced  direct  by  the  atmosphere. 

4.  Biogenic  or  Organic :    Produced  by  the  biosphere  from   material  taken 
from  the  hydrosphere  or  atmosphere. 

We  may  consider  these  rock  types  from  another  point  of  view. 
Were  we  to  imagine  the  rock  material  of  the  earth  deprived  of  its 
solid  character,  that  which  is  essential  to  the  constitution  of  a  rock, 
we  should  have  to  think  of  it  as  existing  in  one  or  more  of  three 
possible  forms. 

i.  It  may  be  turned  into  a  molten  mass  like  lava,  by  the  applica- 
tion of  heat,  and  so  constitute  an  igneous  magma,  in  which  all  the 
minerals  which  go  to  the  making  of  the  rock  would  become  as  it 
were  dissolved  in  one  another,  and  the  compounds  dissociated  into 


72       Classification  and  Principal  Types  of  Rocks 

their  ions.  It  would  then  become  a  part  of  the  pyrosphere,  and 
in  this  state  most  of  the  rocks  of  the  earth  are  believed  to  have 
existed  at  an  earlier  period  according  to  the  believers  in  one  hy- 
pothesis of  the  earth's  origin  (see  Chapter  XXIX)  when  the  earth 
as  a  whole  was  a  molten  mass.  Whether  this  theory  is  correct  or 
not,  the  fact  that  many  of  the  rocks  of  the  earth's  crust  were  at 
one  time  in  this  state  cannot  be  doubted.  By  solidification  igneous 
or  pyrogenic  rocks  are  formed. 

2.  The  rocks  of  the  earth  may  be  turned  into  a  condition  of 
vapor  —  by  the  application  in  most  cases  of  still  greater  heat. 
In  such  a  state  they  would  become  a  part  of  the  earth's  atmosphere. 
The  same  hypothesis  of  earth-origin  holds  that  this  was  a  condition 
of  the  entire  rock  mass  of  the  earth  at  a  still  earlier  period,  and  that 
from  this  condition  of  vapor  was  separated  at  a  later  period  the 
molten  material  of  the  earth,  and  later  still  the  water  of  the  hydro- 
sphere.    Again,  the  truth  of  this  hypothesis  is  not  essential  to  the 
recognition  of  the  fact  that  some  at  least  of  the  rocks  of  the  earth 
were  formerly  in  a  condition  of  vapor,  and  that  they  were  separated 
from  the  earth's  atmosphere  either  by  direct  condensation,  as  we 
see  to-day  in  the  formation  of  snow,  or  by  the  work  of  organisms, 
such  as  has  resulted  in  the  separation  of  the  carbon  of  the  at- 
mosphere, which  now  constitutes  our  coal  beds,  by  the  agency  of 
plants.     Rocks  derived  by  direct  condensation  from  the  atmosphere 
are  atmo genie  rocks,  while  those  separated  by  the  activities  (physio- 
logical) of  organisms,  are  biogenic  or  organic  rocks. 

3.  Finally,  we  may  think  of  the  rocks  of  the  earth,  or  at  least 
some  of  them,  as  dissolved  in  the  universal  fluid  envelope  of  the 
earth  —  the  water  —  and  so  become  a  part  of  the  hydrosphere. 
If  rock  material,  thus  held  in  solution,  is  separated  out  from  the 
water  by  direct  condensation,  by  chemical  reactions,  or  by.  elec- 
trolytic action,  aqueous  or  hydrogenic  rocks  are  produced.     If,  how- 
ever, the  dissolved  material  is  separated  out  by  the  agency  of 
organisms,  as  in  the  formation  of  limestone  masses  on  coral  reefs 
by   polyps   and   lime-secreting  seaweeds    (algae),   it   becomes   an 
organic  or  biogenic  rock. 

There  are  no  other  primary  states  than  those  of  fusion,  solution 
in  water,  or  vaporization,  into  which  the  rocks  of  the  earth  may  be 
changed,  nor  are  there  any  other  known  ways  in  which  rocks  are 
formed  from  the  three  states  of  primary  dissociation  except  by 
direct  precipitation  or  separation  or  by  organic  agencies.  Hence 


The  Unaltered  or  Little  Altered  Rocks 


73 


the  four  rock  types  —  the  igneous  or  pyrogenic,  the  aqueous  or 
hydrogenic,  the  atmogenic,  and  the  organic  or  biogenic  —  are  the 


FIG.  27.  —  Diagram  showing  the  interrelations  of  the  Endogenetic  Rocks. 

only  primary  types  recognized.     These  relationships  are  shown  in 
the  preceding  diagram.     (Fig.  27.) 

The  Fragmented  or  Clastic  Rocks 

(Exogenetic  Rocks) 

These  are  the  rocks  which  are  made  up  of  fragments  of  other 
rocks,  which  may  range  in  size  from  dust  particles  of  microscopic 
dimensions  to  boulders  many  feet  in  diameter.  For  their  produc- 
tion it  is  evident  that  preexisting  rocks  should  be  broken  into  frag- 
ments, and  that  these  fragments  should  become  recemented  or 
bound  together  again  in  some  manner.  The  methods  by  which 
rocks  are  broken  into  fragments,  and  those  by  which  the  fragments 
are  recemented  will  be  taken  up  later,  but  we  may  here  use  as 
an  illustration  of  this  type  of  rock  one  of  the  artificial  rock-making 
processes  carried  on  by  man,  and  which  differs  primarily  from  the 
natural  process  in  the  rapidity  with  which  it  is  carried  forward. 
This  is  the  process  of  manufacture  of  rubble  concrete  for  paving 


74       Classification  and  Principal  Types  of  Rocks 

and  other  construction.  Rocks  like  the  trap  of  the  Palisades,  an 
ancient  igneous  rock,  are  quarried  and  passed  through  the  stone- 
crusher,  where  they  are  reduced  to  rock  rubble  or  particles  of  small 
dimensions.  They  are  assorted  into  different  sizes  by  screening, 
and  bound  together  by  a  cement,  which  is  a  mixture  of  lime,  alu- 
mina, and  silica,  and  of  sand,  which  is  the  finer  rock  material  obtained 
either  from  the  screening,  or  more  commonly  from  natural  sand 
banks.  The  result  is  a  clastic  rock,  but  an  artificial  one,  and  this 
is  produced  in  a  few  days,  whereas  a  similar  rock  produced  in 
nature  might  require  as  many  centuries  or  millenniums  for  its  pro- 
duction. But  trap  rocks  or  other  igneous  rocks  are  not  the  only 
ones  used  in  the  making  of  rubbl^  concrete,  nor  are  igneous  rocks 
the  only  source  of  clastic  material  in  nature,  though  they  are  often 
the  most  common  one.  All  rocks,  —  igneous,  aqueous,  and  organic, 
and  clastic  rocks  as  well,  —  are  broken  into  fragments  by  natural 
agencies,  and  from  these  fragments  new  clastic  rocks  are  made. 
For  the  rocks  of  the  earth's  crust  are  being  constantly  reworked, 
all  of  them,  whether  clastic  or  non-clastic,  furnishing  material  for 
the  formation  of  a  younger  bed  of  clastic  rock.  Let  us  take  an 
example  for  illustration  from  the  eastern  United  States. 

There  is  in  the  foothills  of  the  Catskill  Mountains  a  great  deposit 
of  bedded  clastic  rocks  known  to  the  arts  as  Hudson  River  Blue 
Stone,  and  used  for  the  manufacture  of  flag-stones  for  sidewalks 
in  New  York  City  and  elsewhere,  for  curbings,  and  for  many  other 
purposes.  To  the  geologist  this  rock  is  known  as  a  member  of  the 
Middle  and  Upper  Devonian  series  of  rocks,  of  which  we  shall  learn 
more  in  the  future.  Examination  under  the  microscope  shows 
that  this  rock  is  composed  of  small  particles,  some  of  which  are 
themselves  small  fragments  of  clastic  rocks  (these  are  called  clas- 
toliths)  and  this  shows  that  this  particular  blue-stone  rock  is  made 
up  of  material  derived  in  large  part,  if  not  wholly,  from  an  older 
clastic  rock  which  was  broken  into  fragments,  assorted  according 
to  size  by  natural  agencies,  and  recemented  to  form  new  clastic 
rocks,  the  "  Blue  Stone  "  being  made  up  of  the  assorted  finer  par- 
ticles only.  From  the  character  of  the  small  fragments  of  clastic 
rock  (the  clastoliths),  and  from  a  study  of  the  structural  and  age 
relations  of  the  "  Blue  Stone  "  to  the  rocks  of  greater  age,  it  has 
been  possible  to  determine  that  the  material  of  the  "  Blue  Stone  " 
was  derived  from  the  so-called  Hudson  River  formation,  which  crops 
out  to  the  east  of  the  Blue  Stone  area  and  forms  in  part  the  Taconic 


The  Unaltered  or  Little  Altered  Rocks  75 

Mountain  range  along  the  New  York-Massachusetts  boundary 
line.  This  rock  is  of  much  greater  age  than  the  "  Blue  Stone," 
belonging  to  the  Ordovician  period  of  the  geological  series  (see 
table,  Chapter  XXIV) .  Similar  examination  of  this  Ordovician  clas- 
tic rock  shows  that  it  in  turn  was  derived  from  a  still  older  clastic 
rock,  this  time  of  Pre-Cambrian  age,  some  of  which  can  be  seen  in 
the  ledges  of  Manhattan  Island.  This  clastic  rock  finally  was  de- 
rived from  still  older  igneous  and  metamorphic  rocks  (Berkey). 

A  second  example  from  the  central  region  of  our  country  may  be 
given.  In  Ohio,  Michigan,  and  Western  Ontario  is  a  great  bed 
of  very  pure  sandstone,  a  clastic  rock  made  up  almost  entirely  of 
small,  well-rounded  grains  of  quartz  sand  bound  together.  This 
rock  is  so  pure  that  it  is  used  for  glass-making  in  Toledo,  Ohio.  It 
is  known  by  the  name  of  the  Sylvania  sandstone,  and  it  belongs 
to  the  Silurian  division  of  the  geological  series.  Careful  study 
has  shown  that  the  grains  were  originally  a  part  of  a  still  older 
sandstone,  called  the  Saint  Peter  sandstone,  which  belongs  to  the 
Ordovician  division  and  covers  a  large  area  in  the  Mississippi 
Valley  region  and  east  of  that  in  Michigan,  Canada,  etc.,  though 
its  outcrops  are  found  only  in  restricted  areas.  This  rock  was  in 
turn  derived,  at  least  in  part,  from  the  destruction  of  a  still  older 
sandstone,  the  so-called  Potsdam  sandstone  of  Cambrian  age, 
which  crops  out  farther  to  the  north,  in  Wisconsin,  Minnesota,  etc. 
This  sandstone  finally  was  derived  from  the  still  older  granites, 
gneisses,  etc.,  which  are  seen  in  the  Canadian  region  to  the  north. 
Thus  the  Sylvania  represents  the  third  generation  of  sandstone, 
the  Saint  Peter  the  second,  and  the  Potsdam  the  first. 

Finally  we  may  note  that  the  modern  sands  of  the  Libyan  desert 
are  derived  from  the  breaking  up  into  its  component  grains  of  an 
older  sandstone,  the  Nubian,  and  of  the  rock  from  which  the 
sphinx  has  been  cut  (Fig.  28).  If  these  sands  become  bound 
together  into  a  sandstone  at  some  future  time,  this  sandstone  will 
be  of  the  second  generation. 

We  see,  therefore,  that  clastic  rocks  may  be  of  different  genera- 
tions. The  first  generation  is  always  derived  from  some  igneous 
or  other  non-clastic  rock  or  the  metamorphosed  product  of  such 
a  rock.  The  later  generations  are  derived  successively  from  older 
elastics,  though  new  contributions  from  the  crystalline  source 
may  also  be  made.  This  formation  of  a  new  and  younger  clastic 
rock  from  the  material  of  an  older  one  may  be  compared  with  the 


76       Classification  and  Principal  Types  of  Rocks 

construction,  in  more  recent  times,  of  man-made  structures  and 
monuments  from  the  stones  obtained  by  the  demolition  of  older 
human  monuments  or  structures. 

The  cycle  of  change  which  includes  the  successive  generations  of 
a  clastic  series  may  be  brought  to  an  end  by  the  melting  of  the 


FIG.  28. — The  Sphinx,  cut  from  a  rock  ledge  and  surrounded  (and  for- 
merly partly  buried)  by  desert  sands  derived  largely  from  the  destruction  of  a 
sandstone  in  other  parts  of  the  Libyan  desert.  The  Great  Pyramid  in  the 
background  is  covered  by  slabs  of  Nummulitic  limestone. 

elastics,  and  by  their  incorporation  into  a  new  igneous  mass, 
or  by  their  pronounced  metamorphism.  Then  a  new  cycle  of 
formation  of  clastic  rock  will  begin. 


The  Agents  Active  in  the  Breaking  Up  or  "  Clastation  "  of  Rocks 

We  may  now  consider  the  chief  agents  which  are  active  in  the 
breaking  up  or  clastation  of  rocks,  that  is,  those  responsible  for 
the  formation  of  clastic  material  from  which  clastic  rocks  are 
formed  by  consolidation.  We  may  classify  the  several  types  of 
clastic  rocks  according  to  the  agent  which  produces  the  material, 
or  which  arranges  it  in  the  form  in  which  it  becomes  consolidated. 
The  methods  of  clastation  will  be  considered  in  a  later  chapter. 


The  Unaltered  or  Little  Altered  Rocks  77 

The  Atmosphere  as  a  Rock  Breaker.  —  In  the  first  place,  rocks 
are  broken  up  or  "  clastated  "  by  the  atmosphere  acting  either 
chemically,  by  the  action  of  the  various  gases  and  vapors  of  the  air 
upon  the  rock,  or  mechanically,  as  in  the  case  of  freezing  moisture, 
or  by  the  wind.  Clastic  material  thus  produced  may  be  called 
atmoclastic  material,  and  rocks  formed  by  the  consolidation  of  such 
clastic  material  may  be  called  atmoclastic  rocks.  As  clastic  material 
is  often  accumulated  in  certain  localities  after  transportation  by 
the  wind,  i.e.,  the  atmosphere  in  motion,  and  as  this  material  has 
generally  a  very  definite  form  and  structure,  we  may  further  dis- 
tinguish wind-arranged  or  wind-deposited  clastic  material,  such  as 
is  found  in  sand-dunes.  When  this  is  consolidated  into  a  rock  it 
becomes  a  wind-formed  or  eolian  rock,  a  rock  which  may  also  be 
called  anemoclastic  from  the  Greek  anemos  (ai/e/xos),  the  wind.1  There 
are  many  examples  of  such  wind-formed,  eolian,  or  anemoclastic 
rocks  to  which  we  shall  call  attention  again  in  a  later  chapter. 
We  may  mention  here  as  examples  the  recent  dune-rock  of  the 
Bermuda  Islands,  and  the  much  older  sandstones  of  the  White 
Cliffs  in  the  Colorado  Plateau  region.  The  first  of  these  was 
formed  in  the  modern  or  Holocene  period,  the  second  in  the  Jurassic 
period,  and  this  has  retained  its  structure  ever  since  that  time, 
though  much  of  the  original  rock,  which  had  a  far  wider  distribution, 
has  been  worn  away  again  from  large  areas. 

The  Hydrosphere  as  a  Rock  Breaker. — The  second  great 
agent  which  accomplishes  the  destruction  of  rocks  is  the  hydro- 
sphere. This  destruction,  so  far  as  it  bears  on  our  present  point 
of  view,  is  mechanical  in  its  nature,  although  chemical  work,  the 
solution  of  rocks  (such  as  limestone,  salt,  etc.)  is  also  performed 
by  the  hydrosphere.  Such  solution,  however,  results  in  the 
reincorporation  of  the  rock  material  into  the  material  of  the  hydro- 
sphere from  which  in  turn  it  may  be  deposited  as  an  aqueous  or 
hydrogenic  rock  (stalactites  and  other  cave  deposits).  Such  a 
rock  is,  however,  not  a  clastic  but  a  "  genie  "  rock,  and  the  student 
should  learn  to  make  the  proper  distinction  between  these  two. 

The  mechanical  work  of  water  is  manifested  in  streams,  where 
the  current  moves  the  rock  fragments  which  it  has  broken  from  the 
ledges,  and  grinds  them  down  as  it  carries  them  along.  It  is  also 
seen  on  the  seashore,  where  the  waves  produce  an  analogous  effect. 

1  We  have  this  same  root  in  the  word  Anemone,  the  name  of  the  wind-flower,  in 
anemometer,  the  instrument  for  measuring  the  velocity  of  the  wind,  etc. 


78       Classification  and  Principal  Types  of  Rocks 

But  in  addition  to  the  breaking  off  of  the  rock  fragments  and  the 
grinding  of  them  to  sand  and  pebbles,  moving  water  in  streams 
and  on  the  sea-coast  assorts  clastic  material,  however  produced, 
into  grades  of  various  sizes,  and  more  or  less  according  to  the  weight, 
and  therefore  the  nature,  of  the  material.  Moreover,  clastic 
material  deposited  by  and  in  the  water  will  generally  be  char- 


FIG.  29.  —  Gorge  of  the  Genesee  River  below  the  Lower  Falls  at  Portage, 
N.Y.,  showing  in  the  opposite  banks,  the  cut  edges  of  the  stratified  rocks  (shales 
and  sandstones)  which  formerly  were  continuous  across  the  gorge. 

acterized  by  a  bedded  structure;  that  is,  it  willjexhibit,  if  seen  in 
section,  a  succession  of  layers  or  strata  one  above  the  other,  each 
one  of  which  was  at  one  time  the  topmost  layer  (Fig.  30).  Such 
deposits  are  called  stratified,  and  of  course,  at  any  one  time,  only  the 
top  of  the  topmost  stratum  is  visible.  Where  a  river,  however,  has 
cut  a  gorge  into  an  older  stratified  series,  or  where  the  waves  on  the 
sea-coast  or  lake  shore  again  partly  wear  away  such  a  series  which 
has  been  lifted  above  the  level  of  the  sea  by  natural  disturbances, 
or  rises  above  the  lake-level,  the  cut  edges  of  such  strata  can  be 
seen.  Thus  on  the  opposite  banks  of  the  Genesee  River  (Fig.  29) 
we  see  the  cut  edges  of  the  successive  strata  which  were  formerly 
continuous  across  the  chasm.  On  the  sea-coast  at  Atlantic  High- 
lands, N.J.,  we  likewise  see  the  edges  of  the  strata  which  were 
exposed  because  the  waves  cut  laterally  into  the  old  deposit  which 


The  Unaltered  or  Little  Altered  Rocks 


79 


had  been  previously  uplifted ;  and  a  similar  exposure  of  successive 
strata  is  seen  on  the  shore  of  Lake  Erie  (Fig.  16,  p.  31)  and  on 
many  other  shores  (Fig.  30). 

In  addition  to  the  bedded  structure  called  stratification,  which 
is  characteristic  of  water-laid  deposits  of  clastic  material  (and  to 
some  extent  of  hydrogenic  deposits  also),  there  are  other  features 


FIG.  30.  —  Sea  cliff  of  the  south  coast  of  Helgoland,  showing  the  edges  of 
the  stratified  rocks  of  which  the  island  is  composed,  exposed  by  wave  cutting. 
The  strata  are  of  Permian  age  and  dip  towards  the  north,  but  appear  to  be 
horizontal  in  the  sea  cliff.  (After  E.  Haase,  from  Walther.) 

by  which  water-laid  elastics  may  be  recognized.  These  will  be 
more  fully  discussed  in  a  later  chapter.  At  present  it  is  merely 
desired  that  the  student  should  recognize  that  water-laid  deposits 
have  definite  characters.  Some  of  these  may  point  to  a  deposition 
of  the  material  on  a  river  flood-plain,  an  alluvial  fan,  or  a  delta; 
some  to  deposition  in  lakes,  ponds,  or  playa  basins,  and  some  to 
deposition  in  the  sea.  These  last  may  be  regarded  as  the  most 
typical  examples  of  water-laid  elastics,  and  they  are  generally 
characterized  by  the  inclusion  of  the  shells  and  other  remains  of 
marine  organisms.  A  sandstone  or  mud-rock  with  marine  fossils 
(Fig.  31),  such  as  may  be  obtained  from  numerous  localities  the 
world  over,  will  serve  as  a  typical  example  of  a  water-laid  clastic. 
In  accordance  with  the  general  method  of  naming  such  deposits, 
those  under  consideration  may  be  termed  hydrodastic.  They 


8o       Classification  and  Principal  Types  of  Rocks 

may  also  be  called  aqueous  elastics  or  sediments,  in  distinction  from 
aqueous  precipitates  or  concentrates,  which  belong  to  the  group 
of  hydrogenic  deposits. 

The  Pyrosphere  as  a  Rock  Breaker.  —  As  we  have   seen,  the 
pyrosphere  contributes  pyrogenic  or  igneous  material,   which  on 


FIG.  31.  —  Photograph  of  a  slab  of  sandstone  filled  with  marine  fossils.     (Oris- 
kany  sandstone,  N.  Y.) 

cooling  and  hardening  forms  a  part  of  the  solid  crust  of  the  earth. 
It  also  breaks  up  material  already  rock,  and  this  is   generally 
£^__  brought  about  by  explosive  activities  such  as 

^A\t^^  j     are  characteristic  of  most  volcanoes.     By  these 

fijf&*>  3M1  activities  the  rock,  whether  an  older  lava  or  a 
e'fcwt^B^  rock  °f  °ther  origin,  located  in  and  around  the 
crater,'  may  be  shattered  and  the  material 
thrown  high  into  the  air,  to  descend  as  a  shower 
of  ashes  or  larger  particles.  Such  material, 
which  is  readily  recognized  by  its  character,  is 
called  pyroclastic  material,  and  when  bound  to- 
gether to  form  a  rock,  this  becomes  a  pyro- 

FIG.  32.  — Fault-      clastic  rock.     Examples:  volcanic  tuff,  volcanic 
breccia. 

agglomerate,  etc. 

Shattering  of  Rocks  within  the  Lithosphere  by  Movements.— 
When  one  part  of  the  lithosphere  moves  over  or  against  another 


The  Unaltered  or  Little  Altered  Roeks 


81 


part,  as  happens  when  the  earth's  crust  suffers  disturbances  (see 
Chapter  XXI)  the  rock  along  the  plane  of  movement  is  shattered 
or  ground  fine,  and  clastic  material  with  very  definite  character- 
istics is  produced.  As  such  a  movement  of  adjoining  rock  masses 
generally  produces  a  displacement,  or  fault  (see  Chapter  XIX),  the 
shattered  or  ground-up  material  is  commonly  called  a  fault-breccia 
(Fig.  32).  A  more  general  term  for  such  material  is  autoclastic, 
because  it  is  produced  by  the  self-destruction  or  breaking  up  of  the 
rocks  of  the  earth's  crust. 

The  Biosphere  as  a  Rock  Breaker.  —  That  growing  plants, 
such  as  trees,  arising  from  seeds  which  lodged  in  a  fissure  of  the 
rock,  can  by  expansive  growth  shatter  that  rock,  has  been  fre- 
quently observed  (Fig.  33).  Animals,  too,  break  rocks.  Thus 
where  a  spring  issues 
upon  a  level  surface  in 
a  more  or  less  arid 
country,  vast  herds  of 
hoofed  animals  will  con- 
gregate to  drink,  and 
their  constant  stamping 
of  the  rock  surface  will 
break  it  and  produce 
dust  and  sand  which 
mingles  with  the  water 
and  which  on  drying 
may  be  carried  away 
by  the  wind.  Thus 
hollows  are  produced 
in  the  surface  of  the 
land,  and  these  may 
be  filled  with  water 
and  so  produce  ponds. 
Many  such  are  known  to  exist  in  western  North  America,  in 
north  and  central  Africa,  and  elsewhere.  .  Fish  and  other  animals 
in  the  sea  will  break  off  branches  of  coral  from  reefs  (biogenic  rock) 
and  grind  these  to  powder  to  obtain  for  nourishment  the  animal 
matter  which  surrounds  these  coral  masses.  Thus  much  coral 
sand  and  mud  is  produced,  and  this  is  clastic  rock  material  of 
strictly  organic  origin. 

But  by  far  the  greatest  destroyer  of  rock  is  man.     We  have  seen 


FIG.  33.  —  A  huge  gravestone,  broken  and 
displaced  by  the  growth  of  the  roots  of  a  birch- 
tree.  Hannover.  (After  Walther.) 


82       Classification  and  Principal  Types  of  Rocks 

how  rock  is  broken  down  to  be  used  for  the  making  of  artificial 
rock,  that  is  mart-made  rock  (rubble-concrete,  etc.).  As  man  is  a 
part  of  the  organic  world  or  biosphere,  his  work  must  be  classed 
with  that  of  other  organisms.  Clastic  material  thus  produced 
by  organisms,  and  the  rocks,  whether  natural  or  artificial  (i.e., 
man-made) ,  made  from  these,  may  therefore  be  called  bioclastic. 


Summary  of  Clastic  Rocks 

We  have  thus  five  main  groups  or  classes  of  clastic  rocks,  each 
produced  by  one  of  the  spheres  and  named  after  it.  (See  the  dia- 
gram, Fig.  34.)  In  general  the  rock  is  regarded  as  belonging  to 


FIG.  34.  —  Diagram  of  the  interrelations  of  the  Exogenetic  Rocks. 

that  particular  class,  the  agent  of  which  has  placed  -upon  it  its 
characterization  stamp.  Thus  the  material  of  a  water-laid  rock 
may  have  been  originally  produced  in  another  way  than  by  aqueous 
erosion  —  it  may  have  been  produced  by  weathering,  after  which 
it  was  sorted  by  and  deposited  in  the  water.  Thus  water  has 
stamped  it  as  undeniably  an  aqueous  clastic  or  hydroclastic  rock. 
If  it  is  possible,  as  it  sometimes  is,  to  determine  how  the  material 


.  The  Unaltered  or  Little  Altered  Rocks          83 

originated  before  it  was  deposited  by  and  in  water,  this  may  serve 
to  further  characterize  the  rock  as  of  a  special  division  in  the  hydro- 
clastic  group.  The  types  of  clastic  rocks  then  are : 

1.  Atmoclastic  rocks.     Rocks  produced  by  atmospheric  destruc- 
tion of  rocks,  and  consolidated  without  rearrangement.     Example : 
consolidated  laterites. 

id.  Anemodastic  rocks.  Eolian  rocks.  Material  of  variable 
origin,  transported,  assorted,  and  deposited  by  wind.  Example: 
eolian  sandstone. 

2.  Hydrodastic  rocks.     Rocks  produced  from   material  eroded 
or  sorted  by,  and  deposited  by  and  in  water,  whether  by  rivers 
(fluviatile)  in  lakes  (lacustrine)  or  in  the  sea  (marine).     Examples: 
fossiliferous  marine  sandstones  or  shales. 

3.  Pyrodastic  rocks.     Rocks    formed  from  material  which  has 
resulted  from  shattering  of  older  rocks  by   volcanic  explosions. 
Examples :  volcanic  tuff,  volcanic  agglomerate. 

4.  Autodastic  rocks.     Rocks  formed  of  material  shattered    or 
ground  by  movements  within  the  earth's  crust.     Example :    fault- 
breccia. 

5.  Biodastic  rocks.     Rocks  produced  from  material  broken  or 
arranged  by  animals  (more  rarely  by  plants)  and  by  man.     Includes 
artificial  rocks.     Examples :    some  consolidated  muds  from  coral 
reefs,  rubble-concrete,  etc. 

In  the  succeeding  chapters  we  will  consider  first  the  commoner 
types  of  rocks,  in  which  the  original  characters  are  retained,  and 
their  structures,  leaving  the  metamorphic  derivatives  for  a  future 
chapter. 


CHAPTER  VI 

THE    PRINCIPAL    TYPES    OF    IGNEOUS    OR   PYROGENIC 

ROCKS 

THE  IGNEOUS  MAGMA 

THE  term  molten  magma  is  applied  to  molten  rock  material  or 
igneous  fluid,  which  is  formed  or  exists  within  that  part  of  the 
earth  which  we  have  called  the  pyrosphere,  and  which,  it  will  be 
remembered,  interpenetrates  the  lithosphere  and  the  asthenosphere 
(tectosphere).  When  the  magma  reaches  the  surface  of  the  earth, 
either  in  a  volcanic  eruption,  or  through  large  fissures  in  the  earth's 
crust,  as  in  parts  of  Iceland  to-day,  it  is  called  a  fluid  lava,  from 
which  by  solidification  in  cooling  a  solid  lava  or  lava-rock  is  pro- 
duced. Such  a  rock  mass  constitutes  an  extrusive  or  e/usive 
igneous  rock  mass.  When  the  magma  does  not  reach  the  surface, 
but  cools  within  the  crust  of  the  earth,  into  which  it  has  risen  or 
become  intruded  to  a  greater  or  less  extent,  either  upon  or  by  the 
formation  of  fissures,  or  otherwise,  it  forms  an  intrusive  igneous 
mass.  Finally,  a  magma  'may  be  conceived  as  cooling  essentially 
where  it  was  formed,  and  to  produce  a  deep-seated  or  abyssal  igneous 
rock  mass. 

Outcrops  of  Igneous  Rock  Formed  by  Solidification  from  Magmas 

It  is  obvious  that  only  the  surface-flows  or  lavas  will  be  accessible 
to  man  immediately  after  cooling,  and  of  these  generally  only  the 
superficial  portion.  All  the  other  igneous  rock  masses  are  located 
within  the  earth's  crust,  and  buried  beneath  the  surface  rock  masses, 
sometimes  to  very  great  depths.  Some  of  those  which  closely 
approach  the  surface  may  be  reached  by  deep  borings  or  by  mining 
operations,  but  this  can  occur  only  in  exceptional  cases.  Some  of 
these  may  also  be  conceived  of  as  brought  into  view  on  the  face 
of  a  great  dislocation-block  of  the  earth's  crust,  where  one  side 
of  the  broken  crust  is  lifted  and  the  other  depressed.  This  too, 

84 


The  Igneous  Magma  85 

however,  is  probably  so  exceptional  that  it  may  be  considered, 
in  the  absence  of  other  modifications,  to  be  practically  negligible. 
In  general,  it  is  only  after  prolonged  erosion,  which  results  in  the 
removal  of  much  of  the  surface-covering  of  the  rock,  that  intrusive 
masses  become  exposed,  while  deep-seated  masses  may  require 
the  removal  of  many  thousands  of  feet  of  covering  rock  before  they 
become  visible.  This  removal  of  great  covering  rock  masses 
requires,  of  course,  long  periods  of  time,  and  it  thus  becomes 
evident  that  the  igneous  rock  masses,  other  than  surface  lavas, 
which  are  now  visible  in  outcrops,  are  of  great  age,  and  that  their 
formation  by  cooling  has  taken  place  at  remote  periods  of  time. 
Igneous  intrusions  and  deep-seated  masses  which  are  now  being 
formed  are  entirely  invisible  to  us,  and  will  not  be  exposed  until 
many  ages  have  passed  by,  if  ever.  Nevertheless  there  are  indirect 
ways  in  which  we  can  infer  the  existence  of  igneous  masses  beneath 
the  surface,  which  are  now  undergoing  the  process  of  solidification, 
and  to  some  of  these  we  shall  refer  later. 


Characters  and  Composition  of  Igneous  Magmas 

It  is  not  possible  to  gain  a  complete  knowledge  of  the  composition 
of  an  igneous  magma  from  the  composition  of  the  igneous  rock 
which  has  resulted  by  solidification  from  that  magma,  because 
the  magma  contains  in  addition  to  the  substances  which  make 
up  the  solid  rock  formed  from  it,  large  quantities  of  volatile  gases 
which  are  expelled  upon  cooling  and  relief  of  pressure.  The  volatile 
gases  which  are  expelled  under  these  conditions  are,  water  vapor, 
carbon  dioxide,  hydrochloric  acid,  sulphurous  vapors,  and  the  like. 
Such  expulsion  of  volatile  matter  is  shown  by  all  surface  lavas 
from  which  clouds  of  steam  arise,  and  such  steam  on  analysis  proves 
to  carry  with  it  many  of  the  other  gaseous  emanations.  Others 
quickly  condense  upon  the  surface  and  may  form  salts  of  various 
kinds,  which  either  encrust  the  surface  of  the  cooling  lava, 
or  are  carried  away  in  solution  by  the  condensing  waters  from  the 
water  vapors.  Sometimes  the  expulsion  of  gas  and  vapors  is  so 
rapid  that  violent  explosions  result,  and  this  is  indeed  the  chief 
cause  in  the  production  of  pyroclastic  material.  In  practically 
every  volcanic  eruption  there  is  produced  a  cloud  of  vapor  and 
gases  which  carries  upward  vast  masses  of  finely  divided  rock 
material  —  the  product  of  the  explosive  action  —  and  often  rises 


86      Principal  Types  of  Igneous  or  Pyrogenic  Rocks 


to  heights  bf  many  miles  above  the  volcano  (Fig.  35).  When 
gases  and  vapors  alone  issue  from  a  fissure  in  the  earth,  the 
phenomenon  is  spoken  of  as  a  fumarole.  Fumaroles  are  commonly 

associated  with  de- 
clining volcanic  ac- 
tivities. The  gases 
and  waters  of  cool- 
ing magmas  within 
the  earth's  crust  may 
escape  through  fis- 
sures not  penetrated 
by  the  magma  or 
opened  since  cooling 
began,  and  these 


FIG.  35.  —  Volcano  in  eruption,  showing  the  cloud 
of  steam  and  ashes  projected  high  into  the  air. 


may  reach  the  sur-  • 
face  as  mineral 
springs,  either  hot  or  cold.  In  their  upward  passage,  such  waters 
and  gases  may  deposit  mineral  matter  which  they  carried,  and  this 
is  believed  by  many  to  account  for  vein  and  other  mineral  deposits. 
The  non- volatile  material  of  the  magma  which  solidifies  to  form 
the  lava  or  other  igneous  rocks  consists  predominantly  of  only  a 
small  number  of  substances,  chief  among  which  is  silica  (SiO^), 
which,  however,  is  present  in  variable  amounts,  according  to  the 
nature  of  the  magma.  In  addition  to  this  there  are  the  oxides  of 
the  metals  aluminum  (Al),  iron  (Fe),  magnesium  (Mg),  calcium 
(Ca),  sodium  (Na),  and  potassium  (K).  These,  too,  vary  in 
amounts  in  the  different  magmas.  On  solidification  the  silica 
unites  with  them  to  form  various  silicate  minerals,  which  in  the 
coarser-grained  igneous  rocks  can  be  readily  distinguished. 

In  a  general  way  there  can  be  recognized  a  gradation  in  the  com- 
position of  the  magmas  from  a  point  where  silica  is  most  abundant, 
sometimes  forming  75  per  cent  of  the  whole  mass,  with  alumina 
and  potassium  next,  by  an  increase  in  sodium  and  later  in  calcium, 
magnesium,  and  iron,  and  a  reduction  in  silica  to  a  point  where  this 
constitutes  50  per  cent  or  less  (rarely  35  per  cent)  of  the  entire  mass. 
With  this  occurs  a  reduction  in  the  oxides  of  potassium,  sodium, 
and  aluminum,  sometimes  to  the  complete  or  nearly  complete 
elimination  of  some  of  these.  The  end  high  in  silica,  etc.,  is  called 
the  acid  end  of  the  series ;  the  other  end  is  the  basic  end.  Rocks 
formed  from  the  acid  portion  of  an  igneous  magma  generally  have 


Formation  of  Igneous  Rocks  by  Cooling  of  Magma     87 

light-colored  and  light-weight  minerals  predominating;  those 
formed  from  the  basic  portion  have  mainly  dark-colored  and 
heavy  minerals.  Acid  magmas  (and  lavas)  are  generally  very 
stiff  or  viscous  even  at  high  temperatures  (2000°  C.  or  over),  and 
their  contained  gases  escape  with  difficulty,  and  often  with  explo- 
sive violence,  as  the  magma  approaches  the  surface.  Basic  mag- 
mas (and  lavas),  on  the  other  hand,  are  more  fluid  even  at  much 
lower  temperatures  (1300°  C.),  and  on  this  account  the  gases 
escape  more  readily,  and  explosions  are  less  common.  The  lavas 
of  Vesuvius  and  Mont  Pelee  are  examples  of  the  first,  those  of 
Kilauea  in  Hawaii,  of  the  second. 

FORMATION  OF  IGNEOUS  ROCKS  BY  COOLING  OF  MAGMA 

With  the  cooling  of  the  non- volatile  part  of  the  magma,  solidifi- 
cation takes  place  and  igneous  rocks  are  produced.  The  kind  of 
rock  will  of  course  vary  with  the  variation  in  the  composition  of 
the  magma,  which  must  be  considered  to  be  the  character  of  first 
importance.  Next  to  this  is  the  rate  of  cooling,  which  determines 
the  grain  or  texture  of  the  rock,  and  which  in  turn  is  influenced 
by  the  relative  position  of  the  magma  while  cooling. 

Influence  of  Rate  of  Cooling.     Texture 

It  is  a  well-ascertained  fact  that  a  rapidly  coolirig  magma  will 
tend  to  produce  a  mass  of  glass,  and  that  in  proportion  as  the  rate 
is  slower  will  there  be  opportunity  for  the  growth  of  mineral 
crystals,  the  size  of  which  is,  in  general,  proportional  to  the  slow- 
ness of  the  cooling.  When  the 
entire  rock  becomes  a  mass  of  min- 
eral crystals,  it  is  said  to  be  holo- 
crystalline.  Between  this  and  the 
glassy  type  in  which  there  are  no 
crystals,  there  are  all  gradations. 
The  relative  size  and  arrangement 
of  the  component  crystals  of  a 
rock  form  its  texture.  FlG  ^  _  Poiphyry .  typkal  por. 

Primary  Textures.  —  Two  gen-  phyritic  texture, 

eral  types  of  primary  texture  may 

be  distinguished,  the  homogeneous  or  uniform,  and  the  hetero- 
geneous or  porphyritic.  In  the  first  all  the  crystals  of  each  mineral 


88      Principal  Types  of  Igneous  or  Pyrogenic  Rocks 

are  essentially  of  uniform  size,  though  the  size  %  varies  with  the 
mineral.  In  the  second,  or  porphyritic  type  (Fig.  36),  on  the  other 
hand,  the  crystals  of  certain  of  the  essential  minerals  (generally 
the  feldspars)  are  of  two  sizes,  one  large  and  more  or  less  fully  and 
perfectly  formed,  the  other  small  and  less  perfect.  The  larger 

crystals,  which  are  scattered  through 
the  mass,  are  called  the  phenocrysts ; 
the  finer-grained  mass,  together  with 
other  minerals,  also  in  small  grains, 
forms  the  ground-mass. 

Secondary    Textures.  —  The    tex- 
tures of  the  ground-mass  and  of  the 
homogeneous     textured     rocks     are 
called  secondary.     When  the  grains 
or    crystals   are  all   nearly   uniform, 
FIG.  37.  —  Granitic  texture       the  texture  is  said  to  be  granitic  or 
(holocrystalline),    slightly       granular    (Fig.   37),  and   it   may  be 

coarsely  or  finely  granular,  i.e.,  the 

texture  is  coarse-grained  or  fine-grained  so  long  as  the  individual 
crystals  are  distinguishable.  But  when  the  crystals  are  no  longer 
distinguishable,  the  texture  becomes  dense  or  felsitic,  while  a  step 
further  brings  us  to  the  glassy  texture  in  which  no  crystals  are 
developed. 

Relation  of  Textures  to  Kinds  of  Magmas 

In  general,  the  basic  magmas,  being  more  fluid,  will  tend  to  form 
coarser  crystals,  the  resulting  rocks  therefore  being  more  com- 
monly coarse-grained.  Glassy  textures  are  correspondingly  less 
frequently  developed.  Acid  magmas,  on  the  other  hand,  tend  to 
develop  the  finer  textures  more  frequently,  and  here  glassy  rocks 
are  common. 

The  presence  of  much  gas  and  water  vapor,  too,  tends  to  increase 
the  power  of  crystal  forming,  and  along  certain  fissures,  presumably 
the  pathway  of  escape  of  these  gases  and  vapors,  the  texture  is 
often  of  exceeding  coarseness.  The  most  common  examples  are 
the  dikes  or  other  masses  of  pegmatite,  a  rock  composed  sometimes 
of  huge  crystals  of  feldspar,  quartz,  and  mica,  the  latter  mineral 
being  obtained  in  large  plates  from  this  rock,  which  is  the  chief 
commercial  source  of  this  mineral. 


Formation  of  Igneous  Rocks  by  Cooling  of  Magma     89 

Relation  of  Texture  to  Place  of  Cooling  and  Bulk  of  Magma 

As  the  coarseness  of  grain  is  in  proportion  to  the  slowness  of 
cooling,  it  is  evident  that,  other  things  being  equal,  whatever 
lowers  the  rate  of  cooling  will  cause  an  increase  in  the  size  of  the 
crystals,  and  of  the  coarseness  of  texture.  Thus  a  magma  cooling 
within  the  earth's  crust,  especially  at  great  depth,  will  of  necessity 
cool  slowly,  and  so  become  a  coarsely  crystalline  rock.  Again, 
the  central  part  of  the  magma  will  cool  more  slowly  than  its  outer 
part,  which  is  in  contact  with  the  cooler 
enclosing  rock.  In  like  manner  small  masses 
will  cool  more  rapidly  than  large  ones,  and 
this  is  especially  the  case  when  the  igneous 
mass  is  a  thin  sheet  transecting  other  and 
cooler  rocks  (dikes  (Fig.  38),  sills,  etc.,  see 
beyond).  There  is,  however,  one  modifica- 
tion of  this  general  rule,  which  should  be 
noted,  and  that  is  the  conditions  under  which 
porphyri tic  rocks  appear.  In  these  the  fine  FIG.  38.— Small 

ground-mass  indicates  rapid  cooling,  but  the     dike  of  basalt>  show- 
.    ,      ,  ,  -  mg  dense  texture,  cut- 

presence  of  the  large  phenocrysts  shows  that      ting  syenite  of  finely 

these  crystals  grew  to  their  full  size  before     granular  texture, 
the  ground-mass  solidified.     In  other  words, 
the  phenocrysts  were  floating  crystals  in  a  still  semi-fluid  matrix. 
Typical  porphyries  (Fig.  36)  (with  fine-grained  ground-mass)  are 
most  common  in  certain  lava  flows  (as  in  the  typical  trachyte 
from  Drachenfels,  Germany),  somewhat  less  so  in  the  smaller  in- 
trusive masses,  and  still  less  common,  and  indeed  rather  rare,  in 
the  great  subterranean  masses.     All  types  of  igneous  rocks  may, 
however,  show  a  porphyritic  texture. 


Classification  of  Igneous  Rocks 

From  what  has  been  said  up  to  this  point,  it  appears  that  the 
principles  which  underlie  the  classification  of  igneous  rock  are 
relatively  simple.  The  first  general  division  is  made  upon  a  chem- 
ical basis,  which  in  the  rocks  is  expressed  by  the  mineral  species 
present.  Further  subdivisions  are  based  upon  the  texture  of  the 
rock,  for  the  same  magma  may  furnish  rocks  of  different  textures 
as  the  result  of  cooling  at  different  rates  under  different  conditions. 


go      Principal  Types  of  Igneous  or  Pyrogenic  Rocks 

Since,  as  above  stated,  the  character  of  the  magma  is  most  readily 
ascertained  from  the  minerals  which  crystallize  from  it,  chief 
attention  is  ordinarily  given  to  these  minerals.  Furthermore, 
since  these  are  readily  recognizable  only  in  the  coarse-grained  or 
holocrystalline  products  of  cooling  of  the  magmas,  it  is  customary 
to  designate  the  rock  groups  based  on  chemical  and  mineralogical 
characters  by  the  name  of  the  typical  holocrystalline  member  of 
each  group.  Before  we  consider  these  types,  however,  we  must 
briefly  review  the  essential  minerals  which  enter  into  the  construc- 
tion of  these  rocks. 

The  Essential  Minerals  of  Igneous  Rocks 

We  may  in  general  distinguish  three  groups  of  essential  minerals 
which  make  up  the  bulk  of  the  igneous  rocks.  These  are  the 
following,  arranged  in  each  case  in  the  order  from  acidic  to  basic. 
They  are:  (i)  Quartz,  (2)  The  Feldspars  and  Feldspathoids,  and 
(3)  the  Micas  and  Ferro-Magnesian  Silicates. 

Quartz  (SiO2). — This  generally  occurs  in  glassy  fragments  of 
irregular  and  usually  of  angular  form,  more  rarely  in  crystals. 
It  is  easily  recognized  by  its  hardness  (see  pp.  49  and  61). 

The  Feldspars.  —  In  composition  these  are  all  double  silicates 
of  aluminum  and  an  alkali  metal  (K,Na,  etc.)  or  an  alkaline  earth 
(Ca,Mg,  etc.)  or  both.  In  general  the  feldspars  are  divided  into 
orthoclase1  or  the  feldspar  with  right-angled  cleavage,  and  plagiodase 
or  the  feldspar  with  oblique  cleavages.  The  most  acid  feldspar  con- 
tains only  potassium  in  addition  to  both  alumina  and  silicic  acid 
(potash  feldspar  or  orthoclase),  the  most  basic  only  lime  in  place  of 
the  potash  (lime  feldspar  or  anorthite).  Between  the  two  stands 
the  soda  feldspar  (albite)  where  sodium  is  the  metallic  base  besides 
aluminum.  Between  the  pure  soda  and  the  pure  lime  feldspars  are 
a  number  of  intermediate  types,  consisting  of  different  proportions 
of  both  soda  and  lime ;  and  between  the  pure  potash  and  soda  feld- 
spars there  are  also  mixtures  of  the  two. 

These  relations  may  be  expressed  in  the  following  table,  where  the 
three  main  or  (theoretically)  pure  types  of  feldspars  are  each 
expressed  by  a  molecular  symbol  and  the  mixed  types  by  formulae 
which  indicate  the  proportions  of  each. 

1  Microline  is  a  potash  feldspar  like  orthoclase,  but  crystallizing  in  the  triclinic 
system,  and  therefore  of  the  plagiodase  type. 


Formation  of  Igneous  Rocks  by  Cooling  of  Magma     91 

Table  of  the  Feldspars 


PRINCIPAL  OR 
PURE  TYPES 

MIXED  TYPES 

COMPOSITION 
(Chemical  Formula) 

COMPOSITION 
(Molecular  Symbol 
or  Formula) 

si 

Orthodase 
(Potash 
Feldspar) 

K2O  •  A1203  •  6  SiOa1 

or                | 
KAlSi308 

Or 

Anorthoc'.ase 

to 
OrjAbi.6 

.2 
.H 

Albite 
(Soda  Feldspar) 

Na20  •  A1203  •  6S;02 
or 
NaAlSi308 

Ab 

to 

(AbgAru) 

Oligoclase 

Ab«Ani 
to 
Ab2Ani 

'LAGIOCLASES 
(Triclinic) 

(Andesine) 

Ab3An2 
to 
Ab4An3 

Labradorite 

to 
Ab:An2 

.U 

1 

Bytownite 

AbiAn3 
to 
AbiAn« 

Anorthite 
(Lime  Feldspar) 

CaO  •  A12O3  •  2  SiO2 
or 
CaAl2Si2O8 

An 

to 

(AnsAbx) 

In  ordinary  rock  determination  it  is  very  difficult  to  recognize 
the  intermediate  feldspars,  though  this  may  be  done  with  more  or 
less  precision  when  a  thin  slice  of  the  rock  is  placed  under  the  micro- 
scope, and  examined  by  the  us"e  of  polarized  light.  From  chemical 
analysis,  however,  which  gives  the  amount  of  each  substance  pres- 
ent, it  is  possible  to  calculate  the  types  of  feldspar  (and  other  min- 
erals) present  in  the  rock,  a  calculation  which  must,  of  course,  be 
checked  by  microscopic  examination.  The  plagioclase  feldspars 
usually  show  fine  parallel  (twinning)  striae  on  some  faces. 


92       Principal  Types  of  Igneous  or  Pyrogenic  Rocks 


In  general,  it  may  be  said  that  orthoclase,  albite,  and  oligoclase 
are  characteristic  of  acidic  rocks,  or  rocks  high  in  silica,  with  ortho- 
clase in  the  most  acidic,  while  labradorite,  bytownite,  and  anorthite 
characterize  the  basic  rocks,  with  the  last  at  the  most  basic  end 
of  the  series. 

The  Feldspathoids.  —  Under  this  designation  are  placed  double 
silicates  of  aluminum  and  the  alkalies  or  alkaline  earths,  which 
have  many  characters  in  common  with  the  feldspars  besides  the 
general  similarity  in  their  composition,  but  differ  in  crystal  form 
and  some  other  characters.  Theoretically  we  again  have  three  types, 
the  potash,  soda,  and  lime  feldspathoids,  but  actually  the  first  two 
are  more  commonly  intermixtures.  This  is  shown  in  the  following 
table.  In  the  order  of  their  importance  as  rock  constituents,  these 
minerals  are  nephelite,  leucite,  and  melilite,  the  last  being  very 
rare. 

Table  of  the  Feldspathoids 


MINERAL 

COMPOSITION 
(pure) 

USUAL  MODIFICATION 

Leucite 

K2O  •  A1203  •  4  Si02 
KALSi2O6 

Some  Na2O  replaces 
part  of  K2O 

Nephelite     • 

4  Na2O  •  4  A12O3  •  9  SiO2 
or  (almost) 
NaAlSi04 

Some  K2O  and  CaO 
replaces  part  of  Na^O 

Melilite 

i2CaO-2  Al2O3-gSiO2 
or 

Ca3Al(Si04)3 

In  general,  the  occurrence  of  leucite  in  igneous  rock  implies  a 
magma  rich  in  potash ;  nephelite,  one  rich  in  alumina  and  soda ; 
and  melilite,  one  poor  in  silica  and  alumina  but  rich  in  lime. 
Melilite  occurs  mainly  in  a  few  rare  basalts. 

Common  Mica  and  the  Ferromagnesian  Minerals.  —  The 
name  ferromagnesian  silicates  is  given  to  the  minerals  (usually  dark 
in  color)  which  include  in  their  composition  both  iron  and  magne- 
sium. They  comprise  the  dark  mica,  biotite,  the  hornblendes,  pyrox- 
enes, and  olivine.  The  common  or  white  mica  (muscovite)  has  neither 
iron  nor  magnesium,  but  is  a  silicate  of  potassium  and  aluminum 


Formation  of  Igneous  Rocks  by  Cooling  of  Magma     93 


together  with  some  hydrogen.  Its  usual  formula  is  K(OH)« 
A12O3  •  2SiO2,  or  HKAl2Si2O8  and  it  is  commonly  called  the  potash 
mica.  As  might  be  inferred,  it  occurs  in  rocks  where  other  potash 
minerals  are  common,  such  as  granites,  pegmatites,  etc.  It  is  also 
common  in  metamorphic  schists. 

In  the  ferromagnesian  silicates  we  trace  a  gradation  in  com- 
position from  biotite,  or  black  mica,  in  which  potash  is  present, 
through  the  amphiboles  and  .pyroxenes,  in  which  the  potash  is  re- 
placed by  lime,  to  olivine,  where  no  alkali  or  alkaline  earth  is  pres- 
ent. This  is  shown  in  the  following  table  where  muscovite  is  also 
placed  at  the  acidic  end  and  magnetite,  the  pure  iron  oxide,  at  the 
basic  end. 

Table  of  the  Ferromagnesian  Minerals  and  Their  Two  End  Members 


MINERALS 

COMPOSITION 

USUAL  MODIFICATION 

Muscovite 

H2KAl3(SiO4)3 
to 

(HK)Al2(Si04)2 

Without  iron  or  magne- 
sium. 

Biotite 

(HK)2(MgFe)2Al2(Si04)3 
(nearly) 

Addition  of  magnesium 
and  iron;  reduction  of 
alumina  and  silica. 

Amphibole 
(Hornblende) 

Ca(MgFe)3(Si03)4 
(in  general) 

Substitution  of  calcium 
for  hydrogen,  potassium, 
and  aluminum,  though 
some  of  the  last  may  be 
present. 

Pyroxene 

(Augite) 

Ca(MgFe)(Si03)4 
(in  general) 

Olivine 

(MgFe)2Si04 

Omission  of  calcium,  some 
reduction  of  silica. 

Magnetite 

Fe304 

Omission  of  silica  and  mag- 
nesium. 

It  must  be  understood  that  the  amphiboles  and  pyroxenes  are 
more  complex  in  composition  than  here  stated,  and  that  there  are 
a  number  of  varieties  of  each,  differing  in  composition.  It  is,  how- 
ever, not  necessary  that  these  be  considered  here.  The  common 


94      Principal  Types  of  Igneous  or  Pyrogenic  Rocks 


amphiboles  and  pyroxenes  crystallize  in  the  monoclinic  system,  but 
there  are  also  orthorhombic  members  of  each  group.  The  chief 
means  of  distinction  is  the  (prismatic)  cleavage  form,  that  of  the 
pyroxenes  being  nearly  at  right  angles,  and  that  of  the  amphiboles 
forming  angles  of  nearly  120  and  60  degrees,  respectively,  as  shown 


Amphibole 

FIG.  39.  —  Basal  sections  of  crystals  of  Pyroxene  and  Amphibole,  showing 
characteristic  differences  in  outline,  and  cleavage  as  seen  under  the  microscope. 
A  characteristic  interference  figure  is  shown  in  Pyroxene.  (After  Moses  and 
Parsons.) 

in  the  above  outlines  (Fig.  39).  In  general,  it  may  be  said  that 
whereas  muscovite  occurs  chiefly  in  the  most  acidic  rocks,  biotite 
and  hornblende  indicate  greater  basicity,  pyroxenes  still  greater 
basicity,  while  olivine  is  characteristic  only  of  the  very  basic  igneous 
rocks.  When  pyroxene  and  olivine  are  present,  free  quartz  is 
usually  absent. 

Secondary  or  Accessory  Minerals.  —  There  are  many  minerals 
which  occur  in  small  quantities  in  igneous  rocks  but  are  not  neces- 
sary constituents  of  them.  They  are  called  accessory  minerals, 
and  their  presence  is  most  frequently  detected  by  the  microscope. 
Zircon  and  jtitanite  are  good  examples  of  these.  By  alteration 
many  other  accessory  minerals  are  formed  from  the  primary  or 
original  ones. 

Order  of  Crystallization 

In  general  the  order  of  crystallization  of  the  minerals  from  an 
igneous  magma  follows  the  order  of  their  basicity,  the  most  basic 
separating  out  first,  the  most  acid  last.  Thus  olivine  will  form  in 
perfect  crystals  from  a  basaltic  magma,  the  other  minerals  being 
less  perfectly  crystallized.  The  pyroxenes  and  the  hornblendes 
separate  out  before  the  feldspathoids  and  the  feldspars,  while 


Types  of  Igneous  Rocks  95 

the  quartz,  if  there  is  any  free  silica  remaining,  separates  out  last, 
filling  the  interstices  between  the  other  minerals.  For  that  reason 
quartz  is  practically  never  in  perfect  crystal  form  in  granites  or 
other  igneous  rocks  which  contain  it,  but  forms  irregular  grains, 
the  shape  of  which  is  determined  by  the  form  of  the  cavities  left 
between  the  other  minerals.  It  has  a  crystalline  structure,  how- 
ever, though  not  a  crystal  form. 

TYPES  OF  IGNEOUS  ROCKS  BASED  ON  COMPOSITION 
AND  TEXTURE 

The  table  on  page  96  summarizes  the  more  important  types  of 
igneous  rocks,  beginning  with  the  most  acidic  on  the  left,  and  ex- 
tending to  the  basic  types  on  the  right.  It  will  be  seen  that  there 
are  certain  composition  groups  based  upon  the  kind  of  feldspar  (or 
feldspathoid)  present,  the  kind  of  ferromagnesian  mineral,  and  the 
presence  or  absence  of  free  quartz  or  olivine.  Each  of  these  com- 
position groups  includes  a  series  of  rocks  ranging  in  texture  from 
coarse  (at  the  bottom)  through  fine  and  dense  or  f  elsitic  to  glassy  (at 
the  top).  The  coarser-grained  rocks  are  generally  formed  as  deep- 
seated  masses,  which  have  become  exposed  by  erosion.  The  in- 
termediate types  occur  mainly  as  intrusive  masses,  while  the  dense 
and  glassy  types  (including  the  cellular)  are  primarily  formed  from 
surface  flows.  As  the  mineral  character  of  the  several  groups  is 
most  readily  ascertained  in  the  case  of  the  coarse-grained  varieties, 
the  various  groups  will  be  considered  under  these  respective  types. 
Microscopic  examination  is  generally  necessary  to  ascertain  the 
characters  of  the  finer-grained  and  dense  types,  though  typical 
examples  may  generally  be  recognized  without  such  aid. 

The  Granite-Rhyolite  Series 

This  contains  the  most  acidic  of  the  common  igneous  rocks. 
Common  types  are :  granite,  rhyolite,  quartz-felsite,  obsidian,  and 
pumice,  and  the  porphyritic  varieties  of  these. 

Granite.  —  The  coarse-grained  members  consist  primarily  of 
orthoclase  feldspar  and  free  quartz.  When  these  alone  are  present, 
as  is  sometimes  the  case,  the  rock  is  called  a  binary  granite.  Gen- 
erally, however,  black  mica  (biotite)  and  often  muscovite  and  some 
hornblende  are  present,  these  being  readily  recognized  by  their 
dark  color,  while  the  biotite  is  distinguished  from  the  hornblende 


96      Principal  Types  of  Igneous  or  Pyrogenic  Rocks 


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Types  of  Igneous  Rocks 


97 


by  its  scaliness  and  softness.  The  orthoclase  is  commonly  recog- 
nized by  its  pinkish  or  flesh  color  and  by  the  smooth  cleavage  planes 
which  it  shows,  as  well  as  the  more  or  less  marked  crystal  form. 
The  quartz  is  always  in  irregular,  glassy  masses,  breaks  with  rough 
fracture  and,  as  a  result,  looks  darker  than  the  feldspar  by  reflected 
light. 

Granites  vary  by  the  replacement  of  some  of  the  orthoclase  by 
acid  plagioclase  while  hornblende  may  increase  in  amount,  forming 
a  hornblendic  granite.  With  this  may  go  a  reduction  in  the 
quantity  of  free  quartz  present,  when  the  rock  approaches  a 
syenite  in  composition,  while  with  the  increase  in  plagioclase  the 
quartz-diorites  or  diorites  are  approached.  In  rare  cases,  too, 
augite  may  occur,  showing  an  approach  to  gabbro.  Among  the 
common  accessory  minerals  are  magnetite,  zircon,  and  garnet.  A 
porphyritic  texture  is  sometimes  developed  by  the  formation  of 
large  and  more  or  less  perfect  crystals  of  orthoclase,  which  fre- 
quently show  a  twinned  character  (Carlsbad  twins),  recognizable 
by  the  fact  that  one  half  of  the 
crystal  appears  darker  than  the 
other  in  reflected  light,  the  shade 
being  reversed  with  change  in  the 
position  of  the  light. 

Pegmatite. — This  is  the  name 
given  to  a  coarse  variety  of  a 
granite  which  occurs  in  vein  or 
dike-like  masses,  and  generally 
consists  of  large  crystals  of  ortho- 
clase (sometimes  acid  plagioclase) 
occasionally  up  to  a  foot  or  more 
in  diameter,  large  masses  of  quartz, 
and  large  plates  of  mica  (musco- 
vite).  The  quartz  and  feldspar 
are  sometimes  found  intergrown  in  such  a  manner  that  the  surface 
of  the  feldspar  seems  to  be  scattered  over  with  small,  irregular, 
dark  masses  of  quartz,  appearing  not  unlike  cuneiform  characters. 
On  this  account  such  a  mixture  of  feldspar  and  quartz  is  called 
graphic  granite  (Fig.  40). 

Rhyolite  and  Quartz  Felsites.  —  These  have  essentially  the  same 
mineralogical  composition  as  granite,  though  the  association  may 
vary  more,  producing  greater  variety.  In  color  the  rocks  are  gen- 


FIG.  40.  —  Graphic  Granite, 
slightly  reduced. 


98      Principal  Types  of  Igneous  or  Pyrogenic  Rocks 

erally  light  gray,  with  yellows,  and  pale  reds,  and  occasionally  darker 
shades.  The  texture  varies  considerably,  from  very  finely  crystal- 
line to  dense  or  felsitic,  when  the  rock  consists  of  minute  masses 
of  quartz  and  feldspar  often  with  more  or  less  glass.  A  cellular 
structure  may  also  be  developed  to  a  slight  degree  in  rhyolites 
formed  from  surface  flows.  In  many  rhyolites,  especially  if  they 
are  porphyritic,  quartz  can  be  recognized  (commonly  as  small 
double-sided  pyramids),  giving  the  surface  a  rough  feel.  Pheno- 
crysts  of  feldspar  also  occur  in  the  porphyritic  types.  When  the 
phenocrysts  of  quartz  and  feldspar  make  up  about  half  the  mass 
of  the  rock  it  is  called  a  rhyolite-porphyry  ;  when  it  constitutes  most 
of  the  rock  it  is  called  a  granite-porphyry  —  and  marks  an  approach 
to  granite.  In  both  the  ground  mass  is  generally  dense  or  felsitic. 
The  name  rhyolite  is  given  to  this  rock  because"  of  the  flow 
structure  often  exhibited  (Greek,  /W  =  flow). 

The  Acid  Glasses.  —  These  include  obsidian,  pitchstone,  perlite, 
and  pumice.  In  these  the  excess  of  silica,  which  on  crystallization 
would  produce  free  quartz,  can  generally  be  recognized  only  on 
analysis,  except  when  phenocrysts  are  developed.  As  the  magmas 

which  produce  syenites  on  crystalliza- 
tion also  form  glasses  of  similar  ap- 
pearance, though  poorer  in  silica,  it 
is  evident  that  it  is  not  possible  to 
determine,  except  by  analysis,  whether 
a  given  glass  belongs  to  that  or  to  the 
granite  series. 

Obsidian.  —  This  is  a  homogeneous 
glass  with  a  low  percentage  of  water. 
FIG.   41.  —  Obsidian,    showing    It  is  black  to  red  in  color,  with  trans- 
conchoidal  fracture.  lvcent   edgeSj   and  a  conchoidal  frac- 

ture, so  called  because  the  surface  of 

the  fracture  is  generally  marked  by  a  series  of  concentric  lines,  like 
the  growth-lines  of  a  shell  (Fig.  41). 

Pitchstone.  —  This  is  like  obsidian,  but  contains  from  5  to  10 
per  cent  of  water.  It  has  commonly  a  sheen  or  luster  suggestive 
of  resin,  and  its  colors  are  commonly  reds  and  greens. 

Perlite,  or  pearl  stone  (Fig.  42). — This  is  glass  composed  of  nu- 
merous rounded  masses,  with  concentric  structure  like  the  coats  of 
an  onion,  formed  by  contraction  in  cooling  and  separated  by  larger 
straight  cracks.  It  usually  contains  from  2  to  4  per  cent  of  water. 


Types  of  Igneous  Rocks 


99 


Pumice  (Fig.  43).  —  This  is  a  cellular  or  porous  glass,  its  charac- 
ter being  due  to  the  liberation  of  gases  on  cooling.  It  may  be  re- 
garded as  the  consolidated  sur- 
face froth  of  the  lavas. 

All  the  glasses  form  from  sur- 
face flows  of  lava,  and  the  move- 
ment or  flowing,  after  partial 
solidification,  is  commonly  shown 
by  the  occurrence  in  them  of 
layers  of  denser  or  more  stony 
material,  in  which  minute  crys- 
tals of  feldspar  and  quartz  are  de- 
veloped. Often  these  are  arranged 
in  rosettes  of  radiating  structure 
to  which  the  name  spherulites  is 
given  (Fig.  44).  Cavities  due 
to  the  expansion  of  steam  or 
gases  are  also  formed,  which  are  generally  spherical  and  often 
contain  crystals  of  various  minerals  (topaz,  quartz  feldspar, 
garnet,  etc.).  These  are  called  lifhophysa  or  stone  bubbles 
(Fig.  45),  and  they  vary  in  size  up  to  an  inch  in  diameter. 
Typical  localities  for  obsidian  are  the  Lipari  Islands  and  Yellow- 


FIG.  42.  —  Perlite.  Thin  section 
under  the  microscope,  enlarged 
30  diameters.  Hlinik,  Hungary. 
(After  Rosenbusch.) 


FIG.  43.  —  Pumice.     Surface  of  a 
hand  specimen. 


FIG.  44.  —  Spherulitic  Obsidian. 


stone  Park;  for  pitchstone,  Meissen  near  Dresden,  Saxony, 
the  Island  of  Arran,  west  Scotland,  and  Silver  Cliff,  Colorado; 
while  the  best  known  localities  for  perlites  are  in  Hungary. 


ioo    Principal  Types  of  Igneous  or  Pyrogenic  Rocks 

Devitrified  Old  Glasses.  —  Volcanic  glasses  of  very  early  geologi- 
cal time  have  generally  undergone  a  change  by  the  development  in 
them  of  excessively  minute  crystals  of  feldspar  and  quartz.  From 


FIG.  45.  —  Lithophysae  in  Lithoidite  of  Obsidian  Cliff,-  Yellowstone  National 
Park.     Slightly  reduced.     U.  S.  Geol.  Sur.  Bull.  150. 

this  they  lose  their  glassy  appearance  and  resemble  felsites,  which 
name  is  commonly  applied  to  them.  They  also  have  been  called 
petrosilex.  They  are  not  uncommon  in  the  old  lava  flows  of  the 
New  England  states  and  elsewhere. 

The  Syenite-Trachyte  Series 

This  series  is  primarily  characterized  by  deficiency  in  silica,  so 
that  no  free  quartz  is  formed  on  crystallization.  It  includes  syenite, 
trachyte,  felsites,  and  glasses. 

Syenite.  —  This  rock  typically  consists  of  orthoclase  and  horn- 
blende, without  quartz.  When  biotite  is  present,  the  rock  is  called 
mica  syenite.  In  practically  all  syenites  some  of  the  orthoclase 
is  replaced  by  plagioclase,  and  this  may  occur  to  such  a  degree  that 
it  makes  up  half  of  the  feldspar,  when  the  rock  approaches  a  diorite, 
and  is  called  a  monzonite.  Augite  too  may  replace  the  hornblende, 
occurring  with  orthoclase  and  forming  an  augite  syenite.  Finally, 


Types  of  Igneous  Rocks  •  >     101 

quartz  syenite,  with  a  small  amount  of  free  quartz,  shows  a  transition 
from  granites.  With  the  appearance  of  nephelite,  they  pass  into 
nephelite  syenites.  Porphyritic  syenites  have  large  crystals  of  feld- 
spar. Common  accessory  minerals  are  magnetite,  zircon,  and 
apatite.  The  name  syenite  is  derived  from  the  ancient  Syene  (now 
Assuan)  in  Egypt,  where  the  rock  is,  however,  a  hornblende  granite 
which  was  formerly  used  for  obelisks.  As  now  used  the  name  was 
first  applied  to  a  granite  rock  almost  without  quartz  near  Dresden 
(Plauen'sche  Grund,  or  Plauen  Gorge). 

Trachytes  and  Felsites.  —  These  have  essentially  the  same  com- 
position as  the  syenites,  of  which  they  form  the  crypto-crystalline 
representatives  with  the  same  relationship  that  rhyolite  holds  to 
granite.  Trachytes  (Fig.  46)  differ  from  rhyolites  in  the  absence 
or  great  rarity  of  quartz.  Biotite  is  in  general  the  most  abundant 
dark  mineral,  but  hornblende  and  augite  also  occur,  forming  varie- 
ties. The  texture  varies  from  felsitic  in  the  true  felsites  to  strongly 
and  coarsely  porphyritic,  and  not  infrequently  the  rock  is  somewhat 
cellular.  Felsites,  on  account  of  their  dense  structure,  cannot 
readily  be  distinguished  from  rocks  of  the  same  type  in  the  granite 
or  even  in  the  diorite  group.  In  typical  trachytes,  which  are  gen- 
erally porphyritic,  the  ground-mass  is  made  up  of  fine  rods  of  ortho- 
clase  arranged  more  or  less  parallel  and  in  flowing  lines.  This  is 
the  characteristic  trachyte  texture,  which  can  often  be  seen  with 
the  naked  eye  in  the  coarser  crystalline  varieties.  Large  pheno- 
crysts  of  a  clear  vitreous  variety  of  orthoclase  (called  sanidine)  are 
characteristic  of  the  porphyritic  trachytes. 

When  the  phenocrysts  are  so  abundant  as  to  constitute  about 
half  the  mass  of  the  rock  with  felsitic  ground-mass,  it  is  called  a 
trachyte  porphyry.  When  phenocrysts  form  the  bulk  of  the  rock, 
while  the  ground-mass  becomes  more  coarse-textured,  the  rock  is 
called  a  syenite  porphyry  and  marks  the  transition  to  syenite. 

The  name  trachyte  is  derived  from  its  rough  or  harsh  surface 
feel  (Greek,  rpa^vs,  rough).  The  most  typical  trachytes  come  from 
the  peak  of  the  Drachenfels  on  the  Rhine.  (See  map,  Fig.  98.) 

The  Glasses  of  the  Syenite  Series.  —  Because  the  fusing  point 
of  these  more  basic  rocks  is  lower  than  that  of  the  granite  series, 
being  about  2000°  F.  (1100°  C.)  for  trachytes  as  compared  with 
about  2200°  F.  (1200°  C.)  for  the  rhyolites,  and  2250°  F.  (1240°  C.) 
for  granites,  they  remain  liquid  longer,  and  hence  crystallization 
occurs  more  generally  with  less  frequent  formation  of  glasses. 


102     Principal  Types'  of  Igneous  or  Pyrogenic  Rocks 


When  glasses  are  formed,  they  are  indistinguishable,  except  by 
analysis,  from  those  of  the  granite  series. 

The  N ephelite-Syenite  Phonolite  Series 

These  differ  from  syenites  chemically  in  the  greater  amount  of 
soda,  and  mineralogically  in  the  partial  substitution  of  nephelite 
(eleolite)  for  the  feldspar. 

Nephelite-Syenite.  —  This  corresponds  to  syenite  except  for 
the  presence  of  nephelite  or  sodalite,  of  both  or  of  leucite.  In  some 
cases  (Litchfield,  Maine)  the  feldspar  is  wholly  plagioclase.  Zir- 
con is  a  usual  secondary  mineral. 

Phonolites.  —  (Klingstein,  so  called  because  of  its  ringing  sound.) 
These  rocks  are  generally  dense  and  finely  crystalline,  seldom 

vesicular  or  glassy.  They  are 
mostly  dull  green  or  gray  in 
color,  and  when  light  colored 
they  are  not  readily  distin- 
guished from  trachytes  except 
by  the  microscope,  which 
shows  the  presence  of  the 
nephelite.  The  chief  feldspar 
is  orthoclase,  while  the  com- 
mon dark  mineral  is  augite, 
hornblende  being  rare.  A 
peculiar  character  is  the  fact 

that  .the  rock  breaks  into  thin 
FIG.  46.-Trachyte-phonolite  show-      glabs   which    have   a   musical 
ing    typical    tracmtic    texture    of    the        .  ,     ,  , 

ground-mass  and  a  large  phenocryst.  nng  under  the  hammer.  The 
The  Rhon,  Germany.  Enlarged  24  rock  is  frequently  porphyritic, 
diameters,  seen  under  crossed  nicols.  fofa  augite  and  feldspar 
(After  Rosenbusch.) 

phenocrysts   appearing    in    a 

dense  ground-mass.  When  the  phenocrysts  are  very  abundant  the 
rock  is  called  phonolite-porphyry,  and  when  in  excess,  it  becomes  a 
nephelite-syenite  porphyry.  With  increase  in  orthoclase  and  decrease 
in  nephelite,  the  phonolites  pass  into  trachytes.  (See  Fig.  46.) 

Glasses  of  the  Nephelite-Syenite  Series.  —  These  are  still  rarer 
than  those  of  the  preceding  case,  since  the  fusing  point  of  phonolites 
(somewhat  less  than  2000°  F.  or  1090°  C.)  is  still  lower  than  that 
of  trachytes.  Phonolite  obsidians  are  known  from  the  Peak  of 
Teneriffe. 


Types  of  Igneous  Rocks  103 

The  Quartz- Diorite  Dacite  Series 

The  members  of  this  series  differ  from  those  of  the  granite  series, 
chiefly  in  the  substitution  of  plagioclase  for  orthoclase.  The  plagio- 
clase  can  be  recognized  by  the  striated  character  of  the  cleavage 
surface. 

Quartz-Diorite.  —  This  resembles  granite,  but  is  darker  and 
heavier.  Acid  plagioclase,  quartz,  hornblende,  and  (or)  biotite 
are  the  essential  minerals.  When  biotite  predominates  the  rock 
is  called  a  quartz-mica-diorite.  These  rocks  form  a  transition  from 
granites  to  diorites,  the  intermediate  forms  being  called  grano- 
diorites. 

A  typical  locality  for  quartz-mica-diorite  is  found  in  the  Cortland 
Series  near  Peekskill,  N."  Y.,  one  for  the  hornblendic  quartz-diorite 
in  the  Yellowstone  Park. 

Dacite.  —  This  rock  is  difficult  to  distinguish  from  rhyolite  except 
by  the  use  of  the  microscope.  It  has  the  same  relation  to  quartz- 
diorite  that  rhyolite  has  to  granite.  When  the  texture  is  finely 
felsitic  and  non-porphyritic,  it  can  only  be  termed  felsite.  When 
porphyritic,  the  dacites  are  recognized  by  the  striated  surfaces  of 
the  plagioclase  phenocrysts,  which  predominate.  Glasses  and  cellu- 
lar texture  are  not  uncommon. 

When  phenocrysts  constitute  up  to  half  the  mass  of  the  rock,  it 
becomes  a  dacite-porphyry.  When  they  are  in  marked  excess  over 
the  ground-mass,  it  becomes  a  quartz-diorite-porphyry.  The  name 
is  derived  from  the  old  province  of  Dacia,  now  Transylvania  (Sieben- 
biirgen),  before  the  war  a  part  of  Hungary,  which  is  a  typical  local- 
ity. Dacites  generally  occur  with  andesites. 

Glasses  of  this  Series.  —  The  glasses  of  this  series  are  more  com- 
mon than  those  of  the  syenite  series,  for  the  fusing  point  is  only  a 
little  less  than  that  of  the  granite  series.  They  are  generally  in- 
cluded with  the  glasses  of  the  next  series. 

.» 
The  Diorite- Andesite  Series 

This  differs  from  the  preceding  in  the  absence  of  free  quartz  (in 
the  crystalline  members)  and  from  the  syenite  series  in  the  substi- 
tution of  acid  plagioclase  for  orthoclase. 

Diorites.  —  These  are  granitoid  rocks,  the  chief  feldspar  of  which 
is  acid  plagioclase,  and  they  are  rich  in  hornblende.  When  biotite 
largely  replaces  the  hornblende,  the  rock  is  called  mica-diorite. 


104    Principal  Types  of  Igneous  or  Pyrogenic  Rocks 

Again,  augite  may  replace  part  of  the  other  dark  minerals,  forming 
an  augite  diorite,  which  is  a  passage-rock  to  gabbro.  Typically, 
the  feldspar  is  in  excess  of  the  dark  minerals,  but  in  other  cases 
these  may  lead  in  the  mineral  constituents.  A  rock  of  diorite  compo- 
sition may  also  arise  by  metamorphism  of  gabbros  with  the  change 
of  the  augite  to  hornblende/  Porphyritic  diorites,  occurring  in 
dikes,  have  been  called  camptonites.  Characteristic  secondary 
minerals  are :  magnetite,  titanite,  and  apatite.  The  name  diorite 
was  given  to  this  rock  because  of  the  striking  contrast  between 
the  light  and  dark  minerals  of  which  it  is  composed. 

Andesites.  —  These  are  the  fine-textured  to  felsitic  members  of 
the  diorite  series.  The  acid  plagioclase  feldspars  are  the  most 
abundant  minerals,  but  quartz  is  rare  or  absent.  Biotite,  horn- 
blende, and  augite  are  the  dark  minerals,  the  last  two  predomi- 
nating over  the  biotite.  The  general  colors  of  the  rocks  are  grays  or 
greens.  Typical  andesite  is  felsitic,  sometimes  cellular,  and  com- 
monly porphyritic.  The  felsitic  ground-mass  consists  of  micro- 
scopic rods  of  feldspar,  forming  a  felt-like  aggregate.  From  trachyte 
and  dacite  it  can  generally  be  distinguished  only  by  its  darker  color, 
owing  to  the  greater  abundance  of  the  ferromagnesian  silicates. 
According  to  the  predominant  dark  mineral  we  have  mica  andesite, 
hornblende  andesite,  augite  andesite,  etc. 

In  the  porphyritic  varieties,  the  phenocrysts  are  mostly  feldspar ; 
when  very  abundant  (up  to  half  the  mass)  they  form  an  andesite- 
porphyry ;  when  in  excess,  a  diorite- porphyry.  These  show  an 
increasingly  coarser  ground-mass,  and  grade  into  diorites  proper. 

With  increase  in  orthoclase,  andesites  pass  into  trachytes,  and 
with  increase  in  the  dark  and  more  basic  ferromagnesian  silicates 
and  a  decrease  of  feldspars,  they  pass  into  basalts ;  with  addition 
of  quartz  they  pass  into  dacites.  The  type  localities  for  these 
rocks  are  in  the  Andes  Mountains,  and  they  are  abundant  and 
widespread  in  the  Pacific  Coast  region  of  North  America,  especially 
in  the  old  volcanic  cones  of  Mt.  Hood,  Mt.  Shasta,  Mt.  Rainier, 
and  others. 

The  Glasses  of  the  Diorite-Andesite  Series.  —  Andesites  fuse 
at  temperatures  around  2000°  F.  (1100°  C.),  which  is  about  the 
same  as  for  trachytes.  Like  these,  therefore,  they  do  not  readily 
form  glasses,  and  most  of  those  referred  to  andesites  are  probably 
referable  to  the  dacites.  The  glasses  of  this  type  which  are  recog- 
nized are :  andesite-obsidian,  andesite-perlite,  and  andesite-pumice. 


Types  of  Igneous  Rocks  105 

They  are  distinguished  from  the  more  acidic  glasses  only  by  chem- 
ical analysis,  or  by  their  field  relations  to  recognizable  types  of 
andesites  or  dacites.  Andesite  obsidian  has  been  obtained  from 
Clear  Lake,  Cal.,  andesite  perlite  from  Eureka,  Nev.. 


The  Gabbro-(Pyroxenite,  Peridotite)-Basalt  Series 

This  includes  all  the  basic  igneous  rocks,  the  coarser-grained 
members  of  which  range  through  gabbro,  olivine-gabbro,  pyroxenite 
andperidotites,  while  the  fine-grained  ones  form  diabases  and  basalt. 
The  surface  flows  are  represented  by  scoriaceous,  ropy,  or  other 
lavas,  and  more  rarely  by  glasses  (tachylite,  Peele's  hair,  etc.). 

Gabbro  and  Olivine  Gabbro.  —  These  are  generally  coarsely 
crystalline  dark  rocks,  composed  chiefly  of  basic  plagioclase  and 
monoclinic  pyroxenes,  the  dark  silicates  typically  predominating. 
There  are,  however,  varieties  which  are  almost  wholly  composed 
of  coarse  crystalline  labradorite  (Canada,  Adirondacks)  with  little 
or  no  pyroxene.  These  are  also  called  anortho sites ;  where  the  py- 
roxene is  of  the  orthorhombic  varieties  (enstatite,  bronzite,  hyper- 
sthene),  the  rock  is  called  norite.  Gabbro  may  contain  some  horn- 
blende and  biotite.  When  olivine  is  present  the  rock  becomes 
olivine-gabbro,  olivine-norite,  etc.  In  rare  cases  nephelite  becomes 
an  important  mineral  in  some  basic  gabbros,  forming  the  rock 
theralite,  which  occurs  in  the  Crazy  Mountains,  Montana.  Gabbros 
have  a  wide  distribution. 

Pyroxenites  and  Peridotites.  —  By  the  decrease  of  the  plagioclase, 
the  gabbros  pass  insensibly  into  pyroxenites  and  peridotites,  which 
are  usually  found  associated  with  the  gabbros,  but  also  occur  in- 
dependently. Pyroxenites  generally  contain  little  else  than  pyrox- 
ene, which  maybe  orthorhombic  (enstatite,  bronzite,  hypersthene)  or 
monoclinic  (diallage,  augite),  but  these  are  not  readily  distinguish- 
able by  the  unaided  eye,  though  the  orthorhombic  pyroxenes  show  a 
bronze  luster.  Frequent  accessory  minerals  are  hornblende,  magne- 
tite, and  pyrrhotite.  When  olivine  is  added  the  rock  becomes  a 
pcridotite,  and  when  olivine  predominates  it  bec6mes  a  dunite,  of 
which  the  nearly  pure  olivine  rock  of  North  Carolina  is  an  example. 
Sometimes  magnetite  is  so  abundant  as  to  make  the  rock  almost  an 
iron  ore.  Porphyritic  peridotites  have  been  called  picrites,  especially 
when  occurring  in  dikes.  Sometimes  hornblende  is  abundant,  and 
it  may  even  form  a  rock  by  itself,  which  is  then  called  amphibolite. 


io6    Principal  Types  of  Igneous  or  Pyrogenic  Rocks 

Pyroxenites  and  peridotites  change  with  age  and  by  metamor- 
phism  into  serpentines. 

Diabase.  —  This  is  a  transitional  rock  between  the  gabbros  and 
the  basalts,  being  fine-grained  but  holocrystalline  and  differing 
in  detail  of  texture  from  the  gabbro.  It  consists  of  basic  plagio- 
clase,  augite,  and  often  olivine,  the  first  occurring  as  elongated  rec- 
tangular rods  arranged  in  an  interlacing  manner,  while  the  other 


FIG.  47.  —  a,  Basalt,  showing  typical  diabasic  structure  of  lath-shaped  crys- 
tals of  plagioclase,  Burney  Falls,  Shasta  Co.,  Cal.  b,  Bronzite  diabase, 
York,  Pa.,  showing  lath-shaped  plagioclase  crystals.  Both  sections  are  seen 
under  crossed  nicols  enlarged  24  times  (after  Rosenbusch). 

minerals  are  packed  in  between  them  in  irregular  masses.  This  is 
brought  about  by  the  fact  that  the  plagioclase  crystallized  out  first, 
contrary  to  the  usual  order  of  crystallization,  and  that  the  ferro- 
magnesian  minerals  had  to  adapt  themselves  to  the  remaining 
spaces.  This  diabasic  texture  (Fig.  47)  is  sometimes  modified  into 
an  ophitic  one,  where  the  rods  of  plagioclase  are  included  in  large, 
coarsely  crystalline  masses  of  pyroxene.  Diabase  is  common  as 
an  intrusive  rock,  both  as  dikes  and  sills. 

Basalt.  —  These  are  the  basic  fine-grained  or  dense  igneous  rocks 
which  form  the  surface  flows  of  the  gabbros,  pyroxenites,  and  peri- 
dotites. Typically  the  texture  is  dense  or  felsitic,  often  cellular, 
and  the  rock  may  be  characterized  by  almond-shaped  steam  holes 
which  are  subsequently  filled  with  secondary  calcite,  giving  the 
rock  an  amygdaloidal  structure  (Fig.  48).  Sometimes  these  cav- 
ities are  filled  with  copper,  as  in  the  Lake  Superior  region.  Por- 


Types  of  Igneous  Rocks 


107 


FIG.  48.  —  Amygdaloid. 


phyritic  basalts  show  phenocrysts  of  augite  and  more  commonly 
olivine.  Such  porphyritic  olivine  basalts  are  also  called  melaphyres. 
The  felsitic  ground  mass  is  composed 
of  microscopic  augite,  plagioclase  and 
magnetite  crystals.  Sometimes  a  little 
glass  is  present.  With  abundant  de- 
velopment of  phenocrysts,  of  augite 
and  olivine,  the  rock  becomes  a  basalt 
porphyry  and  with  excessive  develop- 
ment of  phenocrysts  a  gabbro  porphyry 
is  produced. 

In  rare  basalts  nephelite  or  leucite 
may  replace  the  feldspar,  giving  a 
variety  of  types,  distinguishable  only 
under  the  microscope. 

Augitites  and  Limburgites.  —  These 

are  rare  basaltic  rocks,  with  little  or  no  feldspar,  which  correspond 
to  the  pyroxenites  and  peridotites,  respectively. 

The  Basic  Glasses.  —  These  represent  surface  formations  of  the 
basaltic  flows,  but  are  on  the  whole  not  common.  Some  of  the 
porous  or  vesicular  scoria  is  glassy,  but  for  the  most  part  it  has  a 
dense  stony  or  felsitic  texture.  Typical  basic  glass,  corresponding 
to  obsidian,  is  known  by  the  name  of  tachylite,  while  a  fibrous  vari- 
ety found  in  the  crater  of  Kilauea  i£  called  Pele's  hair. 

Special  Field  Names 

Several  names  are  commonly  applied  to  the  basic  igneous  rocks 
when  the  exact  character  cannot  be  determined  in  the  field. 
One  of  these  is  dolerite,  used  for  rocks  which  may  be  diorites  or 
gabbros,  but  in  which  the  dark  mineral  is  undeterminable.  An- 
other is  trap,  in  common  use  for  dark,  dense  basalts  or  diabases. 
It  is  derived  from  the  Swedish  trappar,  a  stairway,  because  the 
sheets  of  this  rock  sometimes  form  successive  steps  in  the  land- 
scape. Old  trap  or  basaltic  rocks  which  have,  by  alteration,  de- 
veloped sufficient  of  the  mineral  chlorite  to  give  them  a  greenish 
cast,  are  called  greenstones. 


io8 


Modern  Volcanic  Phenomena 


CHAPTER  VII 
MODERN  VOLCANIC   PHENOMENA 

EXCEPT  in  the  case  of  comparatively  young  volcanic  eruptions, 
the  structures  and  relationships  of  igneous  masses  can  be  studied 
only  in  regions  where  these  masses  have  been  uncovered  by  erosion 
of  the  rocks  which  formerly  covered  them,  and  their  origin  can  be 
determined  only  from  the  available  facts.  We  shaljl  first  consider 
those  masses  the  formation  of  which  are  open  to  observation  or 
which  have  been  formed  so  recently  that  there  can  be  no  doubt  as 
to  their  origin. 

DISTRIBUTION,  CLASSIFICATION,  AND  DEVELOPMENT    OF 
VOLCANOES 

Volcanoes  are  found  in  many  parts  of  the  world  at  the  present 
time,  but  their  most  extensive  distribution  is  around  the  borders 
of  the  Pacific,  where  they  form  a  "  chain  of  fire  "  which  surrounds 
that  ocean.  The  map  on  the  opposite  page  shows  this  and  their 
occurrence  elsewhere.  Along  the  western  border  of  the  Pacific 
more  than  150  active  volcanoes  are  distributed  through  a  length 
of  16,000  kilometers,  while  about  100  border  the  eastern  margin  of 
that  ocean  from  the  Aleutian  Islands  to  Tierra  del  Fuego.  Typical 
and  perfect  examples  of  the  former  are  Fujiyama  in  Japan  (Fig.  50) 
and  Mayon  in  the  Philippines  (Frontispiece),  while  Bogosloff  and 
Popocatepetl  may  serve  as  examples  of  the  latter.  Within  the 
Pacific  and  arising  from  its  floor  are  other  active  volcanoes,  such 
as  those  of  the  Hawaiian  Islands,  near  its  very  center,  and  those  of 
the  southwest  coral  reef  regions. 

On  the  Atlantic  borders,  active  volcanoes  are  not  common,  being 
chiefly  confined  to  the  Antillean  region  in  the  west,  the  Icelandic  in 
the  north,  and  the  Azores,  Canaries,  and  Cape  Verde  Islands  in 
the  east.  These  are  in  reality  local  or  isolated  groups  of  volcanoes, 
except  those  of  Iceland,  which  are  the  last  manifestations  of  vol- 
canic activity  in  a  vast  volcanic  field  which  extends  from  Greenland 
on  the  west  to  Siberia  on  the  east,  and  which  began  far  back  in 

109 


no 


Modern  Volcanic  Phenomena 


FIG.  50.  —  Fujiyama,  a  perfect  volcanic  cone  in  Japan. 


FlG  5I  — Map  Of  the  Naples  volcanic  region,  with  the  three  principal 
volcanic  centers,  —  Vesuvius,  the  Phlegraean  volcanic  field  west'  of  Naples 
(Pampi  Flegrei),  and  the  Island  of  Ischia;  together  with  the  submarine  vol- 
canoes. (After  J.  Walther,  from  Ratzel.) 


Distribution,  Classification,  and  Development     in 

the  geological  history  of  the  region.  In  the  Mediterranean  are 
three  important  fields  of  volcanic  activity  known  since  ancient 
times :  that  around  Naples,  that  of  Sicily  and  its  islands,  and  that 
of  the  Grecian  archipelago.  The  Naples  volcanic  field  (Fig.  51) 
comprises  Vesuvius  with  Monte  Somma  on  the  east  of  Naples, 
the  Phlegraean  volcanic  field  with  a  number  of  vents  including 


FIG.  52. — View  of  Stromboli  from  the  northwest. 
Kayser's  Lehrbuch.) 


(After  A.  Bergeat,  from 


Monte  Nuovo,  the  Solfatara,  the  Pozzuoli  district,  etc.,  on  the  west, 
and  the  volcanic  island  of  Ischia  on  the  southwest,  besides  many 
smaller  islands,  including  Capri  on  the  south.  The  Sicilian  region 
includes  Mount  Etna  in  the  northeastern  part  of  that  island,  and 
the  volcanoes  of  the  Lipari  or  ^Eolian  group  of  islands  north  of 
Sicily,  among  which  are  Vulcanoand  Stromboli  (Fig.  52),  the  latter 
called  the  "  Lighthouse  of  the  Mediterranean,"  because  of  the 


KIRVN6A-CMA-MOTO 


FIG.  53.  —  Volcanoes  of  the  great  Rift  Valley  of  East  Africa.     (After  Moore.) 

flashes  of  its  explosions,  given  off  at  intervals  of  from  i  to  20  min- 
utes. In  the  Grecian  archipelago  the  island  of  Santorin  has  been 
a  scene  of  volcanic  activity  for  more  than  2000  years.  Volcanoes 
are  also  active  around  the  Indian  Ocean,  especially  on  the  islands 
which  form  its  eastern  margin.  Active  volcanoes  exist  in  the  great 
"  Rift  Valley  "  of  East  Africa,  in  which  lie  many  of  the  large  lakes 
of  that  region  (Fig.  53),  and  recently  extinct  volcanoes  occur  along 


ii2  Modern  Volcanic  Phenomena 

the  northern  margin  of  the  Indian  Ocean  in  Arabia,  while  more 
ancient  volcanic  activities  are  recorded  in  India. 

Active,  Dormant,  and  Extinct  Volcanoes 

Volcanoes  may  be  classed  as  active  when  eruptions  are  occurring 
at  intervals ;  as  dormant,  when  they  have  been  quiescent  for  cen- 
turies, though  the  possibility  of  a  new  eruption  exists ;  and  as  ex- 
tinct, when  volcanic  activity  has  entirely  ceased  and  the  volcano  is 
undergoing  destruction  by  atmospheric  agencies.  There  is  of 
course  every  gradation  between  these ;  some  that  would  be  classed 
as  dormant  may  never  be  revived,  while  others,  after  a  long  sleep, 
burst  forth  again,  as  in  the  case  of  Vesuvius  at  the  beginning  of 
the  Christian  Era.  It  should  be  clearly  understood  that  volcanism 
is  not  a  phenomenon  restricted  to  the  present,  but  that  volcanic 
activities,  often  on  a  much  grander  scale  than  those  of  to-day,  have 
been  going  on  during  all  the  periods  of  the  earth's  history.  Many 
of  the  older  volcanoes  and  their  products,  after  suffering  more  or 
less  destruction  by  erosion,  were  buried  under  more  recent  deposits, 
and  have  been  reexposed  only  in  part  as  the  result  of  the  latest 
phases  of  erosion. 

Formation  of  New  Volcanoes  in  the  Historic  Period 

Most  of  the  modern  volcanoes  came  into  existence  before  the 
time  of  recorded  history,  but  a  few  have  been  formed  during  the 
historic  period,  and  their  growth  and  development  has  been  wit- 
nessed by  man.  The  most  noted  of  these  are :  Monte  Nuovo,  in  the 
Bay  of  Baiae  near  Naples  (1538) ;  Jorullo,  Mexico  (1759) ;  Pochutla, 
Mexico  (1870) ;  Camiguin,  Philippine  Islands  (1871) ;  a  new  cone 
of  the  Ajusco  group,  Mexico  (1881) ;  the  New  Mountain  of  Japan 
(1910)  ;  and  the  submarine  cones  of  Sabrina  and  Graham  Islands 
(1811  and  1831).  A  few  of  these  may  be  considered  in  some  detail. 

Monte  Nuovo.  —  This  volcano  (Fig.  54)  arose  in  the  Phlegraean 
fields  west  of  Naples  on  Sunday,  September  29,  1538,  beginning 
about  one  o'clock  in  the  morning.  It  appeared  mainly  on  the  site 
of  the  ancient  Lake  Lucrinus,  which  itself  was  regarded  as  the 
crater  of  an  older  but  extinct  preexisting  volcano,  filled  with  water. 
Incandescent  gases  burst  open  the  earth,  and  within  a  week  a  cone 
made  up  of  ejected  ashes,  cinders,  and  large  stones,  but  no  lava, 
was  built  up  to  a  height  of  440  feet  above  the  level  of  the  sea.  The 


Distribution,  Classification,  and  Development    113 

depth  of  the  crater  is  421  feet,  reaching  within  19  feet  of  the  sea- 
level,  and  its  basal  circumference  about  8000  feet.  The  eruption 
was  preceded  on  the  day  and  night  before  by  about  twenty  earth- 
quake shocks  in  Pozzuoli  and  the  neighborhood,  and  after  the  erup- 
tion it  was  found  that  the  sea  had  receded  for  some  distance,  owing 


FIG.  54. — Monte  Nuovo,  formed  in  the  Bay  of  Baiae,  Sept.  29,  1538. 
(After  Lyell.)  i.  Cone  of  Monte  Nuovo.  2.  Rim  of  crater  of  same. 
3.  Thermal  spring  called  Bath  of  Nero,  or  Stufe  di  Tritoli. 

to  the  elevation  of  the  region,  and  the  strand  was  covered  with 
quantities  of  dead  fish,  as  well  as  dead  birds. 

The  cone  of  Monte  Nuovo  is  very  regular,  and  is  composed  of 
layers  of  pumaceous  fragments  and  ashes,  and  of  trachytic  blocks, 
the  whole  partly  consolidated  and  dipping  away  from  the  crater 
at  angles  of  26  to  30  degrees.  Fragments  of  marine  shells  and 
Roman  bricks  were  also  included  in  the  beds  as  a  result  of  the 
disturbance  of  the  region  by  the  explosions. 

Jorullo.  —  This  cone  arose  on  the  night  of  September  28,  1759, 
at  a  point  35  miles  distant  from  any  then  existing  volcano,  and 
1 20  miles  from  the  sea.  It  appeared  on  a  level  plain  2000  to  3000 
feet  above  the  sea  in  the  state  of  Michoacan,  Mexico.  A  north- 
easterly fissure  opened  in  the  midst  of  the  sugar  and  indigo  fields 
of  this  plain,  and  ashes  and  rocks  were  thrown  to  a  great  height, 
and  on  falling  these  built  up  six  conical  hills  on  the  line  of  the  chasm, 
the  smallest  300  feet  high,  while  Jorullo  itself  was  built  up  to  1600 
feet  above  the  plain,  or  4265  feet  above  sea-level.  Great  streams 


Modern  Volcanic  Phenomena 


of  basaltic  lava  were  poured  forth  from  Jorullo,  these  including 
fragments  of  granitic  rock.  The  ejection  did  not  cease  until  Febru- 
ary, 1760.  Twenty  years  later  the  lava  was  still  hot  enough,  a 
few  inches  below  the  surface,  to  light  a  cigar.  This  eruption,  too, 
was  preceded  by  earthquakes  and  subterranean  rumblings,  which 
began  in  the  June  preceding.  There  has  been  no  eruption  in  this 
region  since. 

Camiguin.  —  This  volcano  also  started  from  a  fissure  in  a  level 
plain  on  one  of  the  small  islands  north  of  Luzon  in  the  Philippines. 
Beginning  in  1871,  it  continued  to  be  active  for  four  years,  by  which 

time  it  had  reached 
a  height  of  about 
1800  feet. 

Sabrina  Island.  — 
This  submarine  vol- 
canic cone  appeared 
above  the  waters  of 
the  Atlantic  in  the 
Azores  group,  off  the 
coast  of  St.  Michaels, 
on  June  13,  1811, 
and  rose  to  a  height 
of  about  300  feet 
above  the  sea,  gain- 
FIG.  55.—  The  volcanic  eruption  which  formed  j  a  circumference 
Sabrina  Island  in  the  Azores,  June  13,  1811.  (After 

DeLaBeche.)  of     about     a     mile" 

Being,  however, 

largely  made  of  unconsolidated  material,  it  has  since  been  washed 
away  again.  As  observed  from  the  nearest  cliff  on  St.  Michaels, 
the  explosions  resembled  a  mixed  discharge  of  cannon  and  mus- 
ketry and  were  accompanied  by  a  great  abundance  of  lightning. 
The  appearance  of  the  eruption  above  water  is  shown  in  the 
accompanying  figure  from  a  sketch  made  at  that  time  (Fig.  55). 

Graham  Island  (Isle  Julia).  —  This  island,  which  existed  for  only 
about  three  months,  rose  as  a  submarine  cinder  cone  in  1831  in  the 
Mediterranean  between  the  southwest  coast  of  Sicily  and  that  pro- 
jecting part  of  the  African  coast  where  ancient  Carthage  stood.  A 
few  years  before  the  appearance  of  the  island,  soundings  at  this 
locality  showed  a  depth  of  water  of  100  fathoms.  Premonitory 
shocks  were  felt  on  June  28  over  the  spot  and  on  the  adjoining 


Distribution,  Classification,  and  Development    115 


coast  of  Sicily.    About  July  10,  a  column  of  water  60  feet  high  and 

800  yards  in  circumference  was  seen  rising  from  the  sea,  followed 

soon  after  by  dense  clouds  of  steam  which  rose  1800-  feet.     Eight 

days  later  the  same 

observer    noted    at 

this    spot    a    small 

island   12  feet  high 

and  with  a  crater  at 

its       center,      from 

which  volcanic  mat- 


FIG.  56.  —  Supposed  section  of  Graham  Island. 
(After  C.  McLaren,  Geology  of  Fife  and  the 
Lothians,  pi.  41,  Edin.,  1839;  from  LyelFs  Prin- 
ciples.) 


ter  and  immense 
clouds  of  vapor  were 
ejected,  while  the  sea  round  about  was  covered  with  floating 
cinders  and  dead  fish.  By  the  end  of  July  the  island  had  become 
from  50  to  90  feet  in  height  and  three  fourths  of  a  mile  in  circum- 
ference, while  on  August  4  it  was  reported  above  200  feet  high  and 
three  miles  in  circumference.  After  this  it  began  to  diminish  in 
size,  owing  to  the  erosion  by  the  sea,  decreasing  to  two  miles 
in  circumference  by  August  25,  and  to  three  fifths  of  a  mile  and  a 
maximum  height  of  107  feet  by  September  3,  when  the  crater  was 
about  780  feet  in  circumference.  On  September  29  the  island  was 
reduced  to  a  circumference  of  only  about  700  yards,  and  toward 

the  close  of  Oc- 
tober it  had  dis- 
appeared except 
for  a  small  point 
of  sand  and 
scoriae.  By  the 
commencement  of 
1832  .  there  was 
only  a  shoal,  with 
a  mass  of  igneous 
rock  which  ap- 
parently filled  the 
center  of  the 
vent.  The  ap- 
pearance and  sup- 


FIG.  57.  —  Graham  Island,  as  it  appeared  on  Sept. 
29,  1831.  The  apparent  bedding  planes  sloping  towards 
the  center  of  the  volcano  are  not  such  in  reality. 
(From  Lyell's  Principles.) 


posed  structure  of  this  island  and  of  the  cone,  which  rose  thus 
about  800  feet  above  the  sea  floor,  are  shown  in  the  above 
figures  reproduced  from  Lyell  (Figs.  56,  57).  The  lava  core  or 


n6 


Modern  Volcanic  Phenomena 


neck,  which  probably  never  appeared  above  sea-level,  now  forms 
the  highest  part  of  the  submerged  remnant  of  the  volcano.  The 
fragmental  material  included,  besides  volcanic  ash  and  cinders, 
blocks  of  limestone,  dolomite,  and  sandstone. 

Several  other  submarine  eruptions  of  this  type  have  been  re- 
corded from  various  sections  of  the  Atlantic  and  from  the  Medi- 
terranean. 

CHARACTERISTIC  FORMS  AND  ACTIVITIES  OF  TYPICAL  MODERN 

VOLCANOES 

Of  the  many  modern  volcanoes,  a  number  which  have  been  ob- 
served for  long  periods  of  time  may  be  described  somewhat  in  de- 
tail so  that  the  student  will  get  a  grasp  of  the  essentials  of  the  struc- 
ture and  activities  of  volca- 
noes. We  shall  begin  with  a 
type- in  which  liquid  lava  is 
the  chief  product  of  erup- 
tion, then  consider  types  in 
which  both  lava  and  frag- 
mental material  are  pro- 
duced, and  enter  into  the 
building  of  the  cone,  and 
finally  consider  some  ex- 
amples which  are  purely 
explosive,  with  the  produc- 
tion of  only  fragmental  ma- 
terial. The  nature  and  com- 
position of  the  fragmental 
material  will  be  considered 
more  at  length  in  a  subse- 
quent chapter. 

Before  proceeding,  the 
student  should  clearly  un- 
derstand that  the  hill  or 
mountain  which  constitutes 
the  volcano  is  built  up 
around  an  opening  or  vent 

from    the    material    ejected 
FIG.  c8. — Volcanic  bomb.    Vesuvius,      r          ,,  .  j    ,-•     ,    ., 

eruption  of  ,872.  (After  Ratzel,  Die  from  thls  vent-  and  that  ll 
Erde.)  does  not  represent  an  upris- 


Forms  and  Activities  of  Typical  Modern  Volcanoes     117 

ing  or  an  upheaval  of  a  part  of  the  earth's  surface,  as  was  at  one 
time  supposed  to  be  the  case.  Three  kinds  of  material  are  ejected 
from  volcanic  vents;  of  these,  two  or  all  may  be  present.  The 
first  type  comprises  gases  and  water  vapors  mingled  with  fumes  of 
other  substances;  this  is  always  present.  The  second  is  liquid 
magma  or  lava,  and  the  third  consists  of  the  shattered  lava  and 
shattered  rocks  resulting  from  the  explosive  activities,  comprising 
masses  of  all  sizes,  from  large  lava  balls  or  bombs  (Fig.  58),  more 
or  less  spherical  or  elliptical  from  their  rotary  motion  through  the 
air,  to  fine  lava  particles  or  lapUli,  and  fragments  or  blocks  of 
older  igneous  or  other  rocks,  and  dust  produced  by  the  shattering 
of  these.  In  general  we  speak  of  such  material  as  volcanic  ashes 
and  cinders.  The  eruptive  activities  range  from  the  quiet  upwell- 
ing  or  bubbling-up  of  liquid  lava  to  the  most  violent  explosion  due 
to  the  sudden  expansion  of  the  gases  and  vapors. 


Kilauea  (and  Mauna  Loo)  of  the  Hawaiian  Islands 

The  entire  group  of  the  Hawaiian  Islands  (Fig.  59)  in  the  mid- 
Pacific  is  a  series  of  volcanic  cones  built  up  from  the  sea-bottom  in 
former  times.  Most  of  these  volcanoes  are  now  extinct,  and  many 


FIG.    59.  —  Map  of  the  Hawaiian  Islands,  showing  the  principal  craters. 
(After  Dana.) 

of  them  are  undergoing  erosion ;  but  a  large,  active  crater,  that  of 
Kilauea  (4000  feet  above  sea-level) ,  exists  on  the  east  side  of  the 
volcanic  mountain  of  Mauna  Loa  (13,675  ft.  high)  and  about  twenty 
miles  from  its  summit,  which  is  also  marked  by  an  active  crater. 
The  crater  pit  of  Kilauea  (Fig.  60)  is  rudely  oval  in  form,  with  a 
circumference  of  about  nine  miles,  and  its  floor  is  formed  by  a  rough 
stony  crust  of  solidified  lava  —  resting  upon  a  vast  column  of  molten 
rock  which  arises  from  an  unknown  depth  in  the  crust  of  the  earth. 
In  places,  this  floor  is  broken  by  lakes  of  liquid  lava,  red  to  white 


n8 


Modern  Volcanic  Phenomena 


hot,  and  set  into  boiling  activity  by  the  ebullition  of  gases  (Figs. 
61,  62).     From  cracks  in  the  floor  and  on  its  margin  jets  of  lava  are 


FIG.  60.  —  View  of  outline  of  the  crater  of  Kilauea  from  Volcano  House. 
(After  Button.) 

frequently  projected  to  great  heights,  and  some  of  this  material, 
blown  like  spray  by  the  wind,  is  drawn  out  into  slender,  hair-like 
fibers.  This  has  become  known  as  Pele's  hair,  so  named  after  the 
goddess  of  the  mountains. 


FIG.  61.  — The  Lava  Lake  at  one  side  of  the  crater  of  Kilauea. 

The  margin  of  the  crater  is  formed  by  a  precipitous  cliff  which 
varies  in  height  from  time  to  time.  This  is  due  to  the  fact  that 
the  lava  column  which  supports  the  floor  rises  as  the  pressure  in- 
creases below,  until  a  point  is  reached  where  the  wall  may  be  rup- 
tured, and  the  liquid  lava  flows  out,  whereupon  the  column  dimin- 


I2O 


Modern  Volcanic  Phenomena 


ishes,  carrying  the  floor  downward,  until  it  may  be  700  feet  below 
the  edge  of  the  crater  rim.  The  rate  of  rising  may  be  as  much  as 
100  feet  in  a  year,  but  in  modern  times  the  lava  has  not  overflowed 
the  rim,  but  issues  from  lateral  fissures  due  to  cracking  or  fusion, 
the  floor  approaching  only  to  within  300  feet  of  the  edge  of  the 


FIG.   63.  —  A  lava  stream  falling  in  cascades  over  a  cliff  into  the  sea, 
Hawaiian  Islands. 

crater.  During  the  eruption  of  1840  the  lava  first  appeared  on 
the  side  of  the  mountain  five  miles  from  the  main  crater,  after  which 
it  issued  at  successively  lower  levels. 

Similar  conditions  exist  in  the  crater  of  Mauna  Loa  proper,  but 
here  the  top  of  the  lava  column  is  nearly  10,000  feet  above  that  of 
Kilauea. 

The  lava  of  these  volcanoes  is  very  liquid,  being  of  an  extremely 
basic  character  and  forming  basaltic  rocks  on  cooling.  Because 
of  its  liquidity,  it  will  continue  to  flow  for  a  long  time  ;  some  of  the 
lava  flows  of  Hawaii  are  thirty  miles  in  length.  When  it  reaches 
a  cliff,  the  lava  cascades  over  it  like  a  waterfall  (Fig.  63).  The 
surface  of  such  lava  streams  often  shows  local  wave-like  advances, 
which  produce  the  appearance  of  a  series  of  crushed  pillows  piled 
one  against  the  other.  This  type  of  surface  form  is  known  in  Hawaii 


Forms  and  Activities  of  Typical  Modern  Volcanoes     121 

as  Pahoehoe  (pron.  pa-hoi-hoi)  (Fig.  64),  and  it  is  seen  in  older  lavas 
of  this  type  in  many  regions  of  the  world.  A  ropy  surface,  having 
the  Appearance  of  coils  of  heavy  rope,  is  also  commonly  produced. 
Compare  also  with  Fig.  66. 

As  the  lava  stream  moves  along,  it  sweeps  away  forests  in  its 
course,  and  carries  away  masses  of  rock  and  soil  covered  with  vege- 


FIG.  64.  —  Sluggish  lava  flow  forming  pillow  lava  or  "  pahoehoe  "  on 
Mauna  Loa. 


tation  (Fig.  65).  Sometimes  a  stream  will  part  around  a  mass  of 
such  rock  and,  reuniting,  enclose  it  as  an  island.  On  reaching  the 
sea,  the  lava  plunges  into  it  with  loud  detonations  and  becomes 
shivered  into  millions  of  particles  of  glass,  which  may  be  thrown 
in  clouds  into  the  air.  The  light  from  such  an  eruption  has  been 
visible  for  over  a  hundred  miles  at  sea,  and  at  a  distance  of  forty 
miles  fine  print  could  be  read  at  midnight  (Dana). 

When  the  crust  of  the  lava  stream  has  cooled,  the  interior  mass, 
still  in  a  molten  condition,  will  flow  on,  leaving  a  tunnel  behind. 
Such  tunnels  are  common  in  Hawaii,  and  their  roofs  are  frequently 


122 


Modern  Volcanic  Phenomena 


FIG.  65. — End  of  the  lava  flow  of  1881  on  Mauna  Loa.     Note  the  trees 
which  were  killed  but  not  consumed,  and  those  which  escaped. 

incrusted  with  lava  pendants  or  stalactites  up  to  20  or  30  inches  in 
length,  while  on  the  floor  corresponding  drip  mounds  or  stalag- 
mites of  lava  are 
found.  Small  "  spatter 
cones"  may  also  arise 
on  its  surface  (Fig.  66). 
The  form  of  these 
basic  lava  volcanoes  is 
very  characteristic, 
being  a  very  flat  cone 
of  vast  dimensions.  As 
they  arise  from  the  sea- 
bottom,  only  a  small 
part  is  visible,  the  total 
height  of  Mauna  Loa 
being  more  than  30,000 
feet,  although  consider- 
ably less  than  half  of 
this  height  is  seen. 
The  summit  is  nearly 


FIG.  66.  —  Lava  tunnel  formed  by  the  cool- 
ing of  the  outer  surface  of  the  flow,  after 
which  the  lava  within  flows  out,  leaving  the 
tunnel.  On  the  surface  of  the  flow  a  spatter 
cone  was  built  up.  Hawaiian  Islands. 


flat  for  several  square  miles,  and  the  slopes  of  the  sides  do  not 
average  more  than  seven  degrees. 


Forms  and  Activities  of  Typical  Modern  Volcanoes     123 

Craterless  Volcanoes  of  Viscous  Lava 

When  the  lava  is  very  viscous,  such  as  is  characteristic  of  very 
silicious  (acidic)  lavas,  it  may  happen  that  the  lava  mass  rises  as  a 
dome-like  swelling  from  the  surface,  producing  a  mound  or  hill  of 
lava  which  is  not  characterized  by  a  summit  crater.  The  volcano 
of  Chimborazo  in  Ecuador  (20,498  feet  above  the  sea)  appears  to 


Thera 


FIG.  67.  — Map  of  the  ruin  of  the  cone  of  Santorin,  in  the  Greek  Archipelago. 
(From  Kayser's  Lehrbuch.} 

be  of  this  type,  and  an  eruption  of  this  character  occurred  in 
1866  which  formed  the  Isle  of  Asphroessa  at  Santorin  in  the  Grecian 
Archipelago  (Figs.  67,  68).  A  gradual  rising  of  the  bottom  of  the 
bay  occurred,  until  the  island  appeared,  which  apparently  was  due 
to  slow  upward  and  outward  pressure  by  steam,  which  was  escap- 
ing at  every  pore  through  the  scoriaceous  lava  surface.  The  red-hot 
lava  could  be  seen  through  the  fissures,  and  the  whole  mass  was 
undulating  and  swaying  from  side  to  side,  sometimes  appearing 
to  swell  to  nearly  double  its  size,  and  to  throw  out  ridges  like  moun- 
tain spurs.  At  last  a  broad  chasm  appeared  across  the  top  of  the 
cone,  accompanied  by  a  tremendous  roar  of  steam,  while  rocks  and 


I24 


Modern  Volcanic  Phenomena 


FIG.  68.  —  Bird's  eye  view  of  the  Gulf  of  Santorin,  during  the  volcanic 
eruption  of  February,  1866,  looking  west.  (From  LyelPs  Principles.} 
a.  Therasia.  b.  The  northern  entrance,  1,068  feet  deep.  c.  Thera. 
d.  Mt.  St.  Elias,  rising  1,887  feet  above  the  sea,  composed  of  granular  lime- 
stone and  clay-slate;  the  only  non- volcanic  rocks  in  Santorin.  e.  Aspronisi. 
/.  Little  Kaimeni  (Kaymeni  or  Kaemenae).  g.  New  Kaimeni  (Nea  Kaymeni 
or  Kaemenae).  h.  Old  Kaimeni  (Palaea  Kaymeni  or  Kaemenae).  i.  Asphro- 
essa.  k.  George. 

ashes  mixed  with  steam  were  thrown  to  heights  of  50  to  100  feet, 
masses  of  this  material,  30  cubic  feet  in  bulk,  falling  at  distances  of 
600  yards  from  the  new  crater.  Then  the  activity  subsided,  the 
cone  was  lowered,  the  crater  closed  in,  and  after  a  few  minutes  of 
quiet  the  process  recommenced. 


FIG.  69  a.  —  Grand  Puy  of  Sarcoui, 
composed  of  trachyte  and  rising  be- 
tween two  breached  scoria  cones;  a 
typical  example  of  a  pustular  cone  or 
volcanic  blister  formed  of  highly 
viscous  lava.  Auvergne,  France. 


FIG.  69  b.  —  Experimental  illustra- 
tion of  the  mode  of  formation  of  vol- 
canic blister  cones  composed  of  viscid 
lavas. 


An  example  of  such  a  blister  cone  or  volcanic  dome,  now  extinct,  is 
found  in  the  Grand  Puy  of  Sarcoui  (Fig.  69  a)  in  the  old  volcanic 
district  of  Central  France.  This  mountain  is  a  mass  of  trachyte 


Forms  and  Activities  of  Typical  Modern  Volcanoes     125 

lava  having  the  appearance  of  an  inverted  cup,  without  a  crater, 
and  was  apparently  formed  from  a  mass  of  viscous  lava  which  was 
forced  upward  to  form  a  blister  upon  the  surface  of  the  earth.  Blis- 
ters of  this  kind  have  been  reproduced  experimentally,  as  shown 
in  the  preceding  figure  (Fig.  69  b). 

Vesuvius 

A  very  different  type  of  volcano  is  represented  by  Vesuvius, 
probably  the  best  and  longest  known  of  active  volcanoes.     The 


FIG.  70.  — Map  of  Vesuvius  with  its  lava  streams  up  to  1872.  The  darker 
are  the  later,  and  lighter  the  earlier  flows.  Scale  about  i :  250,000.  After 
Le  Herr  and  others.  (From  Kayser's  Lehrbuch.) 

present  cone,  which  lies  east  of  Naples,  is  surrounded  on  three  sides 
by  the  rim  of  the  ancient  cone,  now  called  Monte  Somma  (Figs. 
70,  71),  which  was  perfect  up  to  the  first  century  of  the  Christian 


126 


Modern  Volcanic  Phenomena 


Era.     That  volcano  had  been  dormant  for  so  long  that  its  slopes 
were  clothed  with  vineyards  and  gardens  and  dotted  over  with 

villas,  while  at 
the  foot  of  the 
mountain  lay  the 
populous  cities  of 
Herculaneum  and 
Pompeii.  Even 
the  interior  slopes 
of  the  crater,  of 
which  only  a  part 
remains  in  Monte 
Somma,  were 
covered  with  wild 
vines,  so  Plutarch 
tells  us,  while  the 
floor  of  the  crater 
was  a  sterile  plain. 
On  this  plain, 
from  which  there 
was  only  a  single 
outlet,  a  break  in 
the  wall  of  the 
crater,  the  gladi- 
ators of  Spartacus 
encamped  in  72 
B.C.,  while  the 
praetor  Clodius 
guarded  the  out- 
let and  attacked 
Spartacus  by  low- 
ering his  soldiers 
into  the  crater 
over  the  precip- 
itous walls. 

In  the  year  63 
A.D.  the  first  evi- 
dence of  the  re- 
awakening of  the 
volcano  made 


Forms  and  Activities  of  Typical  Modern  Volcanoes     127 

itself  felt  in  an  earthquake  which  damaged  the  cities  in  the  vi- 
cinity. From  that  time  to  79  A.D.  slight  shocks  were  frequent, 
and  in  August  of  that  year  they  became  more  numerous  and 
violent,  finally  terminating  in  the  first  great  historic  eruption. 
As  described  by  the  younger  Pliny,  a  dense  column  of  vapor  first 
arose  vertically  from  the  crater,  spreading  out  laterally  so  that  the 


FIG.   72.  — Ruins  of  Pompeii. 

upper  part  resembled  the  head,  and  the  lower  the  trunk,  of  a  pine 
tree.  Flashes  of  light,  vivid  as  lightning,  at  intervals  pierced  this 
cloud,  and  ashes  began  to  fall  even  upon  the  ships  at  distant  Mi- 
senum,  shoaling  the  sea  in  places  and  burying  Herculaneum  and  Pom- 
peii (Fig.  72).  The  violent  explosions  shattered  the  crater  rim, 
of  which  only  a  part  remains  in  Monte  Somma,  and  the  later  cone 
of  Vesuvius  proper  was  built  upon  the  floor  of  the  old  crater,  sur- 
rounded on  three  sides  by  its  rim.  No  lava  appears  to  have  been 
ejected  at  this  time,  the  material  being  all  of  the  pyroclastic  or 
f ragmen tal  type,  such  as  lapilli,  sand,  and  fragments  of  older  lava. 
The  first  lava  stream  recorded  from  Vesuvius  flowed  in  the  erup- 
tion of  1036 ;  which  was  the  seventh  since  the  reawakening  of  the 
volcano.  Another  eruption  occurred  in  1049,  and  still  another  in 
1138  or  1139 ;  after  this  the  volcano  rested  for  168  years,  though 
two  minor  vents  opened  at  distant  points,  one  at  Solfatara,  near 
Pozzuoli  (Bay  of  Baiae),  in  1198,  and  the  other  on  the  island  of 


128  Modern  Volcanic  Phenomena 

Ischia  in  1302.  Then  in  '1306  a  minor  eruption  of  Vesuvius  took 
place,  after  which  this  volcano  again  became  dormant  for  325  years 
or  until  1 63 1 ,  with  one  slight  eruption  in  1 500.  During  this  interval 
the  Sicilian  volcano  Etna  was,  however,  in  constant  eruption,  while 
within  the  Phlegraean  volcanic  field  west  of  Naples  arose  the  new 
volcano  Monte  Nuovo  in  1538  (ante,  p.  112).  Between  1139  and 
1631,  or  for  492  years,  there  had  been  no  violent  eruption,  and  the 


FIG.   73. — Eruption  of  Vesuvius  in  1872.      (After  photograph  from  Ratzel.) 

crater,  which  was  five  miles  in  circumference  and  about  a  thousand 
paces  deep,  had  its  sides  covered  with  brushwood  forests  frequented 
by  the  wild  boar,  while  cattle  grazed  on  its  floor.  Three  small 
pools  of  water  remained  upon  the  floor  of  the  crater,  one  being  hot 
and  bitter,  another  more  salty  than  the  sea,  and  the  third  hot  but 
tasteless.  Suddenly,  in  1631,  the  floor  and  sides  of  the  crater  were 
blown  to  fragments  which  the  wind  scattered,  and  in  December  of 
that  year  seven  streams  of  lava  poured  forth  from  the  crater,  over- 
whelming several  villages  on  the  flanks  and  at  the  foot  of  the  moun- 
tain, one  of  which,  Resina,  had  been  partly  built  over  the  ancient 
site  of  Herculaneum.  Great  floods  of  mud,  from  the  condensed 
vapor  and  the  ashes,  also  poured  down  the  sides  of  the  volcano  and 
did  much  damage. 


Forms  and  Activities  of  Typical  Modern  Volcanoes     129 

After  a  brief  rest,  the  eruptions  were  renewed  in  1666,  since  which 
time  they  have  occurred  almost  constantly,  with  only  short  inter- 
mittent periods  of  quiescence  (Fig.  73).  The  last  great  eruption 
occurred  in  1906.  This  began  with  premonitory  explosions  in  1904, 
while  during  the  whole  of  1905  a  narrow  stream  of  lava  flowed  from 
a  fissure  in  the  cone  (Fig.  74).  On  April  4,  1906,  began  the  last 
great  eruption,  which  was  inaugurated  by  the  appearance  of  a 
cloud  of  dust,  carried  aloft  by  the  gases,  and  assuming  the  shape  of 


FIG.   74.  —  Looking  into  the  crater  of  Vesuvius ;  hot  lava  sending  up  clouds 

of  steam. 

a  cauliflower.  At  the  same  time  several  lava  streams  broke  out 
at  successively  lower  levels  in  the  side  of  the  cone.  Three  days 
later  (April  7)  occurred  a  violent  explosion,  and  a  dust  cloud  arose 
vertically  into  the  air  for  a  height  of  four  miles,  this  dust  falling 
in  such  quantities  upon  the  roofs  of  the  houses  in  the  near-by  towns 
as  to  cause  their  collapse.  Larger  streams  of  lava  also  issued  from 
various  openings,  one  of  them  reaching  the  town  of  Boscotrecase 
and  destroying  it.  The  main  lava  stream  descended  the  steeper 
slopes  at  a  rate  of  somewhat  less  then  two  miles  an  hour,  but 
flowed  at  a  much  lower  rate  on  the  gentler  slopes.  The  lava  had  a 
temperature  of  more  than  2000°  F.,  but  owing  to  the  rapid  cooling 


Modern  Volcanic  Phenomena 


on  the  surface  it  did  not  burn  up  the  trees  with  which  it  carne  in 
contact,  but  charred  them,  and  sometimes  broke  them  off  by  its 
weight  and  carried  them  along  on  its  surface. 


FIG.   75.  —  Inside  the  crater  of  Vesuvius.      Note  the  stratified  appearance  of 
the  wall  of  ashes  and  cinders,  and  the  slopes  of  loose  material. 

Both  the  old  cone  of  Monte  Somma,  and  the  later  cone  of  Vesu- 
vius, which  has  a  height  of  about  4000  feet,  are  built  up  of  layers 
of  cinders,  ashes,  and  lava  (Fig.  75).  These  have  a  steep  inclination, 
dipping  away  from  the  rim  of  the  craters  in  all  directions  at  angles 
of  26°  to  40°  or  more.  These  layers  are  cut  vertically  by  numerous 
fissures,  which  are  filled  with  hardened  lavas,  forming  dikes  which 


» 

I 


II 


a. 


i  3. 

'       * 


132 


Modern  Volcanic  Phenomena 


bind  the  entire  mass  together.  Much  of  the  consolidation  of  the 
fragmental  material  is  also  due  to  the  fact  that  it  falls  often  in  a 
half -fused  condition,  and  the  heat  from  the  volcano  tends  to  bind  the 
particles  together.  Material  carried  to  greater  distances,  however, 
remains  incoherent.  Since  the  last  eruption  (1906)  much  gullying 
by  erosion  has  occurred  on  the  slopes  of  the  cone  (Fig.  76).  (See 
also  Fig.  86  a,  p.  142.) 

Etna 

This  famous  volcano,  in  the  eastern  part  of  the  island  of  Sicily, 
rises  almost  11,000  feet  above  the  sea,  and  has  a  nearly  circular 


FIG.  77.— Map  of  Etna  and  the  Val-del-Bove,  or  Valle-del-Bue.  After 
map  of  Italian  general  staff.  (From  Ratzel.)  The  orientation  of  this  map 
is  such  that  north  is  on  the  right.  The  coast  line  runs  in  a  direction  east  of 
north. 

base  with  a  circumference  of  87  miles,  while  its  lavas  cover  an  area 
almost  twice  as  great  (Fig.  77).  The  lower  part  of  the  cone  is  cul- 
tivated ;  higher  up  are  forests,  and  the  upper  part  is  a  barren  lava 


133 


134 


Modern  Volcanic  Phenomena 


waste,  which  terminates  in  a  sort  of  tableland,  from  which  arises 
the  principal  cone,  noo  feet  in  height.  From  this  cone  sulphurous 
vapors  and  steam  constantly  arise,  and  lavas  are  emitted  at  fre- 
quent intervals.  Viewed  from  north  or  south  the  cone  is  very  sym- 
metrical ;  but  on  the  east  it  is  cut  by  a  deep  valley,  the  Val-del-Bove 
or  Valle-del-Bue,  which  is  a  vast  amphitheater  four  or  five  miles  in 
diameter,  and  is  enclosed  by  precipices  between  3000  and  4000  feet 
high  at  the  upper  end.  This  valley  was  probably  formed  in  part 
by  explosions  and  in  part  by  subsidences  (Fig.  78). 

During  the  eruptions  of  Etna,  which  have  been  known  to  be  more 
or  less  continuous  since  the  fourth  century  B.C.,  the  upper  cone 
has  repeatedly  been  blown  away  or  has  been  partly  engulfed  by 

subsidences,  being 
renewed  again  each 
time  by  upbuilding 
from  lava  and  frag- 
mental  material. 
Where  shown  in  sec- 
tions, the  structure 
is  that  of  stratified 
layers  dipping  away 
steeply  from  the 
crater,  but  more 
complicated  than  in 
Vesuvius.  Numer- 
ous dikes  intersect 
the  layers  vertically 
and  bind  them  to- 
gether, such  dikes 
of  the  Val-del-Bove 


FIG. 


79.  —  Dikes  at  the  base  of  the  Serra  del 
Solfizio,  Etna.      (After  Lyell.) 


being  especially  well   shown   in   the   walls 
(Fig.  79)- 

The  formation  of  such  dikes  was  illustrated  by  the  eruption  of 
1669,  when  the  whole  top  of  the  mountain  collapsed.  At  this  time 
a  fissure  six  feet  broad  and  of  unknown  depth  opened  in  the  side 
of  the  mountain,  extending  north  and  south  for  a  length  of  12  miles 
from  the  plain  of  St.  Lio  to  within  a  mile  of  the  summit.  Five 
other  parallel  fissures  opened  one  after  the  other.  The  incandes- 
cent glow  from  these  fissures  showed  that  they  were  filled  up  to  a 
certain  height  with  lava  which  on  cooling  produced  transecting  dikes. 
Near  the  town  of  Nicolosi,  which  lay  near  the  base  of  the  wooded 


Forms  and  Activities  of  Typical  Modern  Volcanoes     135 

region,  about  20  miles  from  the  summit  of  Etna,  and  which  was  de- 
stroyed by  preliminary  earthquakes,  a  gulf  opened  from  which  sand 
and  scoria  were  thrown,  which  built  up  a  subordinate  cone,  Monte 
Rossi,  about  450  feet  high  (see  map,  Fig.  77). 

The  great  lava  stream  of  this  eruption  overflowed  fourteen  towns 
and  villages,  some  having  a  population  of  between  3000  and  4000 
souls,  and  finally  reached  the  walls  of  Catania  by  the  sea  (Fig.  77) 
15  miles  away.  It  accumulated  against  the  walls  of  this  city, 
which  were  60  feet  high,  and  finally  flowed  over  them,  but  without 


FIG.  80.  —  View  of  a  part  of  the  Val-del-Bove  with  parasitic  cones  and  steep 
lava  streams.  The  main  crater,  emitting  steam,  is  in  the  background. 
(After  Sartorius  and  Lasaulx,  from  Kayser's  Lekrbuch.) 

destroying  them,  falling  on  the  inside  in  a  series  of  fiery  cascades 
which  overwhelmed  part  of  the  city.  It  covered  the  first  13  miles 
of  its  journey  in  20  days,  but  required  23  days  for  the  last  two  miles. 
Sometimes  it  moved  at  the  rate  of  1500  feet  an  hour,  at  other  times 
only  a  few  yards  in  several  days.  When  it  finally  reached  the  sea, 
it  was  still  600  yards  broad  and  40  feet  deep.  Its  surface  was 
generally  solid  rock,  but  the  hot  liquid  interior  broke  this  surface 
and  flowed  on  to  be  in  turn  chilled  with  a  repetition  of  the 
process.  The  course  of  this  lava  stream  is  shown  upon  the  map 
(Fig.  77). 
The  formation  of  lateral  cones  or  monticules  from  which  two 


136 


Modern  Volcanic  Phenomena 


eruptions  proceed  for  every  one  that  issues  from  the  main  cone, 
is  a  very  characteristic  feature  of  Etna,  more  than  200  such  cones 
being  known.  One  of  these  (Monte  Minardo,  east  of  Bronte,  see 
map,  Fig.  77)  reached  a  height  of  over  750  feet.  When  new  open- 
ings form,  the  lava  from  these  may  surround  and  even  bury  the 
old  monticules,  so  that  the  volcano  is  covered  with  numerous,  more 
or  less  buried,  extinct,  parasitic  cones. 

In  the  eruption  of  August,  1852,  to  May,  1853,  two  new  cones 
opened  close  together  near  the  head  of  the  Val-del-Bove,  rising  in 
1 6  days  to  a  height  of  about  500  feet  (Fig.  80).  The  lava  poured 
down  the  Val-del-Bove,  in  places  completely  filling  it  from  side  to 
side,  so  that  it  became  a  barren  waste,  no  longer  able  to  support  the 
cattle  from  which  it  had  derived  its  name. 


Mont  Pelee 

This  volcano,  on  the  island  of  Martinique  in  the  West  Indies, 
became  violently  active  in  May,  1902,  the  volcano  Soufriere  on  St. 
Vincent  going  into  activity  at  almost  the  same  time.  The  erup- 
tion of  Mont  Pelee  was  characterized  by  violent  explosions,  pre- 


FIG.  81. — View  of  the  volcano  Mount  Pelee,  on  Martinique,  showing  the 
spine  (a)  with  a  larger  view  of  the  same  (b).  (From  E.  de  Martonne  in 
Geographic  Physique.) 

ceded  by  small  premonitory  symptoms.  There  was  no  actual  out- 
pouring of  lava,  which  was  completely  shattered,  and  the  mass  of 
minute,  incandescent  rock  particles  was  carried  aloft  by  the  highly 
heated  gases,  forming  dense  fiery  clouds,  which  not  only  rose  into 
the  air,  but  rushed  like  a  stream  through  a  gap  in  the  crater  down 
the  slopes  of  the  mountain  into  the  sea,  overwhelming  and  destroy- 
ing all  life,  including  all  but  two  individuals  of  the  30,000  inhabit- 


Forms  and  Activities  of  Typical  Modern  Volcanoes     137 

ants  of  the  town  of  St.  Pierre.  This  ejection  of  incandescent 
clouds  continued  for. several  months.  Another  remarkable  feature 
of  this  eruption  was  the  formation  of  the  great  spine,  which  will 
be  again  considered  somewhat  later  (Figs.  Si,  106,  107). 


Krakatoa  and  Bandai-San 

The  volcano  of  Krakatoa  forms  an  island  in  the  straits  of  Sunda, 
between  Java  and  Sumatra  in  the  East  Indies  (Fig.  82).  It  sud- 
denly became  active  on  August  26  and  27,  1883.  This  activity 


FIG.  82.  —  Map  of  the  straits  of  Sunda  in  the  East  Indies,  showing  the 
location  of  the  volcano  Krakatoa  and  the  rift  lines  which  center  in  it.  (After 
R.  D.  H.  Verbeck,  from  Ratzel,  Die  Erde.) 

began  with  a  series  of  premonitory  convulsions,  after  which  the 
greater  part  of  the  island  was  blown  away  by  a  succession  of  terrific 
explosions,  the  detonations  of  which  were  heard  more  than  150 
miles  away.  A  mass  of  material  estimated  at  a  bulk  of  almost  one 
and  one-eighth  cubic  miles  was  thrown  into  the  air  in  the  form  of 
lapilli,  ashes,  and  the  finest  dust  —  some  of  it  to  the  height  of  seven- 
teen miles — and  the  air  waves  generated  by  the  explosion  traveled 
westward  carrying  the  dust  with  them,  and  are  supposed  to  have 
passed  three  and  a  quarter  times  around  the  earth  (82,200  miles) 
before  they  died  away.  For  many  months  after  the  eruption  the 
dust  in  the  air  caused  a  series  of  brilliant  sunsets  all  over  the  earth. 


138 


Modern  Volcanic  Phenomena 


Dust  fell  in  large  quantities  on  the  decks  of  vessels  1600  miles  dis- 
tant, three  days  after  the  eruption,  and  tracts  of  deep  water  were 
made  so  shallow  from  this  dust  as  to  become  unnavigable.  Great 
sea-waves  (tsunamis)  were  generated,  one  of  which  was  estimated 


Crater  Perbuatan 


Verlaten  I. 


Lang  I. 


f  -  •  •  Crater  Danan 


"• Rakata 


Ancient  Volcano 

lanan    /''Rakata 


Vfrlatenl.  . 


Sea  Level 


FIG.  83  a. — Map  and  section  (on  line  AB)  of  Krakatoa  before  the  explo- 
sion of  1883.  After  Verbeck.  i,  older  andesite;  2,  younger  andesite; 
3,  basalt;  T,  Tertiary  basement.  (From  Kayser's  Lehrbuch.) 

to  have  risen  100  feet,  and  these  destroyed  1295  towns  and  villages 
along  the  shores,  killing  36,380  people.  By  their  force  a  large  ship 
was  carried  inland  for  a  mile  and  a  half  and  left  stranded  30  feet 
above  sea-level.  Great  blocks  of  stone,  weighing  from  30  to  50 
tons,  were  also  carried  inland  for  two  or  three  miles.  Altogether 
this  was  the  most  stupendous  manifestation  of  volcanic  activity 


Forms  and  Activities  of  Typical  Modern  Volcanoes     139 

known  in  modern  times.  The  appearances  before  and  after  the 
explosion  are  shown  in  the  maps  and  sections  here  given  (Figs.  83 
a,  b)  and  the  main  remaining  mass  in  Fig.  84. 

Bandai-San  in  Japan  was  a  volcanic  cone  2000  feet  high  and  had 
been  dormant  for  a  thousand  years.  Suddenly  in  1888  the  greater 
part  of  the  cone  was  blown  away  by  a  terrific  series  of  explosions 


Lang  /. 


FIG.  83  b.  —  Map  and  section  (on  line  CD)  of  Krakatoa  after  the  explosion 
of  1883.  T  to  3,  same  as  in  preceding;  4,  product  of  latest  eruption. 
(From  Kayser's  Lehrbuch.) 


which  continued  through  a  period  of  less  than  two  hours,  leaving 
only  a  remnant  which  terminates  in  a  cliff  about  1500  feet  high 
(Fig.  85).  This  explosion  is  believed  to  have  been  due  to  the  per- 
colation of  surface  waters  into  the  volcanic  interior  and  the  forma- 
tion of  steam,  which  caused  the  disruption.  No  lava  flows  were 
observed  in  this  eruption,  and  the  volcano  has  since  been  in- 
active. 


140 


Modern  Volcanic  Phenomena 


FIG.  84.  —  View  of  the  Rakata  of  Krakatoa,  the  chief  remaining  fragment 
of  an  older  eruption,  showing  the  numerous  dikes  which  bind  the  mass  together. 
(After  Judd  from  Ratzel,  Die  Erde.} 


FIG.  85.  —  Section  of  the  Bandai-San.      (After    Sekiya.)     The   dotted   line 
shows  the  part  destroyed  by  the  explosion  of  1888. 


CLASSIFICATION  or  VOLCANOES  ACCORDING  TO  TYPE  or 
ERUPTION  AND  FORM 

We  have  now  seen  something  of  the  mode  of  eruption  of  several 
distinct  types  of  volcanoes  in  various  parts  of  the  earth.  Ac- 
cording to  their  mode  of  eruption,  we  may  classify  them  as  the 
quiet  type  on  the  one  extreme,  represented  by  the  welling  up  and 
pouring  out  of  liquid  lavas,  as  in  Kilauea,  and  the  explosive  type, 
represented  by  Krakatoa  and  Bandai-San,  where  shattering  of 


Classification  of  Volcanoes  141 

rock  material  occurs,  but  without  the  outpouring  of  liquid  lava. 
In  the  milder  examples  of  this  type  a  cinder  cone  is  built  up 
(Figs.  86  a,  b).  Between  these  two  stand  the  types  which  show  both 
kinds  of  eruption,  with  the  result  that  beds  of  volcanic  ashes  and 
lapilli  alternate  with  beds  of  solidified  lava.  Here  we  place  Vesu- 


FIG.  86  a.  —  Cinder  Cone,  Arizona.  Young  cinder  cone  on  left,  late  mature 
cinder  cone  on  right.  The  young  cone  and  lava  flow  are  but  a  -few  hundred 
years  old  and  are  located  on  the  northern  edge  of  the  Flagstaff,  Arizona,  topo- 
graphic sheet.  (Photo  by  D.  W.  Johnson.) 

vius  and  Etna,  explosive  eruptions  being  more  marked  in  the  former 
and  lava  eruptions  in  the  latter. 

Comparison  of  Form.  —  Comparing  the  form  of  the  cinder  cone 
with  that  of  a  pure  lava  cone,  we  see  a  striking  difference.  The 
former,  illustrated  by  the  wonderfully  perfect  cinder  cone  of  May  on 
in  the  Philippines  (Frontispiece)  has  steep  slopes,  the  angle  being 
determined  by  the  nature  and  coarseness  of  the  fragmental  ma- 
terial. The  lava  cone,  on  the  other  hand,  especially  that  composed 
of  basic  lava,  is  broad  and  relatively  low,  though  the  crater  may  be 
situated  at  a  great  height  above  the  base.  The  slopes  are  very 
gentle,  and  the  top  generally  a  plateau.  This  is  illustrated  by 
Mauna  Loa  in  the  Hawaiian  group. 

In  the  diagrams  on  page  143  is  shown  a  comparison  of  a  number 
of  modern  volcanoes  and  craters  drawn  approximately  to  scale 
(Fig.  87). 


Modern  Volcanic  Phenomena 


Geological  Age  of  Volcanoes  and  Lava  Flows     143 


5  Km 

FIG.  87.  —  Cone  sections  of  various  types  of  volcanoes,  i.  Vesuvius. 
2.  Lake  Laach.  3.  Rocca  Monfina.  4.  Lago  Bracciano.  5.  Krakatoa. 
6.  Peak  of  Teneriffe.  7.  Mauna  Loa.  (From  Kayser's  Lehrbuch.) 

GEOLOGICAL    AGE     OF 

VOLCANOES    AND 

LAVA  FLOWS 

The  geological  age  of 
a  volcano  can  be  deter- 
mined from  the  age  of  the 
associated  formations.  It 
is  obvious  that  a  lava 
flow  is  always  younger 
than  the  formation  upon 
which  it  rests,  and  older 
than  that  which  covers  it. 
In  Fig.  88  is  shown  a  lava 
sheet  which  rests  upon 
river  gravels  of  Pleisto- 
cene age  and  is  .therefore 
younger  than  these.  The 
extensive  erosion  which 
it  has  suffered,  indicates 

that  it  is  probably  of  late  FlG  88  _  Uta  flow  over  Pldstocene  gravel> 
Pleistocene  age.  Utah.    (Photo,  by  F.  J.  Pack.) 


CHAPTER  VIII 

STRUCTURAL  CHARACTERS  OF  VOLCANOES,  AND 
OTHER   IGNEOUS   PHENOMENA 

THE  structural  character  of  volcanoes  is  revealed  in  cones  that 
have  become  extinct,  for  in  these  the  parts  are  not  only  more  easily 
accessible,  but  dissection  has  often  revealed  the  internal  character 
as  well. 

EXTINCT  VOLCANOES 

There  are  many  regions  where  volcanoes  have  been  active  in 
the  recent  geological  past,  and  in  such  cases  enough  of  the  form 
and  character  of  the  volcanoes,  now  extinct,  is  still  retained  to 
enable  one  to  recognize  them  readily.  Such  recently  extinct  and 
partly  dissected  volcanoes  are  not  only  of  interest  as  showing 
former  distribution  of  volcanic  activity,  but  they  have  an  added 
value  because  their  erosion  has  revealed  many  features  which  in  an 
active  volcano  are  not  open  to  view.  Thus  the  study  of  the 
recently  extinct  volcanoes  supplements  that  of  active  ones. 


Extinct  Volcanoes  of  Central  France 

One  of  the  most  notable  fields  of  former  volcanic  activity,  and 
one  that  has  played  a  prominent  part  in  the  history  of  the  science 
of  volcanology,  lies  in  the  central  part  of  the  great  area  of  crystalline 
and  younger  rocks  which  makes  up  the  so-called  Massif  Central 
of  France,  and  which  is  bounded  on  the  north  by  the  Paris  basin 
of  Mesozoic  and  Tertiary  rocks,  on  the  southwest  by  the  basin 
of  the  Garonne,  and  on  the  east  by  the  valleys  of  the  Rhone  and 
Saone.  The  center  of  this  massif  (Fig.  89)  is  dissected  by  the 
river  Allier,  which  flows  north  into  the  Loire,  and  it  is  along  the 
western  border  of  the  Allier  valley,  which  is  bounded  by  a  fault, 
that  the  main  volcanic  district  is  located,  while  a  second  one  lies 

144 


Extinct  Volcanoes 


145 


on  the  southeast,  in  the  region  of  the  headwaters  of  the  Allier  and 
the  Loire. 

The  younger  eruptive  rocks  belong  to  several  geological  epochs, 
namely  the  Pleistocene,  the  Pliocene,  and  the  Miocene.  Of  these 
the  youngest,  of  Pleistocene  age,  form  the  famous  Chaine  des  Puys, 


FIG.  89.  —  Geological  map  of  Auvergne  (after  Michel-Levy,  M.  Boule 
and  Glangeaud).  Scale  about  i :  1,400,000.  Light  gray,  gneiss  and  granite. 
Black,  Carbonic  beds  (mostly  confined  to  the  Loire  valley).  Dark  gray, 
Tertiary  trachytes,  basalts,  etc.  Fine  lines,  Quaternary  eruptives  of  the  Puys. 
White,  Tertiary  and  Quaternary.  (From  Kayser's  Lehrbuch.) 

a  group  of  extinct  volcanoes  which  still  retain  to  a  remarkable 
degree  their  form  and  general  character.  South  of  this  lies  the  vol- 
canic district  of  Mont  Dore,  and  still  farther  south  that  of  Cantal 
-  both  of  Pliocene  age.  To  the  east  of  the  latter  and  between 
the  Allier  and  the  Loire  is  the  Chaine  du  Velay,  of  Pliocene  basalts, 
and  east  of  this  a  series  of  Miocene,  eruptive  hills  (Mezenc  and 
Megal). 


146  Structural  Characters  of  Volcanoes 

The  Chaine  des  Puys.  —  This  is  best  approached  from  Clermont- 
Ferrand,  which  lies  to  the  east  of  the  highest  of  these  old  volcanic 
hills,  the  Puy  de  Dome  (Fig.  90),  from  the  summit  of  which  an 
inspiring  panorama  of  these  silent  volcanic  cones,  some  sixty  in 
number,  may  be  seen.  They  extend  for  a  distance  of  about  90 
kilometers  north  from  the  Mont  Dore  region.  The  Puy  de  Dome 
itself  is  not  a  perfect  volcano,  but  is  formed  by  a  central  dike  or 
neck  of  trachytic  rock  (called  by  the  French  domite),  but  the  great 
majority  of  the  cones  show  each  their  cup-shaped  crater  at  the  top, 


FIG.  90.  —  Puy  de  Dome,  an  extinct  volcano  in  the  Chaine  des  Puys,  central 
France.     (Photo,  by  D.  W.  Johnson.) 

and  produce  a  volcanic  topography  rivaling  that  of  the  surface  of 
the  moon  (Fig.  91).  The  lava  of  these  volcanoes  consists  of  olivine 
basalts  and  andesites,  with  abundant  slags,  scoriae,  and  pumaceous 
material  all  rich  in  ferro-magnesian  minerals,  and  containing  from 
50  to  58  per  cent  of  silica.  The  outpourings  belong  to  the  middle 
and  later  Pleistocene  time.  Besides  the  perfect  crater  cones  there 
are,  however,  great  intumescences,  or  dome-like  blisters  of  trachytic 
rock  without  craters,  and  these  represent  the  type  of  up-swellings 
of  viscous  acidic  lavas  already  discussed.  Such  an  one  is  the 
Grand  Puy  of  Sarcoui,  shown  in  the  distance  on  the  extreme  right 
of  our  illustration  (Fig.  91),  and  in  outline  on  page  124  (Fig.  69  a). 


Extinct  Volcanoes 


147 


The  Massif  of  Mont  Dore.  —  This  mass,  south  of  the  Puys, 
represents  the  remnant  of  a  great  volcano,  active  during  Pliocene 
time,  but  since  then  partly  destroyed  by  erosion.  While  this  makes 


8< 

OQ    s- 


P> 
"I 


possible  trie  detailed  study  of  the  various  parts  of  the  volcano, 
and  gives  us  an  opportunity  to  observe  the  successive  eruptions 
and  their  effects,  it  also  makes  a  more  difficult  problem  for  the 
student  to  comprehend,  for  it  must  be  borne  in  mind  that  the 


148 


Structural  Characters  of  Volcanoes 


sr«g- 


•&, 


Jiff 


§  s 

s  a 

-f 


1-3 


.11 


8* 

If 
8  2 
3.  a 


-  «    «n 

<u  .'Z   -^ 

ill 


c/3  ^  « 

I   T3S 

-^  a 

r5.y    2 


s -a 


many  peaks  and  prominences 
now  seen  are  not  separate  vol- 
canoes as  in  the  Puy  district, 
but  erosion  remnants  of  larger 
masses,  and  the  imagination 
must  be  drawn  upon  to  restore 
again  what  is  missing  and  so 
get  a  picture  of  the  whole  vol- 
cano as  it  Was  during  its  prime. 
In  the  adjoining  diagram  (Fig. 
92)  is  represented  such  a  view 
of  the  relationships  of  the  dif- 
ferent volcanic  rocks  which 
make  up  this  volcano  as  would 
be  obtained  were  the  entire 
massif  cut  through  its  highest 
part  (Sancy)  like  a  round  cake 
cut  through  the  middle,  and  half 
of  it  removed  so  that  the  cut 
side  of  the  other  half  is  visible. 
Such  a  cross-section,  as  it  is 
called,  is  built  up  from  innu- 
merable local  observations  which 
are  then  connected  by  logical 
inferences.  This  section  shows 
that  the  foundation  of  the  vol- 
cano is  ancient  granite  and 
gneiss,  and  that  it  was  built  up 
on  an  almost  level  surface  (a 
peneplane)  which  had  pre- 
viously been  cut  across  the  old 
foundation  rocks  by  natural 
agencies.  Upon  this  floor  lies 
a  mass  of  andesitic  tuff,  the 
product  of  the  first  eruptive 
activity  (upper  Miocene) .  Then 
(early  Pliocene)  followed  an 
eruption  of  trachyte  porphyries 
through  the  Sancy  vents  and 
that  of  the  Capucin,  the  flows 


Extinct  Volcanoes  149 

of  which  extended  laterally  for  some  distance,  finally  thinning 
away.  The  next  eruptions  (middle  Pliocene)  were  those  of  the 
Puy  de  Pailleret  and  the  Puy  de  Cliergue  on  each  side  of  Sancy, 
these  lavas  on  cooling  forming  hornblendic  andesites.  Then  came 
the  late  Pliocene  eruption  of  the  Plateau  basalts,  which  cut  and 
rest  upon  the  others,  and  this  was  followed  by  local  Pleistocene 
eruptions  of  the  basalts  and  the  formation  of  cinder  cones  which 
correspond  to  those  of  the  Puy  Chain.  The  succession  of  events  is 
clearly  indicated  by  the  relationships,  especially  the  superposition 
of  the  various  lavas  and  pyroclastic  products,  and  it  will  be  ob- 
served that  the  eruptions  proceeded  from  acidic  trachytes  (to- 
gether with  rhyolites,  phonolites,  etc.)  to  basic  types,  i.e.  basalts. 
The  tuffs  often  carry  impressions  of  the  vegetation  of  their  time 
from  which  their  geological  age  can  be  determined. 

The  Cantal.  —  This  Tertiary  volcanic  massif  lies  south  of  Mont 
Dore  and  is  connected  with  it  by  a  basaltic  plateau.  It  forms  the 
most  prominent  of  the  Pliocene  volcanoes,  and  it  also  shows  much 
dissection,  so  that  the  structure  and  succession  of  eruptive  events 
can  be  determined.  Its  diameter  is  from  60  to  80  kilometers,  and 
its  mass  about  ten  times  that  of  Mont  Dore.  As  in  the  latter  case 
there  are  many  elevated  peaks,  the  highest  of  which,  the  Plomb- 
du-Cantal,  rises  1858  meters.  These  are  all  erosion  peaks  of 
parts  of  older  eruptions,  only  one  —  the  Puy  de  Griou  (1694  meters) 
—  representing  a  late,  though  not  the  latest,  eruption  from  the  cen- 
ter of  the  volcano,  but  even  it  does  not  show  the  original  height 
of  the  mountain.  In  Fig.  93  two  cross  sections  are  shown,  one 
from  northwest  to  southeast,  the  other  at  right  angles  to  it,  and 
both  passing  through  the  center  of  the  old  volcano.  These  are 
reconstructed  in  the  same  manner  as  that  of  Mont  Dore,  namely, 
from  numerous  local  observations  and  the  combination  of  these. 
The  succession  of  eruptive  events  here  is  of  similar  character  to 
that  of  Mont  Dore,  the  volcano  resting  upon  an  old  erosion  floor 
of  gneiss  upon  which  were  locally  deposited  beds  of  Oligocene  sedi- 
ments (4,  Fig.  93),  which  are  especially  well  shown  on  the  south- 
west. Through  these  came,  first,  eruptions  of  older  basalt  (5) 
(not  positively  known,  though  suspected,  at  Mont  Dore)  and  this 
was  followed  by  eruptions  of  acid  trachytes  and  phonolites  with 
trachytic  tuffs,  still  of  Miocene  age  (6).  These  locally  rest  upon, 
the  gneiss,  or  upon  the  Oligocene  sediments,  and  still  again,  in  one 
part  of  the  section,  upon  the  older  basalts,  showing  that  they 


Structural  Characters  of  Volcanoes 


succeeded  these.  Then  was  thrown  out  from  the  central  orifice 
a  mass  of  breccias,  cinders,  etc.,  of  andesitic  and  basaltic  material 
(7),  intruded  by  dikes  and  interbedded  with  sheets  of  andesite, 


I1*J 

S  ex  H  3 


this  also  forming  a  capping  rock  (8)  and  marking  the  great  early- 
Pliocene  eruption  from  the  central  orifice.  Through  these,  in 
middle  Pliocene  time,  was  forced  the  dike  of  phonolite  which  forms 
the  Puy  de  Griou,  and  finally  came  the  eruption  of  the  last  basalt, 


Extinct  Volcanoes 


152 


Structural  Characters  of  Volcanoes 


Extinct  Volcanoes 


153 


the  so-called  Plateau  basalt,  because  the  great  plateaus  of   the 
region  are  capped  by  it. 

If  the  student  has  succeeded  in  gaining  a  clear  conception  of  the 
succession  of  events  in  these  two  dissected  volcanoes,  he  may  next 
attempt  an  analysis  of  the  two  cross  sections  of  the  southeastern 
district  given  in  the  next  two  diagrams  (Figs.  94  and  95),  where,  as 
a  result  of  eruption  through  numerous  vents,  a  more  complicated 
structure  is  produced.  After  this  he  will  be  ready  to  analyze 


FIG.  96.  —  Map  of  the  volcanic  areas  and  fracture  lines  of  Central  France, 
the  former  shaded,  the  latter  in  broken  lines.  Fault  lines  are  shown  solid. 
(After  Marcellin  Boule.) 

sections  of  older  volcanic  districts,  including  those  of  our  own 
country,  as  given  in  the  various  folios  of  the  Geological  Atlas  of  the 
United  States. 

Arrangement  of  these  Volcanic  Centers  Along  Lines  of  Fracture 
in  the  Earth's  Crust.  —  From  the  study  of  the  volcanic  region  of 
Central  France  it  has  become  apparent  that  all  of  these  volcanic 
manifestations  are  located  along  lines  of  fracture  in  the  earth's 
crust,  these  fractures  making  possible  the  rise  of  the  lavas  and 
gases  which  have  produced  the  phenomena.  The  map  of  the 
region  (Fig.  96)  shows  the  fractures  ascertained  and  their  relation 


154 


Structural  Characters  of  Volcanoes 


to  the  volcanic  manifestations.  This  is  a  very  general  arrange- 
ment of  volcanoes  the  world  over,  and  will  be  referred  to  again 
later  on. 

The  Extinct  Volcanoes  of  the  Rhine  Region 

Extinct  volcanoes  and  volcanic  activity  during  Tertiary  time 
are  shown  in  a  number  of  localities  in  the  mountainous  region 
through  which  the  river  Rhine  has  cut  its  famous  gorge.  The 
most  impressive  of  these  are  the  Seven  Mountains  (Siebengebirge) 


FIG.  97.  —  Volcanic  landscape  of  the  Siebengebirge  after  a  photograph  from 
the  Rodderberg.  (From  F.  Ratzel,  Die  Erde.)  These  low  mountains  are 
slightly  dissected  volcanic  peaks  of  Tertiary  age.  The  high  peak  on  the  left 
is  the  Drachenfels.  (See  map,  Fig.  98.) 

on  the  .right  bank  of  the  river  in  the  Cologne  region  not  far  from 
Bonn  (Fig.  97).  Like  the  Tertiary  volcanoes  of  France,  these 
show  only  in  part  their  former  character,  erosion  having  modified 
them  to  a  considerable  degree.  Nevertheless,  it  can  be  recognized 
that  they  represent  a  series  of  eruptions,  which,  like  those  of  France, 
proceeded  from  acidic  to  basic  lavas. 

The  eruptions  began  in  Miocene  time,  and  the  volcanic  masses 
were  built  up  on  the  old  erosion  surface  of  the  Devonian  shales 


Extinct  Volcanoes 


155 


and  .sandstones,  which  form  the  cliff  of  the  Rhine  gorge  (map, 
Fig.  98).  The  first  outpouring  resulted  in  the  formation  of  light- 
colored  trachyte  of  which  the  typical  trachyte  of  the  Drachenfels, 
already  referred  to  in  the  discussion  of  that  rock  (p.  101),  was  the 
product.  Others  of  the  hills  of  this  region  were  also  formed  by  this 


Devonian  Clay  &  Sand     Tuff 
Basaltic  cinders  Trachyte  Andesite 
Basalt       Diluvium    Alluvium 


FIG.    98.  —  Map   of    the    volcanic   district   of   the   Rhine    (Siebengebirge) . 
(After  Laspeyres,  from  Walther.) 

eruption.  The  next  eruption  was  of  more  basic  lava,  resulting  in  the 
formation  of  andesites,  of  which  rock  another  group  of  these  hills  is 
composed.  Finally,  very  basic  lavas  came  to  the  surface,  forming 
basalts,  the  hills  of  this  rock  being  scattered  among  the  others, 
while  dikes  of  the  basalt  cut  the  older  andesites  and  trachytes. 
This  last  eruption  occurred  in  Pleistocene  time.  There  is  thus  a 


156  Structural  Characters  of  Volcanoes 

succession  from  acid  to  basic  lavas  as  in  the  Auvergne  district. 
One  of  the  last  formed  of  these  volcanoes  is  the  Rodderberg, 
situated  on  the  west  bank  of  the  Rhine  between  Mehlem  and 
Rolandseck.  It  consists  of  basaltic  scoriae  which  in  places  rest 
upon,  and  have  by  their  heat  altered  and  partly  fused,  some  of 
the  older  river  sediments  of  the  Rhine,  and  its  crater,  still  perfectly 
recognizable,  is  filled  with  a  deposit  of  wind-blown  dust  or  loess 
and  has  now  become  the  site  of  a  thriving  farm  (the  Broichhof). 

EXTINCT  CALDERAS  AND  SINKS 

The  term  caldera  has  often  been  applied  to  large  craters,  such 
as  those  of  Kilauea,  but  it  has  recently  been  suggested  that  these 


FIG.    99.  —  Cinder    Cone    within    the    Crater    Lake,    Oregon.     A    volcano 
built  within  the  basin  of  a  sink.     (Photo  by  D.  W.  Johnson.) 

be  spoken  of  as  sinks,  because  they  are  formed  by  subsidences  of 
the  lava  column,  and  that  the  name  caldera  be  restricted  to  explo- 
sion craters  or  hollows  such  as  that  formed  at  Krakatoa  (Daly). 
Both  sinks  and  calderas  are  known  which  were  formed  by  past 
volcanic  activities  in  a  region  not  subject  to  such  disturbances  at 
present.  An  example  of  an  older  sink  is  Crater  Lake,  Oregon 
(Figs.  99,  100  a-d).  This  occupies  the  site  of  a  former  volcano, 
which  has  been  named  Mount  Mazama  (Fig.  100  d)  and  the  sum- 
mit of  which  has  collapsed.  From  this  summit  glaciers  descended 
probably  during  the  Pleistocene  glacial  period,  which  scoured  and 
polished  the  sides  of  the  volcano,  as  is  shown  by  the  marks  still 


Extinct  Calderas  and  Sinks 


157 


lo/     TRAVEL-GUIDE  MAP     '-' 

CRATER  LAKE°FNATIONAL  PARK 

OREGON 

1 o    '         i  Scale  2 3  4  Miles 


FIG.  ioo  a.  —  Map  of  Crater  Lake,  National  Park,  Oregon.     (U.  S.  G.  S.) 


158 


Structural  Characters  of  Volcanoes 


FIG.  ioo  b.  —  Map  of  Crater  Lake,  Oregon.      (U.  S.  G.  S.)     Heights  and 
soundings  in  feet.     (Copied  from  de  Martonne.) 

remaining  on  the  outer  slopes  of  the  lake  rim.  Thus  the  col- 
lapse of  the  mountain  summit  is  shown  to  have  been  a  recent  one, 
and  appears  to  have  followed  upon  an  extensive  outpouring  of 


FIG.  ioo  c.  —  Profile  section  of  Crater  Lake  National  Park.     (U.  S.  Dept. 

Interior.) 

lava.  Many  old  calderas  are  found  in  various  parts  of  the  world, 
those  of  the  Eifel  district  in  western  Germany  (Fig.  101),  where 
they  are  known  as  Maare,  being  the  most  typical  (Figs.  102, 
103).  These  are  readily  recognized  by  their  circular  character, 


Extinct  Calderas  and  Sinks 


and  by  the  fact  that  around  their  margins  are  extensive  deposits 
of  scoriae  and  even  of  small  volcanic  bombs  together  with 
the  fragments  of  shale  and  sandstone  blown  from  the  craters 
(Fig.  104).  Less  frequently  are  lava  flows  of  basalt  trachyte  or 


FIG.  100  d.  —  Section  of  Crater  Lake  and  its  rim,  with  the  probable  outline 
of  Mount  Mazama.  Structural  details  generalized.  (Vertical  and  horizontal 
scale  the  same.)  Smithsonian  Institution. 

phonolite  associated  with  them,  which,  together  with  the  frag- 
mental  material,  built  up  crater  cones.  One  of  the  most  typical  and 
largest  of  these  hollows  is  now  occupied  by  the  beautiful  Laacher 
Lake  (Fig.  105).  The  lapilli  from  these  explosive  eruptions  form 


FIG.  101.  —  Map  of  the  volcanic  district  of  the  Eifel.  (After  von  Deschen.) 
Be.  Bertrich ;  Da.  Daun ;  Dr.  Dires ;  Ge.  Gerolstein ;  Gi.  Gillenfeld ;  H.  Hilles- 
heim;  Klb.  Kelberg;  Ma.  Manderscheid ;  U.  Ulman;  Maare  in  black;  vol- 
canic rocks,  shaded.  (From  Kayser's  Lehrbuch.} 


i6o 


Structural  Characters  of  Volcanoes 


FIG.  102.  —  Gmiinden  Maar,  Eifel. 
An  explosion  crater  converted  into  a  lake. 


FIG.  103.  —  Schalkenmehren  Maar,  Eifel. 
A  Tertiary  explosion  crater  converted  into  a  lake. 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      161 

extensive  deposits  of  "  sand  "  along  the  left  bank  of  the  Rhine, 
readily  visible  from  the  train. 


Tttff- 


Watt 


FIG.  104.  — •  Ideal  section  through  a  Maar  of  the  Eifel  showing  the  old  crater 
funnel  and  pipe,  with  the  lakelet  in  the  upper  part,  and  the  wall  of  tuff  and 
scoriae  surrounding  it.  (From  Kayser's  Lehrbuch.) 


VOLCANIC  FUNNELS  AND  PIPES,  SPINES,  PLUGS,  AND  NECKS 

Funnels  and  Pipes.  —  From  the  mouth  or  rim  of  the  crater 
of  the  volcano  the  slope  is  generally  inward,  forming  a  funnel- 
shaped  depression  to  its  bottom,  this  constituting  the  normal 
crater.  This  differs  from  the  crater  of  Kilauea,  which  is  a  sink 
with  practically  perpendicular  sides.  The  funnel  is  continued 


FIG.  105.  —  The  Laachersee    (Lake  Laach)  Northwest  Germany,  occupying 
an  old  explosion  crater.     (After  Walther.) 

downward  into  the  depths  of  the  earth's  crust  as  a  more  or  less 
cylindrical  tube  or  pipe,  which  is  the  main  conduit  or  vent  through 
which  the  lava  reaches  the  surface.  (See  Fig.  87,  p.  143.) 

The   Spine   of   Mont  Pelee   (Figs.    106,   107).  —  We   have,  of 
course,  no  direct  means  of  knowing  from  observation  that  this  tube 


l62 


Structural  Characters  of  Volcanoes 


is  in  reality  a  cylindrical  one,  nor  that  it  penetrates  vertically 
through  the  country  rock  upon  which  the  volcano  is  built.  That  such 
is  the  case,  however,  may  be  inferred  from  the  remarkable  phenome- 
non which  accompanied  the  eruption  of  Mont  Pelee  in  Martinique 


FIG.  106.  —  The  great  "spine"  of  Mont  Pelee,  Martinique,  from  the  east. 
From  the  old  summit  plateau,  the  basin  of  L'Etang  Sec.  The  spine  rises^  ap- 
proximately 358  meters  above  the  old  crater  rim  in  the  middle  foreground. 
(Photo  by  E.  O.  Hovey,  March  25,  1903 ;  courtesy  of  American  Museum  of 
Natural  History.) 

in  1902,  when  a  columnar  mass  of  extremely  viscous  or  solid  lava 
was  pushed  up  700  to  1000  feet  above  the  crater,  reaching  a  height 
of  over  5000  feet  above  the  level  of  the  sea.  This  remarkable 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      163 

column  appears  to  have  been  the  plug  of  lava  which  filled  the  pipe 
of  the  volcano,  and  which  lacked  the  proper  fluidity  to  flow  over 
the  edge  of  the  crater,  but  hardened  in  the  pipe  and  was  pushed 
upward  by  the  pressure  of  the  gases  beneath,  retaining  essentially 
the  form  of  the  tube  in  which  it  had  solidified.  The  growth  of 
this  spine  of  Mont  Pelee  was  actually  witnessed.  It  began  in 
October,  1902,  and  reached  its  maximum  elevation  in  seven  months. 
After  that  it  slowly  crumbled  away  under  the  influence  of  the 


.  FIG.  107.  —  The  spine  and  upper  part  of  the  new  cone  of  Mont  Pelee,  Mar- 
tinique, from  the  north ;  from  the  crater  rim.  (Photo  by  E.  O.  Hovey,  March 
26,  1903 ;  courtesy  of  American  Museum  of  Natural  History.) 

atmosphere  and  its  own  weight,  and  from  the  explosion  of  gases 
beneath  it. 

Plugs  and  Necks.  —  If  we  speak  of  the  tube  which  descends  from 
the  base  of  the  crater  as  the  volcanic  pipe  or  vent,  the  solidification 
of  the  lava  in  this  pipe  forms  the  volcanic  plug.  When  this  plug 
is  pushed  upward  and  becomes  visible,  as  in  Mont  Pelee,  it  con- 
stitutes a  volcanic  spine.  When  it  becomes  visible  as  the  result  of 
the  removal  of  the  enclosing  mass  of  the  rock  material  which 
constituted  the  volcanic  cone,  it  becomes  a  volcanic  neck.  Necks 
are  often  left  as  the  only  relief  feature  of  a  volcano,  because  of  the 


164  Structural  Characters  of  Volcanoes 

solid  nature  of  the  lava  which  has  hardened  in  the  tube  or  pipe, 
and  the  more  readily  erodible  character  of  the  material  which 
constitutes  the  rest  of  the  volcano.  Frequently,  too,  a  cross- 
section  of  a  plug  still  within  the  pipe  is  seen  when  the  country 
has  been  worn  down  until  both  volcano  and  projecting  neck  have 
been  removed.  Again,  in  an  erosion  cliff,  a  vertical  section  of  a 
part  of  such  a  volcanic  plug  may  sometimes  be  seen,  showing 
the  relation  which  it  assumed  to  the  country  rock  on  cooling. 
From  such  sections  the  form  of  the  plug,  and  hence  that  of  the 
original  pipe,  may  be  ascertained.  Theoretical  considerations, 
too,  would  lead  us  to  infer,  first  that  the  upward  path  of  the 
heated  gases,  vapors,  and  lavas  is  most  likely  a  direct  one,  and 
secondly  that  the  passage-way  would  become  a  more  or  less  cy- 
lindrical one,  even  though  it  was  part  of  an  irregular  fissure  in 
the  first  place. 

The  French  experimental  geologist,  Daubree,  was  able  to  show 
that  gases  and  vapors  under  high  pressure,  when  forced  through 
fissures  in  limestone,  granite,  steel,  or  other  substances,  con- 
verted their  passageway  into  a  cylindrical  canal.  From  this  we 
may  argue  that  the  pipe  or  passageway  of  some,  volcanoes  was 
formed  by  the  advancing  heated  gases  and  vapors  which  are 
liberated  from  the  magma  deep  down  in  the  earth,  and  which, 
under  a  pressure  of  thousands  of  atmospheres,  are  forced  to  find 
their  way  to  the  surface  through  fissures,  which  they  enlarge 
to  cylindrical  canals  and  thus  prepare  the  way  for  the  uprising 
lava  masses. 


Volcanic  Necks  and  Exposed  Plugs  of  Old  Volcanoes 

As  we  have  seen,  the  term  volcanic  neck  is  applied  to  the  hardened 
mass  of  lava  which  filled  the  upper  part  of  the  pipe  of  the  volcano 
and  which  has  been  modeled  out  in  relief  by  the  erosion  of  the 
material  of  the  cone  which  formerly  surrounded  it.  This  is  to 
be  distinguished  from  a  volcanic  plug,  which  is  the  mass  of  lava 
hardened  in  the  pipe  which  is  seen  to  be  still  surrounded  by  the 
material  through  which  it  passed,  whether  this  is  the  material  of 
the  cone,  or  the  country  rock  beneath,  on  which  the  volcano  was 
built  up. 

In  districts  of  comparatively  recent  but  extinct  volcanic  activity, 
volcanic  necks  are  not  uncommon.  Some  of  the  finest  examples 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      165 

are  found  in  the  Tertiary  volcanic  district  of  Haute-Loire,  France, 
the  eastern  part  of  the  volcanic  region  of  Auvergne  already  referred 
to.  Here,  near  the  town  of  Le  Puy,  is  the  famous  Rocher  St. 
Michel  (Fig.  108),  an  almost  perfect  example  of  a  volcanic  neck 
from  which  all  surrounding  material  of  the  cone  has  been  removed. 
This  old  neck,  the  position  of  which  is  shown  in  the  section  on 
p.  152  (Fig.  95),  is  not  a  uniform  mass  of  lava,  however,  but  is  rather 


FIG.  108.  —  Rocher  St.  Michel,  in  the  Bassin  du  Puy,  south  central  France. 
The  modeled-out  neck  of  an  extinct  volcano,  now  crowned  by  a  chapel.  For 
location  and  relation  to  other  rocks  of  the  region,  see  the  section,  Fig.  95,  page 
152.  (After  Tempest  Anderson,  Volcanic  Studies  in  Many  Lands.} 


made  up  of  fragments  of  volcanic  rock  bound  together  by  igneous 
material,  and  represents  the  product  of  an  explosive  eruption,  the 
fragments  having  fallen  back  again  into  the  throat  of  the  volcano 
where  they  were  solidified.  Thus  this  neck  represents  the  filling  of 
the  upper  part  of  the  old  volcanic  vent,  and  although  it  no  longer 
shows  the  perfect  original  form  of  this  vent,  having  been  subjected 
to  narrowing  by  erosion  near  the  top,  it  may  still  be  regarded  as 
nearly  typical  of  such  structures.  Its  summit  is  to-day  crowned 
by  a  chapel.  The  neighboring  and  higher  Rocher  Corneille 
is,  however,  an  erosion  remnant  of  a  brecciated  lava  mass, 


1 66  Structural  Characters  of  Volcanoes 

probably  from  this  same  volcano,  and  rests  on  horizontal  strata 
(Oligocene) . 

Typical  examples  of  volcanic  necks  from  our  own  country  are 
the  Leucite  Hills  of  Wyoming,  and  peaks  of  the  Mount  Taylor 
region  of  New  Mexico  (Fig.  109) .  Another  example,  from  Colorado, 
is  shown  in  Fig.  no.  In  diameter  such  necks  may  vary  from  several 
hundred  feet  to  several  miles,  and  the  material  may  be  either  solid 
lava  or  fragments  of  the  same  bound  together,  that  is,  a  breccia 
of  lava  fragments.  When  the  necks  have  been  exposed  to  atmos- 
pheric influences  for  some  time,  especially  in  a  dry  climate,  they 


FIG.  109.  —  Great  Neck,  a  volcanic  neck  in  the  Mount  Taylor  region,  New 
Mexico.     (Photo  by  D.  W.  Johnson.) 

are  apt  to  crumble,  and  a  mass  of  loose  debris  will  accumulate 
around  them,  forming  a  conical  hill  or  tepee  butte,  from  which  the 
summit  of  the  much  reduced  neck  may  project.  Such  a  hill  must 
not  be  mistaken  for  the  original  volcanic  cone  which  surrounded  the 
central  plug  of  lava  before  it  was  modeled  out  in  relief  as  a  neck. 
It  is  merely  a  conical  heap  of  fragmental  material  forming  talus- 
slopes  around  the  central  core,  from  the  destruction  of  which  the 
material  was  derived. 

Not  all  projecting  neck-like  masses  of  volcanic  rock,  however, 
can  be  interpreted  as  old  volcanic  necks.  We  have  already  seen 
that  the  Rocher  Corneille  at  Le  Puy,  France,  is  an  erosion  remnant 
of  a  brecciated  lava  sheet  resting  on  older  rocks,  although  its  form 
is  not  unlike  that  of  the  old  neck  of  the  Rocher  St.  Michel  close  by 
(see  Fig.  95).  The  famous  Devil's  Tower,  or  Mato  Tepee,  of 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      167 

Wyoming  (Fig.  in)  has  long  been,  and  by  some  is  still,  regarded 
as  an  ancient  volcanic  neck,  but  others  consider  it  as  a  tower-like 
erosion  remnant  of  an  old  lava  sheet  intruded  beneath  the  sur- 
face (probably  a  laccolith,  see  beyond).  The  peculiar  columnar 
structure  of  the  rock  of  this  tower  favors  the  last  interpretation, 
as  will  be  more  fully  shown  in  the  next  chapter.  The  determina- 
tion of  the  neck  character  of  such  a  mass  depends,  of  course,  on 
the  possibility  of  showing  that  the  igneous  rock  continues  down- 
ward through  the  rocks  of  the  earth's  crust,  whereas  an  erosion 
remnant  of  a  lava  sheet  or  intruded  mass  would  rest  upon  the 


FIG.  no. —  Conical  butte  formed  by  a  typical  volcanic  plug,  —  a  pillar  of 
basalt  formed  by  cooling  in  the  vent  of  an  extinct  volcano,  and  modeled  out 
in  relief  as  a  neck  by  erosion  of  the  volcanic  material  which  formerly  surrounded 
it.  It  is  now  surrounded  by  basaltic  debris  due  to  weathering,  and  this  forms 
talus-slopes.  One  mile  north  of  Adair  station,  Colorado  and  Southern  R.  R. 
Elmoro  quadrangle.  Colorado.  (G.  W.  Stose,  photo  from  U.  S.  G.  S.) 


rocks  of  the  crust  which  are  continuous  beneath  it,  and  would 
not  necessarily  have  direct  connection  with  the  deeper  parts  of 
the  earth. 

Volcanic  plugs,  i.e.,  the  hardened  lava  which  still  fills  the  old 
pipes,  are  exposed  in  many  regions  in  horizontal  or  in  vertical 
sections  as  the  result  of  erosion.  Practically  all  of  these  are  found, 
not  within  the  old  volcanic  cone,  for  sections  of  such  cones  are 
seldom  if  ever  exposed,  but  in  the  rock  beneath,  upon  which  the 
now  vanished  volcano  had  been  built.  They  therefore  represent 
the  lower  part  of  the  filling  of  the  old  volcanic  pipe  or  conduit, 
and  those  that  are  exposed  belong  to  the  older  volcanoes  of  the 


i68 


Structural  Characters  of  Volcanoes 


world.  The  presence  of  the  solid  lava  plug  is  of  course  inferred 
in  all  uneroded  volcanoes,  and  indeed  it  is  a  necessary  part  of  an 
extinct  example.  .  The  part  it  takes  in  the  determination  of  a 
volcano-like  hill  as  an  extinct  volcano  is  illustrated  by  the  classical 
case  of  the  Kammerbiihl,  a  small  hill  in  northern  Bohemia,  which 
played  a  leading  role  in  the  days  when  geologists  still  discussed 
the  question  of  the  origin  of  beds  of  basalt,  —  one  group,  the  follow- 


FIG.  in.  —  Mato  Tepee,  Devil's  Tower,  Wyoming,  a  supposed  volcanic  neck 
showing  vertical  columnar  structure.     (Photo  by  N.  H.  Barton,  U.  S.  G.  S.) 


ers  of  Werner,  holding  that  this  rock  was  precipitated  from  water, 
while  another  group  argued  for  its  volcanic  origin. 

The  hill  in  question  (Fig.  112  a)  is  composed  mainly  of  cinders  and 
altered  sediments,  with  a  small  basalt  stream  on  one  side.  Werner 
and  his  followers  contended  that  the  material  of  this  hill  and  others 
like  it  was  formed  by  the  combustion  of  beds  of  coal,  which  had 
not  only  burned  the  older  slates  and  the  younger  rock  material, 
but  had  also  in  part  melted  a  layer  of  basalt  of  aqueous  origin,  and 
so  produced  the  cinders.  The  poet  Goethe,  however,  believed 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      169 


this  hill  to  be  an  extinct  volcano,  and  argued  that  if  a  tunnel  were 
driven  into  it  the  hardened  plug  of  lava  in  the  old  pipe  would  be 
encountered  near  its  center.  To  prove  the  correctness  of  the  view 
of  the  poet-naturalist, 
his  friend  Count  Cas- 
par von  Sternberg 
had  the  tunnel  driven 
in  1837,  with  the  re- 
sult that  the  volcanic 
plug  was  found  and  its 
connection  with  the 
basalt  layer  proved 
(Fig.  1126).  This 
virtually  ended  the 
controversy  about  the 


FIG.  112  a.- 


The  Kammerbiihl,  an  old  volcanic 

hill  in  Bohemia, 
origin  of  basalt. 

Sometimes  old  lava  plugs  may  be  modeled  out  in  relief  by  the 
erosion  of  the  older  sediments  which  were  penetrated  by  the  vol- 
canic pipe.  In  such  a  case  the  resulting  hill  is  essentially  a  neck, 
not  readily  distinguishable,  except  perhaps  in  the  material  of 
which  it  consists,  from  necks  produced  by  the  removal  of  the 
volcanic  cone  only.  An  example  of  such  an  older  neck  is  found 
in  Edinburgh,  Scotland,  where  it  forms  the  famous  peak  known 
as  Arthur's  Seat,  near  Holyrood  Castle.  The  material  of  this 
peak  consists  of  hardened  basaltic  lava  and  fragmental  rock  in 
which  not  infrequently  are  found  pieces  of  wood  from  the  trees 

which  clothed  the 
slopes  of  the  ancient 
volcano.  These 
wood  fragments, 
now  silicified,  were 
buried  in  the  frag- 
mental material 
which  filled  the  old 
throat  of  the  vol- 
cano, and  which 
probably  extends  to  some  depth  because  of  the  sinking  of  the  old 
lava  plug  on  solidifying. 

Sections  of  volcanic  plugs  in  the  older  rocks  are  often  exposed 
by  erosion.  An  illustration  of  one  such  from  the  coast  of  Ireland 


FIG.  112  b.  —  Section  of  the  Kammerbiihl,  show- 
ing the  probable  former  outline  of  the  volcano  and 
the  old  volcanic  plug,  a,  metamorphic  rocks; 
b,  basaltic  scoriae;  c,  plug  of  basalt;  d,  stream  of 
basalt ;  e,  alluvial  beds. 


170 


Structural  Characters  of  Volcanoes 


— Chatk 


is  here  given  in  Fig.  113  a,  and  a  second,  still  joined  to  the  basaltic 
lava  sheet,  in  Fig.  113  b.     The  basaltic  lava  penetrates  the  chalk, 
which  was  altered  to  marble  by  the  heat  of  the  basalt.     It  is  prob- 
able that  these  are 
not  the  plugs  filling 
the  pipes  of  sepa- 
rate volcanoes,  but 
that  they  represent 
fissures     through 

FIG.  1 13  a.  — Section  of  volcanic  plug  (basalt)  in     wnicn  lava  reached 
chalk.     Coast  of  Antrim,  Ireland.     (After  Geikie.)       the  surface   in    the 

form     of    a    great 

sheet,  as  described  later.  A  similar  plug  may  be  seen  on  the 
Nova  Scotia  coast  at  Wasson's  Bluff,  not  far  from  Parrsborough. 
Here  the  volcanic  material  penetrates  old  sandstones  and  gypsum 


A    A    A    A    A    A   A 


A    A    A    / 

Basalt 


FIG.  1136.  —  Marmorization  of  chalk  beds  by  basalt.  Island  of  Rathlin 
on  coast  of  Antrim,  Ireland.  (Leonard.)  The  marble  is  dotted.  (From 
Kayser's  Lehrbuch.) 

beds  of  late  Palaeozoic  age,  and  belongs  to  the  eruptions  of  Triassic 
time  which  produced  the  great  lava  sheet  of  Cape  Blomidon  and 
adjoining  regions. 

Sections  of  old  volcanic  plugs  are 
abundant  in  some  districts.  Thus  on 
the  famous  shore  of  County  Fife  in 
Scotland,  no  fewer  than  eighty  are 
found  in  a  space  twelve  miles  in  length 
by  from  six  to  eight  in  width.  These 
plugs  (Fig.  114)  penetrate  sandstones, 
shales,  limestones,  and  coal  beds,  and  FIG.  114.  —  Ground-plan 

are  consequently  of  more  recent  date     section  of  the  Plus  of  an  old 

'  .    ,       ,      .  ,  .  ,      Al  volcano  on  the  shore  of   St. 

than  the  periods   during  which   these     Monans>      Fife>      Scotland. 

sediments   were  deposited   (Mississip-     (After  Geikie.) 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      171 


plan,  see  Chapter  XXXV I).  The  sections 
of  the  plugs  are  roughly  circular  or  ellip- 
tical, varying  in  size,  and  from  them  dikes 
and  sheets  of  igneous  material  not  infre- 
quently penetrate  the  adjoining  rocks,  after 
the  manner  of  the  radiating  dikes  seen 
in  modern  volcanoes,  as  already  described. 
The  plugs  of  Fife  are  not  placed  along 
lines  of  great  fissures,  as  might  be  sup- 
posed, but  penetrate  the  rocks  apparently 
without  any  relationship  to  its  structure. 
That  necks  are,  however,  ranged  along 
great  lines  of  fissures  in  the  earth's  crust 
is  frequently  found,  and  is  seen  in  the 
alignment  of  some  of  the  necks  of  the 
Auvergne  region  in  France  (see  ante)  and 
elsewhere,  and  may  be  inferred  from  the 
arrangement  of  the  volcanoes  of  Iceland. 
Here  the  lava  first  welled  up  through  a 
recognizable  fissure  in  the  earth's  crust, 
which  subsequently  was  partly  closed  by 
the  hardening  of  the  lava,  so  that  only  the 
broader  portions  remained  open.  These 
were  converted  into  volcanic  pipes,  above 
which  individual  volcanoes  were  built  up 
(Fig.  115).  The  Japanese  volcanoes,  too, 
seem  to  be  arranged  along  great  lines  of 
fissures,  and  this  is  probably  true  for  the 
majority  of  existing  volcanoes,  as  is  in- 
dicated by  their  linear  arrangement  (Fig. 
116). 

The  material  filling  the  volcanic  vents, 
and  .  therefore  the  material  of  necks  and 
plugs,  ranges  from  basalt  to  rhyolite,  ac- 
cording to  the  acidity  of  the  volcanic 
mass.  When  deeper  portions  of  the  necks 
are  exposed  they  show  rocks  of  coarser 
crystalline  type,  even  gabbro  and  grano- 
phyre,  which  have  resulted  from  slow 
cooling.  In  other  cases  fragmental  ma- 


/.u'&. 


172 


Structural  Characters  of  Volcanoes 


LO  S    ALTOS 

(deMatagalpa) 


S\j 


F.IG.  1 1 6.  —  Volcanoes  in  Nicaragua,  showing  linear  arrangement  apparently 
along  a  fissure  line.  This  line  is  parallel  to  the  trend  of  the  mountains  and 
ridges,  and  to  the  coast  line.  (After  Karl  von  Seebach,  from  F.  Ratzel,  Die 
Erde.) 


Miocene  Volcano 
==•=== 


GrPheuttaL 
Fault 


FIG.  117.  —  Diagrammatic  section  through  the  Freiburg  region  (i.  Br.) 
showing  the  former  and  present  topography  and  the  extensive  erosion.  (After 
Th.  Lorenz,  from  Kayser's  Lehrbuch.) 


Volcanic  Funnels,  Pipes,  Plugs,  and  Necks      173 

terial  (agglomerate  and  tuff)  fills  the  ancient  vent,  as  in  the  case 
of  the  Nova  Scotia  plug  already  cited.  A  mixture  of  both  may 
occur,  and  the  structure  may  show  evidence  of  successive  erup- 
tions through  the  same  vent,  and  the  formation  of  a  cylinder  of 
lava  within  a  cylinder.  Finally,  as  already  noted,  the  lava  may 
enclose  fragments  of  the  wall  rock,  generally  from  a  deeper  hori- 
zon. The  relationship  of  a  plug  to  the  old  volcano  in  a  much- 
eroded  region  is  shown  in  the  section  on  the  preceding  page 
(Fig.  117). 


FIG.  1 1 8.  —  Diamond  mine  in  old  volcanic  plug,  South  Africa. 

In  South  Africa,  old  volcanic  plugs  have  become  of  great  eco- 
nomic importance,  for  it  is  in  them  that  the  great  diamond  mines 
are  located.  These  plugs,  which  are  often  of  funnel  form,  broaden- 
ing upward  in  diameter  to  300  and  in  exceptional  cases  to  685 
meters,  are  composed  of  brecciated  rock,  the  so-called  "  blue 
ground"  which  contains  the  gems  (Fig.  118).  Whether  the  dia- 
monds represent  the  crystallized  carbon  of  wood  included  in  the 
plugs  as  in  those  of  Scotland,  or  whether  the  carbon  is  of  sub- 
terranean origin,  is  an  open  question.  The  character  of  the  dia- 
mond-bearing material  suggests  that  it  may  have  risen  from  great 
depths.  ** 


174 


Structural  Characters  of  Volcanoes 


SHEET  LAVAS  FORMED  BY  FISSURE  ERUPTION 

Not  all  lava  reaches  the  surface  through  volcanic  pipes  above 
which  volcanoes  are  built.     Indeed,  we  have  seen  that   some  of 

these  pipes,  as  in  the 
Icelandic  volcanoes,  are 
merely  the  reduced  ex- 
pression of  large  fissures 
which  opened  in  the 
earth  and  through  which 
the  lava  at  first  reached 
the  surface,  after  which 
volcanoes  were  built  up 
on  those  spots  where 
the  fissure  became 
broken  up  into  discon- 
nected pipes.  In  some 


L 


UTAH 


FIG.  119  a.  —  Map  of  the  Columbia  and  Snake 
River  lava  fields.     (After  Bowman.) 


of  the  grandest  erup- 
tions of  molten  rock,  — 
generally  basalt,  —  no  volcanoes  are  built  up,  or  at  least  not  until 
long  after  the  eruption  begins.  Instead,  great  flat  surfaces  of  lava 


FIG.  119  b.  —  "Gordon  Craters,"  Malheur  County,  S.  E.  Oregon.  Irregu- 
larities in  surface  of  recent  lava,  produced  by  pressure.  (Photo  by  Russell, 
U.  S.  G.  S.  Courtesy  of  D.  W.  Johnson.) 


Sheet  Lavas  Formed  by  Fissure  Eruption       175 


—  lava  plains  or  plateaus  —  are  produced.  Fissure  eruption,  ^in 
which  the  lava  wells  up  along  the  entire  length  of  the  fissure,  is 
seen  to-day  in  Iceland.  Along  the  great  Eld  Cleft,  lava  has 
welled  up  in  historic  times,  spreading  on  both  sides  of  the  fissure 
to  form  a  flat  basalt  plain,  270  square  miles  in  extent.  Along  the 
southern  prolongation  of  the  fissure,  however,  where  it  was  nar- 
rower, a  row  of  low 
cones  has  been 
formed,  through  iso- 
lation of  a  series  of 
vents. 

One  of  the  largest 
lava  plains  of  this 
type  is  the  Columbia 
and  Snake  River 
plain,  which  covers 
an  area  of  200,000 
to  250,000  square 
miles  in  Washing- 
ton, Oregon,  Idaho, 
and  California  (Fig. 
1 19  a).  The  lava 
apparently  spread 
over  a  region  of 
varying  topography, 
filling  the  valleys 


FIG.  IIQC.  —  Canon  in    the   lava   of    the    Snake 
River  plateau. 


and  burying  the 
smaller  hills,  sur- 
rounding the  larger 

ones,  and  penetrating  the  valleys  in  their  sides.  A  succession  of 
outpourings  occurred,  sometimes  at  short  intervals,  sometimes  long 
enough  apart  for  the  production,  by  weathering,  of  old  soil  beds 
upon  the  preceding  sheets,  which  were  occasionally  covered  with 
forests  before  the  next  lava  sheet  was  poured  out.  The  present 
surface  of  this  lava  plain  is  in  many  places  barren  and  desolate 
beyond  description,  but  in  some  places  low  cones  rise  above  it, 
formed  probably  during  the  later  stages  of  volcanic  activity,  or  the 
surface  is  broken  by  pressure  (Fig.  ngb).  Where  canons  are  cut 
by  rivers,  the  nature  of  the  material  is  well  shown  (Fig.  119  c). 
Another  extensive  flat  lava  plain  formed  by  fissure  eruption 


i76 


Structural  Characters  of  Volcanoes 


covers  Central  India,  and  forms  what  is  known  as  the  Dekkan 
Plateau.  This  has  a  present  area  of  some  200,000  square  miles,  but 
probably  was  originally  much  larger.  The  thickness  of  the  basalt 
sheet,  generally  spoken  of  as  the  Dekkan  Trap,  is  in  places  more 
than  a  mile.  No  cones  have  been  built  upon  its  surface. 

Remnants  of  an  equally  extensive  lava  sheet  of  this  type  are  found 
in  western  Europe,  where  they  occur  on  the  west  coast  of  Ireland 
(Giant's  Causeway,  etc.),  Scotland,  Staffa,  the  Orkney  and  Faroe 
Islands,  and  perhaps  Iceland.  The  active  work  of  the  sea,  aided 
no  doubt  by  the  weather,  has  removed  large  parts  of  this  plateau, 


FIG.  120.  —  Basaltic  columns  of  the  Giant's  Causeway,  on  the  coast  of  Ireland. 

while  others  have  probably  subsided  beneath  the  sea-level  by 
recent  disturbances  of  that  part  of  the  earth's  crust.  This  lava 
sheet,  too,  is  composite,  and  beds  with  plant  remains,  from  which 
the  age  of  the  eruption  can  be  ascertained,  lie  between  successive 
flows. 

Remnants  of  much  older  (Triassic  or  Jurassic)  lava  flows  probably 
of  this  type  are  found  in  New  Jersey  (Watchung  Mountains),  in 
the  Connecticut  Valley  of  Connecticut  and  Massachusetts,  and 
along  the  shores  of  the  Bay  of  Fundy  in  Nova  Scotia.  In  New 
Jersey  and  elsewhere  the  peculiar  character  of  the  base  of  the 
sheet  sometimes  indicates  that  the  lava  overflowed  bodies  of 


Sheet  Lavas  Formed  by  Fissure  Eruption      177 

standing  water,  with  the  result  that  steam  pipes  and  a  comminuted 
basal  structure  were  produced,  the  mud  from  the  bottom  of  the 
lake  or  pond  being  forced  up  into  the  lava.  In  cavities  thus  pro- 
duced many  famous  mineral  deposits  (zeolites,  etc.)  were  subse- 
quently formed.  This  structure  is  also  found  in  lava  sheets  of 
basic  volcanoes. 

A  characteristic  feature  of  many  of  these  basaltic  sheets  is  the 
development  of  prismatic  columns,  which  generally  stand  vertical, 


FIG.  121.  —  Near  view  of  a  portion  of  the  columnar  basalt  of  the  Giant's 
Causeway,  coast  of  Ireland.  The  upper  surfaces  of  the  transverse  joints  are 
seen  to  be  convex  in  most  of  the  columns,  but  in  a  few  cases  they  are  concave, 
holding  small  pools  of  water. 

as  so  finely  shown  in  the  Giant's  Causeway  on  the  north  coast  of 
Ireland  (Figs.  120,  121).  Sometimes,  however,  these  columns  are 
curved,  as  is  seen  in  some  parts  of  the  island  of  Stafla  off  the  Scot- 
tish coast,  while  in  other  parts  they  are  vertical  as  at  the  entrance 
to  Fingal's  Cave  (Figs.  122  a-d).  This  columnar  or  basaltic  joint- 
ing is  also  seen  in  some  of  the  basalt  sheets  of  the  Columbia  lava 
plateau  and  the  basalt  of  the  Watchung  Mountains,  and  is  in- 
deed a  fairly  common  structure  in  basaltic  lavas.  The  name  joint 
is  unfortunate,  as  these  structures  have  little  in  common  with 
the  true  joints,  which  are  fractures  in  the  earth's  crust,  and  of 


i78 


Structural  Characters  of  Volcanoes 


FIG.  122  a. —  General    view   of    the  FIG.  122  b.  —  The  island  of  Staffa, 

Island  of  Staffa  from  the  sea.     Note  showing  the   basalt  columns    capped 

the  columnar  basalt  capped  by  massive  by  massive  basalt  on  the  left  and  an 

basalt.     Entrance     to    Fingal's    Cave  eroded  surface  of  basalt  columns  on 

near  the  center  of  the  view.     (Photo  the   right.      Curved    columns   in   the 


by  author.) 


distance.     (Photo  by  author.) 


FIG.  122  c.  —  Fingal's  Cave  on  the  island  of  Staffa,  showing  the  columnar 
jointing.  This  is  a  sea-cave,  eroded  by  the  waves  which  remove  the  columns. 
The  floor  of  the  cave  is  covered  by  sea-water  even  at  low  tide,  though  a  nar- 
row path  has  been  constructed  on  one  side.  The-  roof  of  the  cave  is  formed 
by  the  non -columnar  capping-mass  of  basalt,  as  is  shown  in  Fig.  122  d.  (See 
further:  Chapter  XXIII,  and  Figs.  720,  721.)  (Photo  by  author.) 


Sheet  Lavas  Formed  by  Fissure  Eruption      179 

secondary  origin.     The  columnar  structure,  on  the  other  hand, 
is  a  primary  structure,  and  is  caused  by  a  radial  contraction  of  the 


FIG.  122  d. —  Wall  and f  roof  of  Fingal's  Cave,  Staffa,  showing  the  cross- 
jointing  of  the  columnar  basalt,  and  the  irregular  appearance  of  the  massive 
basalt  which  forms  the  roof  of  the  cave.  (Photo  by  author.) 

cooling  lava  mass  about  a  series  of  equally  spaced  centers  (Fig.  123). 
As  a  result  six-sided  prisms  are  produced.  A  characteristic  feature 
of  these  prisms  is  the  curved  form  of  the  horizontal  or  transverse 


FIG.  123.  —  Diagram  illustrating  the  formation  of  contraction  prisms. 
The  centers  of  attraction  are  connected  by  solid  lines.  The  prisms  formed  are 
dotted.  (From  Grabau,  Principles  of  Stratigraphy.) 

planes,  which  generally  separate  them  into  component  blocks 
(Figs.  121,  122  d).  Prismatic  structure,  though  best  developed 
in  basalt,  is  not  confined  to  this  rock,  but  occurs  even  in  such  acid 
lavas  as  obsidian  (Fig.  124).  The  position  of  the  columns  is,  in 


i  So 


Structural  Characters  of  Volcanoes 


general,  at  right  angles  to  the  surface  of  the  mass  in  which  they 
are  developed,  while  diverging  and  curved  columns  are  often  de- 
veloped just  below  the  surface  of  the  lava  sheet.  Examples  of 


FIG.  124.  —  Columnar  jointing  in  obsidian,  Obsidian  Cliff,  Yellowstone 
National  Park.     (U.  S.  G.  S.) 


such  curved  columns  are  seen,  as  before  noted,  in  parts  of  the 
island  of  Staffa  and  in  parts  of  the  basalt  sheets  of  the  Watchung 
Mountains,  New  Jersey. 


Minor  Phenomena 


181 


MINOR  PHENOMENA   GENERALLY  ASSOCIATED  WITH  CLOSING 
STAGES  OF  VOLCANICITY 

There  are  a  number  of  igneous  phenomena  which  are  manifested 
upon  the  surface  of  the  earth  to-day,  which  are  regarded  as  pri- 
marily associated  with  the  dying  stages  of  volcanicity.  These  may 
be  enumerated  in  the 
order  of  their  importance 
as  follows : 

a.  Solfataric  action. 

b.  Fumarolic  activity. 

c.  Mofettes  and  effer- 
vescent springs. 

d.  Mud-volcanoes. 

e.  Geysers. 

/.   Hot  springs. 

Solfataric  action.  — 
The  volcano  of  Solfatara 
(Fig.  125),  in  the  Phle- 
graean  fields  near  Naples, 
represents  a  dying  stage 
in  volcanicity,  giving  off 
steam  and  gases  only, 
since  its  last  eruption  in 
1198.  The  crater  of  Sol- 
fatara is  very  wide,  but 
its  walls  are  only  about 
100  feet  high,  while  its 
floor  is  marshy  and  salt- 
encrusted,  with  occa- 
sional pools  of  boiling 

water.  On  one  side,  at  the  foot  of  the  crater  wall,  a  jet  of  steam 
escapes  from  an  opening  and  rises  to  a  height  of  6  or  7  yards.  In 
Iceland,  Java,  New  Zealand,  the  Andes,  and  elsewhere,  such 
solfataric  vents,  as  they  are  called,  are  common,  indicating  the 
dying  condition  of  those  particular  volcanoes.  Besides  the  steam 
many  other  gases  are  given  off,  these  including  hydrochloric  acid 
gas,  sulphur  dioxide,  sulphureted  hydrogen,  ammonium  chloride, 
and,  in  the  case  of  the  Italian  Saffioni,  boracic  acid,  which  forms 
the  source  of  the  important  borax  industry  of  Tuscany. 


FIG.  125.  —  The  Solfatara  at  Pozzuoli,  Italy. 
The  steam  and  gases  issue  in  one  corner  of 
the  nearly  extinct  volcano. 


182 


Structural  Characters  of  Volcanoes 


Fumaroles.  —  These  are  typically  emanations  of  steam,  from 
fissures,  or  from  cooling  lava  surfaces,  but  they  also  vary,  espe- 
cially with  the  decrease  in  temperature  of  the  lava  or  other  source. 
Above  350°  C.  only  dry  fumaroles  exist,  these  giving  off  chiefly 
anhydrous  chlorides.  Between  this  and  a  temperature  of  100°  C. 
fall  the  acid  fumaroles,  which  give  off  hydrochloric  acid  and  sulphur 
dioxide,  with  some  steam.  At  about  100°  C.  occur  the  alkaline 
fumaroles,  which  give  off  steam  and  ammonium  chloride,  and 


FIG.  126.  —  Active  mud- volcanoes  near  Volcano  Lake,  Cerro  Prieto,  Delta 
of  the  Rio  Colorado.  The  largest  of  the  group,  seen  in  the  distance,  is  now 
quiescent.  (Courtesy  of  the  American  Geographical  Society,  Broadway  at 
157  St.,  New  York.  From  the  Geographical  Review.}  (For  location,  see  map, 
Fig.  166.) 

below  100°  C.  fall  the  cold  fumaroles  with  nearly  pure  steam. 
The  famous  valley  of  Ten  Thousand  Smokes,  in  the  Katmai  Penin- 
sula of  Alaska,  is  a  prominent  example  of  fumarolic  action  on  a 
large  scale. 

Mofettes.  —  These  give  off  only  carbon  dioxide,  nitrogen,  and 
oxygen  at  the  temperature  of  the  atmosphere.  Caves  in  which 
such  gases  are  given  off  exist  in  many  volcanic  districts ;  and  they 
are  sometimes  exhibited  to  tourists  with  the  lowering  into  them  of 
dogs  or  other  luckless  animals,  which  quickly  become  unconscious, 


Minor  Phenomena 


183 


after  which  they  are  drawn  up  and  revived.  The  carbon  dioxide, 
on  account  of  its  weight,  remains  near  the  bottom  of  the  cave. 
Effervescent  springs,  i.e.  water  charged  with  CO2,  occur  in  many  old 
volcanic  regions. 

Mud-Volcanoes.  —  Among  the  subordinate  phenomena  asso- 
ciated with  or  compa^ble  to  igneous  activities,  is  the  formation  of 
mud-volcanoes  (Fig.  126).  These  are  cones  built  up  of  mud  with 
small  craters  at  the  summit,  resembling  miniature  volcanoes. 
Their  height  varies  from  a  few  feet  to  a  hundred  feet  or  more, 


FIG.  127.  —  The  Great  Geyser  Basin  of  the  Madison  River  in  Yellowstone 
National  Park.     (Guyot.) 

and  their  activity  is  either  constant  or  intermittent,  quiet  or  ex- 
plosive. They  are  formed  by  highly  heated  steam  or  by  gases, 
which  rise  through  a  superficial  layer  of  mud  and  originate  from 
underlying  lava  beds  or  from  chemical  reaction.  The  mud  is  built 
up  into  cones  of  the  volcanic  type.  As  the  material  is  soft,  how- 
ever, these  cones  are  readily  destroyed  by  rains,  etc. 

Examples  are  known  from  the  Colorado  Desert,  from  lower  Cali- 
fornia, the  Yellowstone  Park  (the  Paint  Pots)  and  elsewhere  in 
this  country.  They  also  occur  at  Baku  on  the  Caspian  Sea  and  in 
the  Crimea,  where  some  of  them  rise  to  250  feet  in  height,  and 
are  provided  with  apical  craters. 

Geysers  and  Hot  Springs.  —  Geysers  are  springs  of  hot  water 
which  erupt  violently  at  intervals,  with  periods  of  quiescence  be- 
tween. They  have  essentially  the  same  relation  to  ordinary  hot 


184 


Structural  Characters  of  Volcanoes 


springs  that  volcanoes  of  intermittent  explosive  eruptions  have  to 
those  of  quiet  and  constant  lava  flows.  Geysers  are  abundant  in 
Iceland,  in  New  Zealand,  and  in  the  Yellowstone  Park,  where  there 

are  about  100  active  examples 
and  more  than  3000  hot 
springs  (Fig.  127).  A  geyser 
consist^  typically  of  a  more 
or  less  circular  basin  sur- 
rounded by  a  rim  of  silicious 
material  (geyserite),  and  this 
commonly  forms  the  upper 
part  of  a  cone  comparable  to  a 
volcanic  cone  and  its  crater. 
From  the  center  of  the  basin 
a  pipe  of  circular  section  and 
with  smooth  wall  descends, 
this  and  the  basin  being  filled 
with  water.  In  the  typical 
Great  Geyser  of  Iceland,  the 
cone  is  about  120  feet  in  di- 
ameter and  13  feet  high,  while 
the  crater-like  basin  at  the 
top  is  60  feet  in  diameter  and 
5  feet  deep.  The  central 
tube  or  pipe  has  a  diameter 
of  about  10  feet,  with  smooth, 
cylindrical,  vertical  walls. 
The  temperature  of  the  water 
which  fills  the  pipe  and  basin 

ranges  from  75°  to  90°  C.,  but  at  a  depth  of  70  feet  in  the  pipe 
the  temperature  is  about  130°  C.  The  eruption  occurs  at  almost 
twenty-four  hour  intervals,  and  the  water  of  the  basin  is  thrown 
to  a  height  of  nearly  100  feet.  The  characters  of  the  other  geysers 
are  similar,  though  differing  in  detail.  The  appearance  of  the 
Giant  Geyser  of  the  Yellowstone  is  shown  in  Fig.  128,  and  of  Old 
Faithful  in  Fig.  129.  Diagrammatic  sections  of  the  two  main 
Icelandic  examples,  "  The  Geyser "  and  "  Strokr,"  are  given  in 
Fig.  130. 

The  eruption  is  due  to  the  heating,  above  the  boiling  point,  of  the 
water  at  a  depth,  while  at  the  same  time  the  pressure  of  the  column 


FIG.  128.  —  Giant  Geyser  of  the  Yellow- 
stone. (After  Hayden.)  The  eruptions 
of  this  geyser  occur  at  intervals  of  7  to  12 
days,  and  last  for  full  60  minutes  at 
each  eruption.  The  column  is  thrown  to 
heights  of  200  to  250  feet. 


Minor  Phenomena  185 

of  water  keeps  it  from  changing  to  steam.  With  increased  heating, 
however,  the  point  is  reached  where  the  water  will  change  to  steam 
in  spite  of  the  pressure,  and  the  expansion  of  the  steam  raises  the 


FIG.  129.  —  Old  Faithful  Geyser  in  eruption,  Yellowstone  National  Park. 
(Photo  by  D.  W.  Johnson.)  The  eruptions  of  this  geyser  occur  at  intervals  of 
60  to  75  minutes,  each  eruption  lasting  4  minutes.  The  height  to  which  the 
column  is  thrown  is  125  to  150  feet.  The  eruptions  are  heralded  by  loud 
rumblings,  with  spasmodic  outbursts  of  jets  10  to  20  feet  in  height,  then  the 
column  is  suddenly  thrown  up  with  a  loud  roar,  maintaining  a  height  varying 
from  90  to  150  feet  for  two  or  three  minutes  with  occasional  steeple-shaped 
jets  rising  still  higher,  the  jets  varying,  and  giving  off  great  rolling  clouds  of 
steam;  then  the  jets  gradually  decrease  in  altitude,  and  in  five  minutes  the 
eruption  is  over,  the  tube  apparently  empty  but  emitting  occasional  puffs  of 
steam  for  a  few  minutes  longer.  During  the  eruption  the  water  falls  in  heavy 
masses  about  the  vent,  filling  the  basins  surrounding  it  and  running  off  in  all 
directions.  The  estimated  discharge  is  3000  barrels  at  each  eruption. 

column  of  water,  and  so  lowers  the  pressure,  with  the  result  that  a 
large  amount  of  steam  is  suddenly  formed  from  the  super-heated 
water,  and  the  eruption  takes  place.  The  source  of  the  water  is 
believed  by  some  to  be  the  ground-water,  and  by  others  it  is  re- 
garded as  new  or  juvenile  water,  given  off  by  subterranean  igneous 
masses  in  the  process  of  cooling.  The  heat  is  probably  in  all  cases 
of  volcanic  origin.  The  geysers  of  Iceland  and  New  Zealand  are 
situated  in  regions  of  still  active  volcanicity,  while  those  of  the 


i86 


Structural  Characters  of  Volcanoes 


Observed  Temp. 


Strokr 


FIG.  130.  —  Semidiagrammatic  sections  of  the  Icelandic  geysers,  "The 
Geyser"  (the  type)  and  "Strokr."  Strokr  has  a  funnel-like  pit  36  feet  deep 
and  8  feet  across,  expanding  into  a  saucer-like  basin.  The  tube  is  generally 
filled  to  within  6  feet  of  the  top  with  clear  water,  which  boils  furiously,  owing 
to  the  escape  of  great  bubbles  of  steam  coming  from  two  openings  in  opposite 
sides  of  the  tube.  When  the  eruption  took  place,  the  jets  rose  in  a  sheaf-like 
column  to  a  height  of  100  or  more  feet.  Eruptions  took  place  at  very  irregular 
and  long  intervals;  but  they  could  be  produced  in  a  short  time  by  "putting  a 
lid  on  the  great  kettle,"  by  dumping  in  large  pieces  of  turf.  "The  Geyser" 
is  a  pool  of  limpid  green  water,  whose  surface  rises  and  falls  in  rhythmic  pulsa- 
tions. The  usual  temperature  is  170°  F.  (76.6°  C.)  or  200°  F.  (93.3°  C.)  but 
varies,  being  greater  immediately  before  eruption.  The  shallow  saucer-like 
basin  is  about  60  feet  across  and  slopes  gently  to  a  cylindrical  shaft  10  feet 
in  diameter,  forming  the  pipe  of  the  geyser;  this  being  about  70  feet  deep. 
The  tube  is  very  regular.  Before  an  eruption,  bubbles  of  steam  entering  the 
tube  suddenly  collapse  with  loud  but  muffled  reports  and  a  disturbance  of  the 
quiet  surface  of  the  water.  During  this  "simmering,"  the  water  rises  in  dome- 
like mounds  over  the  pipe  and  overflows  the  basin,  running  down  the  terraced 
slope  and  wetting  the  cauliflower-like  forms  of  sinter  that  adorn  it.  Domes  of 
water  rise  in  quick  succession,  and  finally  burst  into  play,  followed  by  a  rapid 
succession  of  jets  increasing  in  height  until  the  column  is  90  to  100  feet  high, 
accompanied  by  dense  clouds  of  steam.  This  lasts  a  few  moments  and  then 
ceases,  and  the  basin  is  empty  and  apparently  lined  with  a  smooth  coating  of 
white  silica.  The  great  geysers  of  the  Yellowstone  surpass  this  in  magnitude 
of  eruption,  but  not  in  beauty.  (Weed.) 


Minor  Phenomena 


187 


Yellowstone  are  in  a  region  where  volcanoes  have  recently  become 
extinct. 

The  hot  waters  commonly  carry  silica  is  solution,  and  this  is 
deposited  partly  by  the  cooling  and  partly  by  the  aid  of  low  organ- 
isms, especially  algae,  and  so  the  cone  of  silicious  sinter  is  built  up 
(Fig.  131).  The  quiet  flowing  hot  springs  more  commonly  deposit 
carbonate  of  lime.  These  deposits  will  be  discussed  in  a  subsequent 
chapter. 

Geysers  are  commonly  affected  by  earthquakes  which  disturb 
the  arrangement  of  the  rocks  of  the  region.  The  Icelandic  geyser 


FIG.  131.  —  Spike  Geyser,  Witch  Creek,  Yellowstone  Park.  Showing  an 
exceptionally  fine  mass  of  silicious  sinter  (geyserite)  built  up  around  the  basin. 
The  sinter  shows  botryoidal  surfaces.  (Photo  J.  P.  Iddings,  from  U.  S.  G.  S.) 

Strokr  is  said  to  have  come  into  existence  during  the  earthquake 
of  1789,  and  the  earthquake  of  1896  put  an  end  to  its  activity. 
It  has  not  been  in  eruption  since.  A  new  geyser  or  hot  spring  ap- 
peared after  the  shocks  of  the  first  day  (Sept.  15)  of  the  earth- 
quake of  1896,  this  spring  throwing  water  and  rocks  to  an  estimated 
height  of  600  feet,  but  in  a  few  hours  it  subsided  to  a  height  of  10 
or  12  feet.  It  ceased  to  flow  altogether  after  ten  days. 


CHAPTER  IX 

FORM   AND   STRUCTURE   OF   OLDER  IGNEOUS 

MASSES 

TYPES  OF  OLDER  IGNEOUS  MASSES 

LEAVING  now  modern  volcanoes  and  those  which  have  so  recently 
become  extinct  that  their  characteristics  can  still  be  readily  rec- 
ognized even  though  modified  by  erosion,  we  must  next  turn  our 
attention  to  those  rock  masses  and  structures  of  igneous  origin 
which  were  formed  at  a  depth  in  the  earth's  crust,  and  have  become 
visible  only  as  the  result  of  erosion  of  the  overlying  rocks,  or  through 
dislocation  of  the  earth's  crust.  These  may  be  grouped  under  the 
following  divisions :  i.  Dikes;  2.  Stocks;  3.  Sills;  4.  Laccoliths 
and  related  types ;  5.  Bysmaliths;  6.  Bosses;  7.  Batholiths. 

The  first  five  groups  are  classed  as  intrusive  or  hypabyssal  masses, 
to  which  group  the  volcanic  plugs  may  also  be  referred,  especially 
that  part  which  penetrates  the  older  rocks  beneath  the  volcano. 
Bosses  and  batholiths  are  classed  as  deep-seated  or  abyssal  igneous 
rocks,  and  they  may  in  general  be  regarded  as  representing  the 
hardened  pools  of  igneous  material  from  which  both  the  intrusive 
and  the  extrusive  or  volcanic  masses  are  derived. 


Intrusive  or  Hypabyssal  Igneous  Masses 

Dikes.  —  In  modern  volcanoes,  as  we  have  seen,  the  eruption 
does  not  always  take  place  through  the  crater,  but  a  fissure  may 
open  in  the  mountain  side  through  which  the  lava  wells  out,  and 
upon  it  there  are  generally  built  up  subsidiary  or  parasitic  cones  or 
series  of  cones,  the  so-called  monticules.  This  fissuring  of  the  moun- 
tain side  is  due  to  the  fact  that  the  material  of  the  mountain,  which 
is  often  in  large  part  volcanic  ash  only,  is  shattered  with  compara- 
tive ease  by  the  pressure  of  the  rising  lava,  while  the  hardened  plug 
of  lava,  which  fills  the  throat  of  the  crater,  cannot  be  lifted  except 
by  a  much  greater  force. 

188 


Types  of  Older  Igneous  Masses  189 

When  lava  hardens  in  the  fissures  formed  in  the  sides  of  the 
volcano,  a  dike  is  produced.  This  is  so  called,  because  erosion 
often  removes  the  soft  tuffs  and  other  material,  leaving  the  hardened 
lava  mass  projecting  as  a  wall.  Fine  examples  of  such  dikes  cutting 
stratified  tuffs  are  seen  on  the  flanks  of  the  Sicilian  volcano  Etna, 
especially  in  the  Val-del-Bove,  as  already  noted  (Fig.  79,  p.  134). 

Such  dikes  are  also  found  in  many  districts  of  recently  extinct 
volcanoes,  but  they  are  not  confined  to  the  cones  of  volcanoes.  In- 


FIG.  132.  —  West  Spanish  Peak,  Colorado,  from  the  northwest.  In  the  fore- 
ground, a  large  dike  and  several  small  dikes  weathered-out  in  relief.  (After 
Stose,  from  U.  S.  G.  S.)  (See  map,  Fig.  138,  p.  195.) 

deed,  where  eruption  takes  place  through  fissures  in  the  earth,  with- 
out the  building  of  volcanoes,  the  hardening  of  the  lava  in  the  fissure 
produces  a  dike.  As,  in  such  cases,  the  country  rock  is  often  much 
older,  the  contrast  which  the  dikes  form  with  it  is  generally  very 
marked.  Of  course,  such  dikes  will  not  become  visible  until  the 
lava  sheet  of  which  they  formed  the  vents  is  removed,  or  until  a 
marginal  section  is  cut  into  the  igneous  mass  by  the  sea  or  other 
agent  with  the  consequent  uncovering  of  the  dike. 

Over  much  of  western  North  America  the  great  lava  sheets  are 
still  in  place,  and  the  dikes  which  connect  with  them  are  not  visible. 
Over  much  of  Great  Britain,  however,  the  basalt  sheet  of  similar 
age,  and  which  probably  extended  to  Iceland,  has  been  removed  by 


1 90    Form  and  Structure  of  Older  Igneous  Masses 

erosion,  in  part  by  the  sea,  and  in  part  by  atmospheric  agencies. 
As  a  result,  the  manifold  types  of  rocks  which  make  up  the  British 
Isles  are  found  transected  by  numerous  dikes  of  basaltic  material, 
most  of  which  have  no  apparent  relation  to  any  existing  masses  of 
solidified  lava.  Some  of  these  have  been  traced  for  a  distance 
of  60  or  even  90  miles.  Dikes  connecting  with  remnants  of  the 
basaltic  sheets  are  also  exposed  on  the  seashore.  In  some  cases 
the  dikes,  being  more  resistant  than  the  enclosing  rock,  have  with- 


FIG.  133.  —  View  of  the  great  dike,  running  north  from  West  Spanish  Peak, 
Colorado.  This  dike  originally  cut  nearly  horizontal  strata  which  have  weath- 
ered away,  leaving  a  continuous  wall  100  feet  high.  The  horizontal  markings 
along  the  side  of  the  wall  indicate  the  original  contact  with  the  stratified  rocks. 
(Stose,  photo;  from  U.  S.  G.  S.) 

stood  erosion,  while  the  country  rock  has  been  worn  away.  As 
a  result,  they  stand  above  the  country  like  stone  walls,  a  fact  which 
first  gave  rise  to  the  name  dike,  since  they  are  not  unlike  the  arti- 
ficial dikes  built  in  some  portions  of  western  Europe,  especially  the 
Netherlands,  to  keep  out  the  sea  from  the  low-lying  lands  (Figs. 
132-134). 

Along  the  border  of  the  great  trap  sheet  which  forms  the  Dekkan 
Plateau  of  Central  India,  where  the  sea  or-  other  agents  have  eaten 
away  a  part  of  the  outer  margin  of  the  sheet,  dikes  are  not  infre- 
quently exposed,  penetrating  the  basement  rock  upon  which  the 
trap  sheet  lies.  For  the  most  part,  however,  the  dikes  cannot  be 
traced  directly  to  the  trap,  though  the  connection  is  indicated  by  the 


Types  of  Older  Igneous  Masses 


191 


similarity  of  the  material.  Dikes  of  probably  much  older  origin 
are  found  abundantly  along  the  Massachusetts  coast,  and  to  a 
lesser  extent  along  the  Maine  coast  and  elsewhere.  Here  they  gen- 
erally consist  of  the  type  of  basalt  known  as  diabase  (see  p.  106), 
a  lava  which  has  cooled  with  sufficient  slowness  to  permit  the  devel- 
opment of  recognizable  crystals.  We  must,  therefore,  conclude 
that  these  dikes  represent  the  deeper  portions  of  fissures  filled  by 
lava,  where,  on  account  of  the  depth  below  the  surface  at  that  time, 
the  cooling  was  slow.  Often  a  columnar  structure  is  found  in  these 


FIG.  134.  —  Devil's  Wall  (Teufelsmauer)  near  Oschitz,  Bohemia.  A  basaltic 
dike,  25  km.  long,  2  meters  thick  —  with  horizontal  prismatic  joints.  Weath- 
ered in  relief,  wall-like.  (From  Kayser's  Lehrbuch.) 

dikes,  the  columns  extending  across  them  from  wall  to  wall  (Fig. 
134).  Where  the  sea  has  access  to  them  after  they  have  become 
exposed  on  the  surface  from  the  erosion  of  large  masses  of  rock, 
these  columns  are  frequently  removed  by  the  waves,  leaving  a 
deep,  parallel-sided  fissure,  into  which  the  sea  penetrates  with  great 
force  during  storms  and  at  high  tide.  Sometimes  caverns  are 
formed  by  the  removal  of  the  lower  columns  only,  when  the  com- 
pression of  the  air  in  these  caverns  under  the  impact  of  the  wave 
will  result  in  producing  a  regular  series  of  water  spouts  after  each 
inrush  of  the  waters.  Such  phenomena  are  common  on  dike-in- 
fested coasts  and  are  known  as  "  spouting  horns,"  etc.  Chasms 
left  by  the  removal  of  dikes  are  characteristic  features  of  the  rock- 
bound  New  England  coast  (Fig.  135).  Many  other  igneous  rocks 


192     Form  and  Structure  of  Older  Igneous  Masses 


besides  basalt  and  diabase  occur  in  the  form  of  dikes,  and  it  may  be 
inferred  that  a  majority  of  them  never  reached  the  surface  as 
molten  lava  but  cooled  in  blind  fissures  of  the  earth's  crust.  Their 
present  exposure  is  therefore  brought  about  by  removal,  through 
erosion,  of  the  rock  which  once  concealed  them.  Among  the  more 
frequent  occurrences  are  those  of  granite  (aplite,  etc.),  and  especially 
of  the  coarse  or  giant  granite,  pegmatite  (Fig.  136) .  These  pegmatite 
dikes  are  often  of  great  width,  and  form  the  source  of  many  rare 
mineral  deposits,  besides  yielding  feldspar,  quartz,  and  mica  in 

sufficiently  large  masses  to  be  com- 
mercially valuable.  In  some  cases  dikes 
of  this  and  other  kinds  have  branches, 
and  sometimes  they  are  extremely  ir- 
regular, having  perhaps  eaten  their 
way  into  the  country  rock.  Such  ir- 
regular dikes  are  also  spoken  of  as 
igneous  veins,  though  they  have  little 
in  common  with  true  veins  (see  p. 
265). 

The    essential    character    of    dikes 
may  be  summed  up  by  saying  that 
FIG.  135.  —  An  eroded  dia-    they    are    generally    of    considerable 
base  dike  in  granite;  west  side    lineal    extent,    continuing    sometimes 
Rockport    Point,     Cape    Ann.     -  .,  ,     r        ..  .  ,  , 

Mass.  In  the  bottom  of  the  for  many  miles>  and  of  uniform  width, 
chasm  the  dike  is  seen  covered  ranging  from  a  fraction  of  an  inch  to 
by  boulders  which  were  formed  many  feet  When  they  cut  bedded 
by  the  waves  from  the  dike  ,.  , 

material.  At  present  the  chasm  strata>  they  maY  do  SO  at  any  angle> 
is  above  high-water  mark,  show-  but  in  general,  dikes  have  an  approxi- 
ing  recent  elevation  of  the  coast.  mately  vertical  position,  though  this 
(After  Shaler,  U.  S.  G.  S.) 

may  be  changed  by  subsequent  move- 
ments of  the  entire  mass.  Around  old  volcanic  centers  dikes 
are  often  radially  arranged,  as  shown  on  the  island  of  Mull,  west 
coast  of  Scotland  (Fig.  137),  and  in  the  ancient  volcanic  center  of 
the  Spanish  Peaks  in  Colorado  (Fig.  138),  the  dikes  of  which  have 
already  been  referred  to  (Figs.  132, 133).  Frequently  dikes  of  differ- 
ent ages  intersect  one  another,  in  which  case  the  younger  can  be 
recognized  by  the  fact  that  it  cuts  the  older.  In  the  broader  dikes 
the  rock  texture  of  the  marginal  portion  is  commonly  finer  than  that 
of  the  center,  because  more  rapid  cooling  took  place  where  the 
igneous  rock  was  in  contact  with  the  cool  country  rock  which  it 


Types  of  Older  Igneous  Masses 


193 


penetrated.  This  is  especially  well  recognized  in  the  case  of  the 
dike  rocks  of  coarser  grain.  Sometimes  the  marginal  texture  is 
even  glassy,  and  the  color  and  composition  may  also  vary  from  the 
margin  to  the  center.  The  wall  rock  of  dikes  and  the  basement 
rock  of  lava  flows  frequently  show  a  certain  amount  of  alteration 
due  to  the  heat  of  the  lava  mass.  This  contact  metamorphism 
(see  p.  207)  is  seldom  very  extensive,  however,  dying  out  a  short 
distance  from  the  igneous  mass,  especially  when  this  cooled  rapidly 
near  the  surface.  Lava  streams  have  indeed  been  known  to  flow 
over  ice  masses  without  completely  melting  them.  This  argues 


FIG.  136.  —  Pegmatite  dike  in  crystalline  dolomite,  New  York  City. 

for  a  comparatively  rapid  cooling  of  the  lava  exposed  on  the  surface 
of  the  earth. 

Stocks.  —  These  are  dike-like  intrusions  which  are  of  short  ex- 
tent, sometimes  more  or  less  regular  and  cylindrical,  at  others  ir- 
regular. They  are  similar  to  volcanic  plugs  or  cores,  such  as  fill 
the  conduits  of  old  volcanic  pipes,  but  differ  from  them  in  not  reach- 
ing the  surface,  though  the  name  has  been  indifferently  used  for 
the  intrusions  of  plug-like  character  of  moderate  size,  even  those 
that  are  in  reality  the  filling  of  ancient  volcanic  conduits.  It  is 
true  that  it  is  not  always  possible  to  determine  in  any  particular 
case  whether  the  intrusion  reached  the  surface  and  formed  a  vol- 
cano, or  whether  it  extended  only  part  way  up  into  the  rocks  of  the 


194    Form  and  Structure  of  Older  Igneous  Masses 

earth's  crusts.  In  such  a  case  the  name  stock  is  best  applied  to 
these  structures.  In  character  they  partake  of  the  deeper  portions 
of  volcanic  plugs  or  pipe  fillings,  and  to  some  extent  of  those  of 
dikes  as  well,  especially  if  they  are  irregular. 


123  456 

FIG.  137.  —  Map  and  section  (on  AB)  of  the  Island  of  Mull,  west  coast  of 
Scotland.  (After  Judd.)  i,  Non- volcanic  basement  beds;  2,  granite; 
3,  basalt  flows;  4,  gabbroitic  dikes ;  5,  acid  flows;  6,  volcanic  tuffs  and  breccias. 
Note  the  radiating  and  branching  dikes. 

Intrusive  Sheets  or  Sills.  —  In  many  regions  of  the  world  we 
find  sheets  of  basalt  and  other  igneous  rock,  the  chemical  and  min- 
eralogical  characters  of  which  indicate  that  they  were  formed  by 
the  cooling  of  a  magma,  often  with  considerable  slowness,  and  which 
are  interbedded  with  rocks  of  a  clastic  character,  the  latter  evi- 


Types  of  Older  Igneous  Masses 


dently  of  non-volcanic  origin.  Some  of  these  have  been  intruded 
between  the  layers,  but  in  other  cases  igneous  sheets  of  this  type 
may  be  old  flows,  which  were  poured  out  over  a  surface  composed 
of  horizontal  strata,  and  which  were  subsequently  covered  by  other 
strata  of  non-volcanic  origin.  It  is  important  that  the  two  types 
be  distinguished.  In  the  cases  of  interbedded  lava  sheets,  we 
should  expect  to  find  evidence  of  this  succession  of  deposits 
(stratification)  not  only  in  the  igneous  sheet  itself,  but  also  in  the 
enclosing  rock. 


FIG.  138.  —  Map  of  a  part  of  the  Spanish  Peaks  region,  Colorado,  showing 
the  numerous  dikes  radiating  from  the  volcanic  necks  or  ancient  centers  of 
volcanic  action.  (See  Figs.  131,  132,  pp.  189,  190.) 

Lava  streams  have  very  definite  and  recognizable  surface  charac- 
ters, as  we  have  seen,  and  these  are  different  from  the  structures 
found  in  the  bottom  of  the  sheet.  In  most  cases,  not  only  is 
the  surface  form  of  a  sheet  distinct  (ropey,  pillowy,  rough,  etc.), 
but  the  lava  itself  is  compact,  or  even  glassy  near  the  surface,  and 
commonly  layers  of  steam  holes,  either  empty  or  filled  secondarily 
by  mineral  deposits,  are  found  for  some  distance  down  from,  and 
parallel  to,  the  surface.  The  structure  of  the  enclosing  rock  also 
is  distinctive,  for  whereas  the  under  layer  over  which  the  lava  was 
poured  out  shows  some  effect  from  the  heat  of  the  stream  (meta- 
morphism),  and  may  actually  have  furnished  fragments  which  are 
included  in  the  lava  mass,  the  overlying  layer  will  show  no  such 
contact  phenomena,  for  the  lava  will,  in  most  cases,  have  cooled 


196    Form  and  Structure  of  Older  Igneous  Masses 


sufficiently,  before  being  covered  by  sediments,  not  to  produce  any 
such  effect.  A  still  more  convincing  argument  of  the  contem- 
poraneity of  the  lava 
flow  is  furnished  when, 
as  is  frequently  the  case, 
fragments  of  the  lava  are 
included  in  the  overlying 
stratum,  having  been 
broken  from  the  surface 
of  the  sheet  before  or  at 
the  time  of  the  deposi- 
tion of  the  covering  layer. 
Weathering  of  the  sur- 
face of  the  sheet  and  the 
formation  of  soil  layers 
is  also  a  clear  indica- 
tion of  the  exposure  of 
the  lava  sheet  for  a  time 
before  it  was  buried  by 
sediment.  Such  old  soil 
surfaces  are  seen  in  nearly 
all  of  the  fine  series  of 
ancient  lava  flows  which 
are  to-day  exposed  on 
the  east  coast  of  Scotland 
(Fife),  and  the  length  of 
time  of  the  exposure  is 
shown  by  the  fact  that 
roots  and  stumps  of  an- 
cient trees  (Calamites)  are 
found  in  these  old  soils, 
and  were  completely 
buried  by  the  sediments 
which  followed.  A  suc- 
cession of  such  ancient 
forest  beds  is  found  in 


FIG.  139.  —  Section  of  Amethyst  Moun- 
tain, Yellowstone  National  Park.  The 
mountain  consists  of  a  base  of  Archaean 
granite  and  Carbonic  limestone,  overlain  dis- 
conformably  by  2700  feet  of  Tertiary  strata, 
chiefly  of  volcanic  origin.  The  coarse  beds 
are  conglomerates  and  breccias,  and  alternat- 
ing with  beds  of  finer  material  are  sandstones 
and  shales  bearing  the  abundant  silicified  re- 
mains of  fossil  forests.  There  are  at  least 
fifteen  successive  forests,  indicating  that  num- 
ber of  volcanic  outbursts,  separated  by  suf- 
ficient time  to  allow  the  growth  of  forest 
trees  varying  in  diameter  of  trunks  from  two 
to  ten  feet.  (After  Holmes.)  For  photo- 
graph of  several  of  these  trees  see  illustration 
in  Chapter  XLV. 


the   series   of  lava  flows 
exposed  by  erosion  in  the  Yellowstone  National  Park  (Fig.  139). 

Intruded  igneous  sheets  or  sills,  however,  show  not  only  a  basal 
but  also  an  upper  igneous  contact,  and,  indeed,  may  include  frag- 


Types  of  Older  Igneous  Masses  197 

ments  of  the  overlying  as  well  as  the  underlying  strata.  More- 
over, the  sheets  themselves  show  a  similar  fine-grained  or  dense 
character  on  both  surfaces,  while  toward  the  center  they  become 
more  coarsely  crystalline.  Such  sheets  are  evidently  of  more  re- 
cent origin  than  the  enclosing  rocks,  and  were  intruded  between 
and  parallel  with  them,  forcing  them  apart,  and  cooling  thus  within 
the  earth's  crust  without  ever  reaching  the  surface.  Such  intru- 
sive sheets  are  also  called  sills,1  and  they  are  well  represented  by 


FIG.  140.  —  Near  view  of  the  Palisades  of  the  Hudson,  showing  jointed 
trap  (simulating  columnar  jointing)  at  top  and  talus  slopes  below.  (Photo  by 
D.  W.  Johnson.) 

the  rocks  which  now  form  the  Palisades  of  the  Hudson  River  op- 
posite New  York  City,  though  these  constitute  only  a  small  part 
of  the  former  extent  of  the  intruded  sheet  (Fig.  140).  This  is  sev- 
eral hundred  feet  in  thickness  and  has  been  traced  for  a  distance  of 
about  100  miles.  A  similarly  extensive  example,  well  known  to 
foreign  geologists,  and  indeed  the  type  from  which  the  term  sill 
has  been  derived,  is  the  "  Great  Whin  Sill  "  of  the  north  of  England. 
This  can  be  traced  for  a  distance  of  80  miles  between  the  enclosing 
rocks,  its  resistance  to  erosion  helping  to  produce  the  great  cliff 
known  as  the  Pennine  escarpment,  which  bounds  the  Vale  of 

1  The  name  sill  was  originally  applied  by  the  miners  in  the  north  of  England  to 
any  prominent  or  hard  projecting  bed  or  stratum.  The  type  of  the  volcanic  sills  is 
the  great  Whin-sill  of  northern  England  mentioned  below,  this  being  a  prominent  bed 
or  sill  of  whin-stone,  a  name  given  to  any  hard,  fine-grained  rock,  such  as  basalt, 
quartzite,  etc. 


198    Form  and  Structure  of  Older  Igneous  Masses 


Eden  on  the  east  (Fig.  141).  This  sill  varies  from  20  to  150  feet 
in  thickness,  the  average  being  from  80  to  100  feet,  and  it  forms 
a  prominent  ledge  wherever  exposed  in 
section  (Fig.  142).  It  covers  an  area 
probably  not  less  than  1000  square 
miles  in  extent.  Other  sills,  mostly 
of  lesser  thickness  and  extent,  are  found 
in  widely  distant  regions  of  the  world. 
Among  these  may  be  mentioned  the 
one  forming  Salisbury  Crags  in  Edin- 
burgh, Scotland,  which  overlooks  Holy- 
rood  Castle,  and  which  figured  in  the 
disputes  over  the  origin  of  basalt  in 
Werner's  time  (Fig.  143  c). 

Through  erosion,  the  sill  is  exposed  in 
various  ways  as  shown  in  the  following 
diagrams  (Fig.  143).  Sills  which  were 
originally  intruded  between  horizontal 
strata  may  become  inclined  by  the  arch- 
ing of  these  strata,  as  in  the  case  of  the 
Palisade  sill  (Fig.  143  a),  or  they  may 
even  become  folded  with  the  strata. 

A  characteristic  feature  of  sills,  and 
one  which  aids  greatly  in  distinguishing 
them  from  contemporaneous  flows,  is 
seen  in  the  lack  of  absolute  conformity 
to  the  enclosing  strata.  Though  in  any 
given  locality  this  conformity  may  be 
unquestioned,  it  will  be  found  that  on 
tracing  the  sill  for  some  distance,  it  gen- 
erally breaks  across  some  of  the  layers, 
passing  either  to  a  higher  or  lower  level, 
and  there  continuing  for  a  time  parallel  to 
the  enclosing  strata  (Fig.  144).  The  rocks 

of  the  sills  are  generally  massive  or  not  very  coarse-grained,  though 
those  of  the  center  of  large  sills,  such  as  that  of  the  Palisades,  are 
sometimes  of  moderate  coarseness.  They  are  generally  darker- 
colored  and  denser-grained  on  both  upper  and  lower  margins,  and 
the  change  in  grain  toward  the  center  is  often  a  very  regular  one. 
In  the  tunnels  which  have  been  cut  through  the  Palisade  sill,  it  has 


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Types  of  Older  Igneous  Masses 


199 


been  possible  to  ascertain,  from  the  size  of  the  grain,  the  distance 
from  the  upper  or  lower  margins  at  any  selected  locality.  Colum- 
nar structure  is  not  a  characteristic  feature  of  sills,  the  apparent 


FIG.  142. — View  looking  up  Hilton  Beek,  northern  England,  showing  the 
out-crop  of  the  Whin  Sill,  which  forms  the  prominent  cliff  on  either  side,  and 
rests  on  Carboniferous  limestone.  (Photo  by  the  Author.) 

columnar  structure  of  the  Palisade  sheet  being  due  to  a  series  of 
subsequently  formed  intersecting  fissures  or  true  joints. 

Laccoliths.  —  The  Henry  Mountains  of  Utah  represent  rem- 
nants of  a  peculiar  type  of  intruded  mass  which,  instead  of  spread- 


FIG.  143.  —  Diagrammatic  sections  of  volcanic  sills,     a,  Palisades  of  Hudson; 
ft,  Whin  Sill,  North  England ;  c,  Salisbury  Crag  in  Edinburgh,  Scotland. 


ing  out  between  the  strata,  is  localized,  each  separate  intrusion 
swelling  into  a  semi-lenticular  or  dome-like  mass,  space  for  which 
is  made  by  the  lifting  into  an  arch  of  the  rocks  which  overlie  the 


2oo    Form  and  Structure  of  Older  Igneous  Masses 


FIG.  144.  —  Base  of  Palisade  diabase,  showing  lateral  ascent  of  the  diabase 
across  the  strata  of  the  Newark  group.  Kings  Point,  Weehawken,  N.  J., 
looking  west.  (From  photographs,  U.  S.  G.  S.) 

intruded  mass  (Figs.  145,  146).  These  structures  were  first  de- 
scribed by  -an  American  geologist,  the  late  G.  K.  Gilbert,  and 
the  type  was  by  him  designated  a  laccolith.  Other  laccoliths 


FIG.  145.  —  Ideal  restoration  of  the  laccoliths  of  Mt.  Holmes,  Henry  Moun- 
tains, Utah. 

have  since  been  found  in  many  parts  of  the  world,  the  best  known 
examples  among  these  being  in  Colorado  and  Montana.  Lac- 
coliths become  visible  by  the  erosion  of  the  covering  rock,  and 


FIG.  146.  —  Ideal  cross-section  of  the  laccoliths  of  Mt.  Holmes,  after  restoration. 

then  they  constitute  hills  of  igneous  material,  thick  in  the  exposed 
center,  but  thinning  away  in  all  directions  where  they  pass  be- 
neath the  remnants  of  the  original  covering  sheet  or  interpenetrate 


Types  of  Older  Igneous  Masses 


201 


it  in  a  series  of  wedges  (Figs.  147,  148).     It  is  important  to  bear 
in  mind  that  where  the  contact  of  the  thin  edge  with  the  cover- 


FIG.  147.  —  Hesperus  Mountain,  showing  wedges  and  sheets  of  trachyte 
intruded  into  the  shales,  from  the  laccolith  of  Mt.  Moss. 

ing  rock  is  exposed  around  the  margin  of  the  hill,  this  contact  is 
seen  to  be  an  igneous  one,  clearly  proving  the  rock  to  be  intrusive. 
Moreover,  it  must  be  shown  that  the  igneous  rocks  rest  upon  rock 


E. 

FIG.  148.  —  Ideal  section  of  La  Plata  Mountain,  Colo.,  showing  the  sup- 
posed original  form  of  the  laccolith  of  Mt.  Moss.  The  line  a,  a  is  the  present 
profile  which  cuts  Hesperus  Mountain  and  Mt.  Moss. 

of  a  different  type,  and  that  they  are  not  deep-seated  igneous  masses 
which  have  eaten  their  way  into  the  overlying  rock  (bosses,  etc.). 
When  a  laccolithic  intrusion  has  been  completely  isolated  by  erosion 
so  that  no  part  of  the  original  mass  is  in  contact  with  the  overlying 


202     Form  and  Structure  of  Older  Igneous  Masses 


rock,  the  recognition  of  the  laccolithic  origin  of  the  mass  becomes 
very  difficult,  and  the  proof  of  such  an  origin  is  sometimes  incon- 
clusive. The  Mato  Tepee  or  Devil's  Tower  of  Wyoming  (Fig.  in, 
p.  1 68)  has  also  been  interpreted  by  Jaggar  as  the  remnant  of  a  lac- 
colith rather  than  a  volcanic  neck. 

Small  laccoliths  may  be  exposed  in  cross-section  in  a  cliff,  when 
their  character  is  undoubted.  Laccoliths  range  in  maximum 
thickness  from  less  than  a  hundred  to  several  thousand  feet, 
and  their  diameter  varies  from  a  few  hundred  yards  to  several 


N.W. 


B 


FIG.  149  a.  —  Section  of  Corndon  Hill,  in  Shropshire,  England ;    the  type  of 
the  phacolith.     (After  Harker.) 

miles.  As  in  the  case  of  sills,  which  may  be  regarded  as  the 
extreme  in  one  direction  of  laccoliths,  the  upper  and  lower  margins 
are  generally  finer  grained  and  may  be  darker-colored  and 
lower  in  silica  content,  besides  being  richer  in  ferro-magnesian 
minerals,  than  the  center.  Columnar  structure  is  sometimes  de- 
veloped, the  columns  standing  vertically.  This  is  shown  in 
the  Mato  Tepee,  already  referred  to  as  probably  an  erosion 
remnant  of  a  thick  laccolith. 

Phacoliths.  — Corndon  Hill  of  Shropshire,  England  (Fig.  149  a), 
appears  to  represent  a  peculiar  form  of  lenticular  igneous  intrusion 
in  which,  however,  instead  of  forcing  the  overlying  strata  upward 

into  a  dome,  these  masses  occupy  the 
axes  of  both  upward  (anticlinal)  and 
downward  (synclinal)  folds  of  the 
enclosing  strata.  These  cavities,  it  is 
inferred,  were  formed  as  the  result  of 
the  spreading  of  the  strata  by  lateral 
compression  during  the  folding,  and 
thus  the  igneous  masses  merely  oc- 
cupy the  spaces  made  for  them  by 

other  agencies,  instead  of  actively  forcing  the  strata  apart  (Fig. 
149  b).  Or  it  may  be  that  the  compression  of  the  stratified  rocks 
into  folds  produced  lines  of  weakness  along  which  the  igneous  mass 


FIG.  149  b.  —  Diagram  illus- 
trating the  formation  of  phaco- 
liths.  (After  Harker.) 


Types  of  Older  Igneous  Masses  203 

found  it  easy  to  enter.     Such  types  of  intrusions  have  been  called 
phacoliths  (Harker)  (Greek  phacos,  <£aKo's,  a  lentil). 

Chonoliths.  —  Another  type  of  this  intrusion  has  been  described 
from  the  Aletschhorn,  a  mountain  in  the  Aar  Massif  of  the  Alps, 
and  from  Ascutney  Mountain,  N.  H.  Here,  during  the  folding 
of  the  strata,  there  were  formed  irregular,  instead  of  lenticular, 
cavities  into  which  the  lava  from  below  found  its  way.  To  a  greater 
or  less  degree  the  igneous  mass  may  also  have  forced  apart  the  rock 
in  an  irregular  manner,  partaking  of  the  characters  of  both  the 
laccolith  and  the  phacolith.  The  most  striking  feature  of  such 


, 


........ 

,  ^i'1'1^'1  ..  ............  -'.. 


FIG.  150.  —  Section  of  the  Mt.  Holmes  bysmalith.   •  (After  Iddings.) 

an  igneous  mass  is,  however,  the  great  irregularity  of  the  spaces 
which  it  occupies.  To  such  masses  the  name  chonolith  (choanolitti) 
has  been  applied  by  Daly,  because  the  cavities  formed,  acted  as  a 
mold  into  which  the  igneous  rock  was  poured  (Greek  choanos, 
xo'avos  or  x^vos  a  mold).  It  is  obvious  that  small  intrusions  of  this 
type  grade  into  igneous  veins. 

Bysmaliths.  —  Still  another  type  of  igneous  intrusion  has  been 
found  in  the  old  volcanic  center  of  the  southern  end  of  the  Gallatin 
Range  in  the  Yellowstone  National  Park  in  northwestern  Montana 
(Fig.  150).  Here  the  mass  which  forms  Mt.  Holmes  has  the  nature 
of  a  huge  core,  resembling  a  giant  volcanic  neck,  but  connected 
either  with  no  surface  flow,  or  with  flows  of  only  secondary  signifi- 
cance. It  is  a  laccolith  in  which  the  upward  force  was  so  great  as 
to  rupture  the  overlying  rock  mass,  and  carry  it  upward  for  a  great 


204    Form  and  Structure  of  Older  Igneous  Masses 

distance.  This  is  therefore  a  large  plutonic  plug  or  core  which  has 
forced  its  way  upward  as  a  compact  mass  into  the  overlying  rock, 
and  the  contact  of  this  mass  with  the  rocks  around  its  margin  shows 
the  evidence  of  such  upward  movement.  On  this  account  it  has 
been  called  a  bysmalith  1  (Iddings) ,  a  rock  rising  from  the  depths 
(Gr.  /Wcros,  the  deep) .  A  bysmalith  represents  the  other  extreme 
of  laccolith  formation,  with  the  exaggeration  of  the  vertical  dimen- 
sions. Bysmaliths  are  also  called  plutonic  plugs  in  distinction  from 
volcanic  plugs,  which  are  the  filling  of  pipes  that  reach  the  surface. 
They  differ  from  stocks  in  the  manner  of  origin,  having  forced  their 
way  into  the  rock  by  lifting  the  obstruction  in  their  path,  while 
stocks  are  intrusive  into  fissures,  which  they  may  widen  and  alter 
by  pressure  and  otherwise. 

It  is  evident  that  the  recognition  of  the  particular  type  of  igneous 
intrusion  which  any  given  mass  represents,  can  only  be  determined 
from  careful  examination  of  both  the  mass  itself  and  of  the  enclos- 
ing rock,  and  that  many  cases  may  occur  where  erosion  has  ren- 
dered the  interpretation  at  best  a  doubtful  one.  If  the  student 
keeps  in  mind  the  types  here  given  and  the  essential  characters  of 
each,  he  may,  by  elimination,  in  most  cases  be  enabled  to  reach  a 
conclusion  regarding  the  nature  of  an  igneous  mass  with  which  he 
becomes  confronted  in  the  field.  Extended  examination  of  all  the 
exposed  parts,  however,  and  especially  of  their  contacts  with  the 
enclosing  rock,  will  be  necessary  before  such  an  interpretation  is 
possible. 

It  must  be  emphasized  again,  that  the  types  so  far  discussed  show, 
from  the  nature  of  the  rock,  that  they  have  cooled  at  some  depth 
below  the  surface,  but  that  they  do  not  belong  to  the  great  mass  of 
deep-seated  igneous  material  which  formed  the  reservoir,  so  to 
speak,  from  which  these  intrusions  were  fed.  This  group  of  deep- 
seated  rocks  will  be  discussed  in  the  next  section. 

Deep-Seated  or  Abyssal  Igneous  Masses 

In  many  portions  of  the  world,  especially  in  such  regions  of  an- 
cient rock  as  eastern  Canada,  the  Adirondack  Mountains,  parts  of 
Sweden  and  Finland,  and  elsewhere,  igneous  rocks  of  coarse  grain 
(holocrystalline)  are  found,  which  may  be  interpreted  as  a  part  of 
the  reservoir  of  igneous  material  from  which  ancient  volcanoes 

1  More  corrrectly  byssolith. 


Types  of  Older  Igneous  Masses 


205 


were  fed,  and  from  which  the  other  types  of  intrusions  (the  hypa- 
byssal)  emanated.  That  these  are  now  exposed  upon  the  surface 
is  due  to  prolonged  erosion,  which  has  removed  great  thicknesses  of 
overlying  rock  beneath  which  the  magma  cooled.  At  the  same 
time  these  igneous  magmas  forced  or  ate  their  way  to  some  extent 
into  the  overlying  rocks,  which,  therefore,  when  still  preserved,  show 
an  igneous  contact,  that  is,  a  contact  of  a  cool  with  a  molten  rock 
mass.  These  igneous  masses  consist  of  granite,  syenite,  diorite, 
gabbro,  or  of  the  more 
basic  types  of  rocks,  and 
they  have  been  divided 
on  the  basis  of  their  form 
and  size  into  bosses  and 
batholiths. 

Bosses.  —  These  are 
deep-seated  igneous 
masses  which  show  a 
dome-like  surface  rather 
than  the  form  of  a  plug, 
and  their  section  as  re- 
vealed by  erosion  is  a 
more  or  less  circular  one 
(Fig.  151).  They  are  sur- 
rounded by  other  rocks,  often  sediments,  which  show  alteration 
from  contact  with  the  heated  igneous  mass,  and  such  altera- 
tion appears  often  in  concentric  zones  around  the  boss,  the  most 
strongly  altered  zone  being  next  to  the  igneous  mass. 

Batholiths.  —  These  are  huge  bosses  of  very  irregular  form,  the 
exposure  of  which  can  sometimes  be  traced  over  many  square 
miles.  They  are  particularly  characteristic  of  the  older  parts 
of  the  earth's  crust,  and  they  have  a  very  variable  relation 
to  the  rocks  into  which  they  are  intruded.  The  granite  head- 
lands of  Cape  Ann,  Mass.,  of  Mount  Desert,  Maine,  and  of 
Halifax,  Nova  Scotia,  are  examples  of  more  or  less  eroded  rock 
masses  of  this  type. 

Since  these  masses  cooled  very  slowly,  they  became  coarsely 
crystalline,  while  the  prolonged  heat  greatly  affected  the  rocks  with 
which  they  came  in  contact.  In  batholiths,  as  in  bosses,  we  may 
generally  trace  a  series  of  zones  of  alteration  in  decreasing  intensity 
from  the  igneous  mass  outward,  those  immediately  in  contact  with 


FIG.  151.  —  Ground-plan  of  a  granite  boss, 
with  the  ring  of  contact  metamorphism. 
a,  sandstones,  shales,  etc.,  dipping  at  high 
angles  in  the  direction  of  the  arrows ;  b,  zone 
or  ring  within  which  these  rocks  are  meta- 
morphosed; c,  granite,  sending  out  veins  or 
apophyses  into  b. 


206    Form  and  Structure  of  Older  Igneous  Masses 

the  igneous  mass  showing  most  profound  alteration,  while  each 
zone  generally  has  developed  minerals  peculiar  to  it. 


Subordinate  Igneous  Masses 

Apophyses.  —  This  name  is  applied  to  offshoots  from  any  in- 
trusive igneous  mass  whether  a  dike,  sill,  laccolith,  or  deep-seated 
magma.  Apophyses  are  generally  irregular  in  form  and  die  out 
in  a  short  distance.  Sometimes  they  may  have  the  character  of 
small  dikes  for  some  distance  of  their  extent.  They  form  the  surest 
means  of  distinguishing  an  intruded  mass  from  a  surface  flow. 

Contemporaneous  Veins.  —  As  all  magmas  contain  more  or  less 
water-vapor,  this  may  become  locally  segregated  during  the  pro- 
cess of  solidification  by  crystallization  of  the  magma,  thus  ren- 
dering portions  of  the  magma  very  fluid,  because  of  the  abundance 
of  the  water  in  it.  If  the  main  mass  of  the  magma  which  has  al- 
ready solidified  but  is  still  highly  heated,  is  fissured,  as  may  happen 
especially  near  the  margin  of  the  mass,  this  more  liquid  magma  will 
flow  into  the  fissures,  and  there  solidify.  Because  the  acidic  min- 
eral combinations  will  be  the  last  to  form,  these  contemporaneous 
•veins,  as  they  are  called,  will  consist  of  increasingly  lighter  minerals, 
toward  the  outer  part  of  the  cooling  mass.  Thus  it  will  happen  that 
a  magma  which  solidified  to  form  a  dark,  dioritic  rock,  for  example, 
will  become  intersected  near  its  margin  with  irregular  veins  of  light- 
colored  rock,  consisting  mainly  of  orthoclase,  feldspar,  and  quartz, 
and  still  farther,  near  the  margin,  by  more  or  less  pure  quartz  veins. 
This  is  well  shown  in  the  rock  ledges  of  the  Massachusetts  coast 
some  distance  north  of  Boston,  where  such  a  dark  dioritic  rock  is 
interpenetrated  in  all  directions  by  veins  filled  with  a  pinkish,  fine- 
grained, granitic  rock  (chiefly  orthoclase  and  quartz),  forming  a 
very  striking  contrast,  from  which  that  portion  of  the  coast  has 
received  its  name  of  "  Marble  Head." 

Pegmatite  Veins.  —  By  the  local  concentration  of  much  water- 
vapor  and  the  gases  from  the  solidifying  of  the  main  mass  of  the 
igneous  body,  exceptionally  fluid  magmas  may  be  produced  which 
contain  much  silica  and  the  substances  which  go  to  make  up  the 
acid  minerals,  together  with  the  rarer  mineral  substances  of  igneous 
magmas.  This  very  fluid  magma  will  occupy  fissures  and  cavities 
in  the  main  mass,  from  which  it  is  differentiated,  and  will  also  be 
injected  into  fissures  in  the  adjoining  rock.  Slow  solidification 


Contact  of  Igneous  Masses  with  Other  Rocks     207 

produces  coarse  crystals,  often  many  feet  or  yards  in  diameter,  and 
these  will  be  largely  the  more  acidic  (potash  and  soda)  feldspars, 
the  lighter  colored  micas,  and  much  free  quartz.  Such  a  coarse 
rock  of  acidic  minerals  is  known  as  a  pegmatite,  and  in  it  the  inter- 
growth  of  quartz  within  the  feldspar  produces  the  peculiar  structure 
known  as  graphic  or  pegmatitic  structure  (see  p.  97,  Fig.  40).  Rocks 
of  this  type  are  known  as  pegmatites,  but  they  are  not  always  of 
such  coarse  texture.  In  size  too,  the  pegmatite  masses  may  vary 
from  dike-like  intrusions  hundreds  of  meters  across,  to  veins  only 
a  few  millimeters  thick.  In  such  pegmatites  there  are  commonly 
found  many  minerals  formed  of  the  rarer  element?,  most  common 
among  which  are  tourmalines  (commonly  the  black  variety,  but 
also  the  red,  green,  or  blue  gem  types),  huge  crystals  of  spodumene, 
of  beryl,  etc.,  and  not  infrequently  many  metals  as  well. 

CONTACT  or  IGNEOUS  MASSES  WITH  OTHER  ROCKS 

Igneous  Contact 

By  the  term  igneous  contact  we  mean  the  junction  which  has 
been  produced  by  a  mass  of  igneous  magma  while  still  hot  with  the 
rock  over  which  it  is  poured  out,  if  it  is  a  lava,  or  with  the  rock  of 
the  earth's  crust  into  which  it  is  intruded.  In  the  first  case  the 
cold  rock  over  which  the  lava  flow  is  poured  is  called  the  basement 
rock,  in  the  second  case  the  rock  into  which  the  igneous  mass  is 
intruded  is  called  the  country  rock.  In  either  case  the  older  rock 
may  be  assumed  to  have  been  cold  when  the  hot  magma  came  into 
contact  with  it,  and  the  structures  which  have  resulted  from  such 
a  coming  together  of  highly  heated  with  cold  rocks,  are  contact 
phenomena,  and  the  alterations  produced  in  either  rock  are  called 
contact  metamorphism. 

In  the  case  of  surface  flows  of  lava,  the  phenomena  of  contact 
metamorphism  are  seen  only  at  the  base  of  the  lava  sheet,  though 
a  certain  change  is  produced  where  the  surface  of  the  sheet  is  in 
contact  with  the  atmosphere,  or  with  water  in  the  case  of  a  sub- 
marine outpouring  of  lava,  and  these  changes  in  the  lava  sheet  may 
also  be  referred  to  as  metamorphic  changes  in  a  very  literal  inter- 
pretation of  the  term. 

Contact  metamorphism  is  very  slight  in  the  case  of  most  intru- 
sive sheets  or  sills,  in  laccoliths,  and  generally  in  dikes,  though  in 
the  case  of  such  large  sills  as  the  Palisades  it  may  extend  for  many 


208    Form  and  Structure  of  Older  Igneous  Masses 

yards  from  the  contact.  In  all  cases  it  must  be  remembered  that 
in  intrusive  masses  the  contact  phenomena  are  found  on  all  sides 
of  the  intruded  mass.  Around  volcanic  plugs  and  stocks  contact 
phenomena  are  well  marked,  especially  in  the  former,  where  there 
was  continuous  or  repeated  passage  upward  of  igneous  material 
with  surface  eruption.  Again,  where  dikes  are  numerous  and  close 
together,  much  contact  metamorphism  is  observable.  But  such 
phenomena  are  most  marked  in  the  large,  deep-seated  igneous 
masses  of  coarse  grain,  which  cooled  slowly  and  which,  therefore, 
subjected  the  adjoining  rocks  for  a  long  time  to  the  heat  of  the 
igneous  body  and  the  action  of  the  gases  given  off  from  it. 


Kinds  of  Contact  Metamorphism 

We  may  distinguish  two  kinds  of  contact  metamorphism,  that 
produced  upon  the  igneous  mass  itself  from  contact  with  a  cool 
wall  rock,  and  that  produced  upon  the  wall  or  country  rock.  The 
first  is  spoken  of  as  an  endomorphic,  and  the  second  as  an  exomorpkic, 
change. 

Endomorphic  Effects.  —  The  effect  produced  by  the  contact  of 
a  magma  with 'the  wall  or  country  rock  upon  the  resulting  igneous 
rock  itself  is  in  the  first  place  a  change  in  texture  along  the  con- 
tact, as  we  have  already  seen.  This  is  shown  by  finer  grain  or  even 
a  glassy  character  of  the  igneous  rock  at  the  junction  with  the  wall 
rock.  A  porphyritic  texture  with  fine  ground-mass  and  coarse 
phenocrysts  may  also  develop  as  the  result  of  more  rapid  cooling. 
When,  however,  the  enclosing  rock  is  thoroughly  heated  by  the 
igneous  mass,  as  in  a  volcanic  conduit,  no  perceptible  change  may 
result  in  the  marginal  portion  of  the  igneous  mass  on  cooling,  while 
the  effects  on  the  wall  rock  itself  (the  exomorphic  effects)  are  the 
more  marked. 

In  the  second  place,  new  minerals,  not  found  in  the  main  mass 
of  the  igneous  intrusion,  may  be  formed  near  the  contact,  from 
the  chemical  activities  of  vapors  and  gases  which  tend  to  be  ex- 
cluded from  the  main  mass  as  it  solidifies  and  to  escape  toward  the 
margin  of  the  mass  and  thence  into  the  surrounding  rock.  In 
granitic  intrusions  tourmaline  is  not  an  uncommon  mineral  thus 
formed. 

Exomorphic  Effects.  —  The  effects  of  the  heated-  intrusion  upon 
the  wall  or  country  rock  are,  however,  the  most  marked.  Among 


Contact  of  Igneous  Masses  with  Other  Rocks     209 

these  the  most  notable  are  the  baking  or  hardening  and  toughening 
of  the  rock  near  the  contact,  from  the  heat,  and  its  frequent  change 
to  a  more  crystalline  condition.  Next  in  importance  is  the  develop- 
ment of  new  minerals  on  the  contact  zone,  these  being  generally 
formed  by  the  chemical  activity  of  the  gases  and  vapors  which 
enter  the  rock  and  constitute  the  mineralizers. 

The  width  of  the  zone  subject' to  contact  metamorphism  varies 
with  the  size  and  the  heat  of  the  igneous  mass,  and  with  the  amount 
of  mineralizing  gases  and  vapors  given  off.  It  also  varies  with  the 
character  of  the  enclosing  rocks,  some  being  more  easily  altered  than 
others,  while  some  are  more  permeable  to  heat  and  mineralizing 
vapors  than  others.  In  general,  older  igneous  rocks  into  which 
younger  ones  are  intruded,  are  less  altered  than  are  sediments  of 
chemical,  organic,  or  clastic  origin,  in  which  the  chemical  composi- 
tion is  such  as  to  permit  ready  change,  or  in  which  the  conduc- 
tivity, texture,  and  other  characters  allow  easy  entrance  of  the 
heat  and  the  vapors.  The  special  effects  on  a  few  types  of  these 
may  be  noted. 

Effects  on  Limestones.  —  Limestone  is  the  general  name  applied 
to  rocks  consisting  of  carbonate  of  lime  or  of  carbonate  of  lime  and 
magnesia.  They  may  be  of  aqueous  (chemical),  organic,  or  of 
clastic  origin,  as  more  fully  discussed  in  later  chapters.  Lime- 
stones are  seldom  pure,  there  being  commonly  an  admixture  of  clay 
or  of  silica  in  the  form  of  flint,  chert,  intimately  admixed  grains 
(sand),  or  other  particles.  The  first  and  most  general  effect  of  the 
igneous  mass  upon  limestone  is  the  crystallization  of  the  latter, 
with  the  result  that  a  marble  is  produced.  Great  masses  of  marble 
are,  however,  not  produced  in  this  manner,  but  by  more  extended 
(regional)  metamorphism,  during  mountain-making  disturbances 
(see  Chapter  XX).  Changes  in  the  mineral  character  also  occur 
by  recombination  of  the  various  substances  present.  Thus  if  silica 
is  present,  the  lime  will  combine  with  it,  with  the  separation  of 
carbon  dioxide,  and  a  lime  silicate  (the  mineral  wollastonite)  is  pro- 
duced. The  change  may  be  expressed  in  the  following  formula : 

CaCO3  +  SiO2     =     CaSiO3   +  CO2 

Carbonate  of        Silica  Lime  silicate      Carbon 

Lime  (Wollastonite)       dioxide 

If  the  limestone  is  magnesian,  then  a  double  silicate  of  lime  and 
magnesium  is  produced,  this  giving  a  mineral  of  the  pyroxene 
group,  known  as  diopside,  abundant  crystals  of  which  are  found  in 


210    Form  and  Structure  of  Older  Igneous  Masses 

the  northern  part  of  the  city  of  New  York  (Inwood  region),  where 
the  magnesian  limestones  are  penetrated  by  numerous  pegmatite 
dikes  (see  Fig.  136,  p.  193).  The  formula  expressing  this  change 
may  be  written  as  follows :  — 

Ca  Mg(CO3)2  +2  SiO2  =  Ca  Mg(SiO3)2  +2  CO2 

Lime  magnesium      Silica         Lime  magnesium      Carbon 
carbonate  »       silicate  (diopside)      dioxide 

When  both  clay  and  quartz  are  present,  a  double  silicate  of  lime 
and  alumina  may  result,  the  aluminum  being  furnished  by  the  clay. 
This  new  compound  will  then  crystallize  out  in  the  form  of  the 
mineral  garnet,  while  both  water  and  carbon  dioxide  are  given  off. 
The  following  formula  expresses  this  change :  — 

3  CaCO3  +H4Al2Si2O9  +SiO2  =  Ca3Al2Si3O12  +3  CO2  +2  H2O 

Lime  carbonate  Clay  Silica        Lime  aluminum      Carbon      Water 

(Calcite)  (Quartz)     silicate  (Garnet l)     dioxide 

Good  illustrations  of  the  formation  of  garnets,  sometimes  of  con- 
siderable size  and'  in  large  numbers,  but  of  no  great  value,  may  be 
seen  in  the  impurer  parts  of  the  same  limestones  (Inwood  marble) 
in  the  northern  part  of  New  York  City,  near  the  contacts  with 
dikes. 

Many  substances  may  be  carried  into  the  limestones  by  gases 
and  vapors,  and'  so  produce  a  variety  of  minerals.  Those  men- 
tioned are,  however,  the  most  important. 

Effects  on  Mud- Rocks.  —  This  is  a  general  name  applied  to  sedi- 
ments composed  most  commonly  of  microscopic  particles  of  quartz 
and  of  clay,  sometimes  the  one  and  sometimes  the  other  substance 
predominating.  Lime  particles  may  also  be  very  abundant  and 
intimately  disseminated  among  the  clay  and  quartz,  and  other 
substances  in  a  fine  state  of  division  may  also  occur.  Such  mud- 
rocks  may  be  massive,  or  they  may  have  a  fine  and  irregularly 
bedded  or  laminated  structure,  when  they  are  called  shales.  By 
baking  along  the  contact,  such  a  mud-rock  may  be  changed  into  a 
dense  mass  comparable  to  artificially  baked  clay  (porcelain), 
forming  a  rock  called  porcelanite  (porcellanite)  or  a  hornfels,  which 
may  have  the  appearance  and  hardness  of  a  dense  basalt  or  other 
igneous  rock  and  may  be  mistaken  for  such.  Another  feature  pro- 
duced in  such  mud-rocks,  a  short  distance  from  the  contact,  is  a 
series  of  spots  or  knots  of  mineral  matter  or  even  of  crystals  of 

1  This  is  the  variety  Grossularite.  There  are  several  others  of  different  composi- 
tion. See  Table,  p.  62. 


Contact  of  Igneous  Masses  with  Other  Rocks     211 

minerals,  among  which  the  commonest  is  a  silicate  of  alumina  known 
by  the  mineral  name  of  andalusite  (A^SiOs) ;  other  minerals  may 
also  be  developed. 

Effect  on  Quartz  Rocks  (Sandstones,  etc.).  —  When  the  country 
rock  consists  largely  of  quartz,  commonly  in  fine  grains  (sandstone) 
the  effect  of  the  intrusion  of  an  igneous  mass  is  not  so  marked  as 
in  the  case  of  other  rocks.  Close  to  the  contact,  the  sandstone 
may  be  hardened  into  a  quartzite,  and  if  clay,  lime,  or  other  mineral 
substances  are  present,  new  minerals  may  be  produced. 

Alteration  by  Gases  and  Vapors 

Where  the  volcanic  activity  has  subsided  into  the  solfataric  or 
fumarolic  stage,  with  the  emission  only  of  vapors  and  gases,  the 
country  rock  around  the  vents  and  along  fissures  penetrated  by 
these  gases  and  vapors  may  be  profoundly  altered.  New  minerals 
may  be  produced  in  this  zone  of  alteration  and  deposits  of  older 
minerals  may  be  enriched  by  the  addition  of  new  material  from 
the  gases  and  vapors.  In  this  wise,  important  mineral  and  ore 
deposits  may  be  produced,  to  some  of  which  reference  will  again 
be  made  in  a  later  chapter. 

Ancient  Igneous  Masses  in  Sedimentary  Contact  with  the 
Overlying  Rock 

The.  granite  mass  of  Pikes  Peak,  in  Colorado,  differs  from  the 
granite  masses  previously  discussed  in  the  fact  that  the  contact 
with  the  overlying  rock  is  not  an  igneous,  but  a  sedimentary  one. 
To  be  sure,  when  the  granite  was  formed  by  the  cooling  of  a  deep- 
seated  igneous  magma,  it  was  in  igneous  contact  with  the  over- 
lying rocks  of  that  time.  But  these  covering  rocks  were  entirely 
removed  by  erosion,  and  later  sediments  (sand,  then  limestone) 
were  spread  over  the  eroded  granite  surface,  and  at  a  much  later 
date  still,  after  the  core  of  the  Rocky  Mountains  was  elevated, 
these  younger  rocks  were  again  partly  removed  by  erosion,  and  in 
the  various  canons  which  cut  the  lower  slopes  of  the  mountains 
these  sediments  are  seen  to  rest  upon  the  eroded  surface  of  the 
granite  (Figs.  152,  153). 

Evidently  the  relationship  thus  seen  between  the  granite  and 
the  sediments  does  not  permit  our  classifying  the  Pikes  Peak  granite 
mass  as  belonging  to  any  of  the  igneous  types  so  far  discussed,  and 


212    Form  and  Structure  of  Older  Igneous  Masses 


the  name  abyssolith  has  been  proposed  for  it  by  Grabau.    An  abysso- 
lith,  then,  is  a  mass  of  rock,  generally  granite,  or  one  of  the  more 

basic  rocks  of  this  type, 
which  was  originally  a 
boss,  or  a  batholith,  or 
may  even  have  been  a 
bysmalith,  laccolith,  or 
stock,  which  has  been 
exposed  by  erosion,  then 
covered  by  sediments 
which  are,  of  course,  un- 
affected by  the  igneous 
mass  because  it  was  cool 
at  the  time,  and  after  a 
more  or  less  domelike 
uplift  the  sediments  were 
again  eroded  from  the 
surface  of  the  dome,  re- 
exposing  the  igneous  core 
around  which  unaltered 


FIG.  152.  —  Contact  of  the  stratified  basal 
Palaeozoic  beds  and  the  granitic  basement  rock 
of  the  Pikes  Peak  mass,  in  Williams'  Canon, 
Colorado.  (Photo  by  Author.) 


sediments  crop  out.     In 
addition    to    the    Pikes 

Peak  region  we  may  note  as  an  example  of  this  type  the  Black 
Hills  Dome,  with  its  center  of  old  igneous  rocks.     Many  small 


----  v.v.v.  -.  ................  -.•/*  +  \+T^\^  +>7+>T++s+,  +  +\V+\' 

A*+  *&&*>•+*  *&&-r*v 


^^*^3^Jv[J?vFj^^^ 

•*\-i-  •*•  ^<»'7.JSf4.+^JL+    O.X-+-   +X*~+4 

/I- 

f\ 


Vt 

'inch.    JOjeet. 

FIG.   153.  —  Details  of  contact  of   Palaeozoic   sediments  and  Pre-Palaeozoic 
granites  in  Williams'  Canon,  Colorado.     (After  Crosby.) 

examples  of  such  resurrected  igneous  rocks  of  dome-like  form  sur- 
rounded by  unaltered  sediments  are  known  from  this  country  as 
well  as  from  Europe. 


Contact  of  Igneous  Masses  with  Other  Rocks     213 

Relative  Age  of  Igneous  and  Enclosing  Rock 

If  we  then  recognize  the  nature  of  the  contact  between  an  ig- 
neous mass  and  the  sediments  which  once  covered  it,  we  can  de- 
termine the  relative  ages  of  the  two  series.  If  the  contact  is 
igneous,  the  sediments  are  older  than  the  intruded  igneous  mass. 
If  the  contact  is  a  sedimentary  one,  the  igneous  mass  is  the  older, 
and  there  is  a'  long-time  interval  lost,  between  the  two  —  an  in- 
terval during  which  erosion  removed  the  older  covering  rocks,  with 
which  the  mass  was  in  igneous  contact,  and  this  erosion  occurred 
before  the  sediments  now  seen  in  contact  with  the  igneous  mass 
were  deposited.  This  shows  how  important  it  becomes  to  deter- 
mine whether  a  contact  is  igneous  or  sedimentary. 


CHAPTER  X 
THE  AQUEOUS  OR  HYDROGENIC  ROCKS 

GENERAL  CHARACTER  AND  VARIETIES 

Source  of  the  Material.  —  Practically  all  natural  waters  contain 
mineral  matter  in  solution,  the  common  illustration  being  ocean 
water,  every  liter  of  which  contains  about  35  grams  of  mineral 
matter,  more  than  27  grams  of  this  being  common  salt  (sodium 
chloride),  which  means  that  every  cubic  mile  of  sea- water  contains 
about  131,526,000  short  tons  of  this  important  substance  in  solu- 
tion. Sea-water  is  therefore  spoken  of  as  salty  or  saline,  and  this 
saltiness  is  readily  recognized  by  taste.  The  other  dissolved  sub- 
stances are  also  called  salts,  but  their  presence  is  not  so  readily 
recognized.  Some  water  bodies,  like  the  Baltic  Sea,  contain  only 
one  fourth  or  less  of  this  amount  of  salts  in  solution,  and  such  waters 
are  called  brackish.  The  waters  of  the  Hudson  River  some  dis- 
tance above  New  York  City  are  brackish,  because  they  are  formed 
by  a  commingling  of  the  fresh  water  from  the  upper  river  with  salt 
water  entering  from  the  sea. 

When  the  water  contains  so  little  substance  in  solution  that  it 
can  be  used  for  drinking  purposes  (whether  it  is  contaminated  by 
organic  matter  or  not)  it  is  called  fresh  water,  but  fresh  water  in 
nature  always  contains  some  substances  in  solution,  lime  usually 
predominating,  this  when  present  in  sufficient  quantity  forming 
hard  water.  When  carbonate  of  soda  and  similar  substances  are 
present  in  such  quantities  as  to  render  the  water  unfit  for  human 
consumption,  it  is  called  alkaline,  and  all  travelers  in  the  semi-arid 
regions  of  western  North  America  are  familiar  with  such  water. 
Finally  there  are  water  bodies  like  Great  Salt  Lake,  the  Dead  Sea, 
and  others,  in  which  the  quantity  of  common  salt  in  solution  ex- 
ceeds many  times  that  in  the  ocean  water,  and  such  waters  are 
spoken  of  as  super-saline,  or  as  brines.  Brines  may  also  be  pro- 
duced by  partial  evaporation  of  ocean  water,  just  as  brackish  waters 
are  produced  by  the  dilution  of  ocean  water. 

214 


General  Character  and  Varieties  215 

Separation  of  Material.  —  When  the  substances  which  waters 
hold  in  solution  are  separated  out  so  as  to  form  solid  material,  this 
material  constitutes  rock  masses  to  which  in  general  we  apply  the 
name  of  salts,  after  the  example  of  the  commonest  of  these,  the 
ordinary  salt.  If  the  separation  is  produced  by  organisms,  the 
product  is  called  organic  salts,  and  these  belong  in  the  division 
of  organic  or  biogenic  rocks,  where  they  will  be  discussed.  If 
the  separation  is  by  inorganic  activities,  the  product  is  a  normal 
aqueous  or  hydrogenic  rock,  to  which  the  present  chapter  is 
devoted. 

Separation  by  inorganic  means  is  accomplished  in  a  variety  of 
ways,  of  which  the  following  are  the  most  important : 

1.  Separation  by  the  condensation  or  complete  evaporation  of 
the  water  by  heat,  drying  winds,  etc.,  during  which  process  a  stage 
is  reached  when  the  water  becomes  saturated,  that  is,  it  holds  in 
solution  as  much  of  a  given  salt  as  it  can  hold  for  that  temperature 
and  pressure.     If  that  stage  is  passed,  the  excess  of  salt  separates 
out,  often  in  a  more  or  less  crystalline  form.     The  point  of  satura- 
tion varies  with  the  nature  of  the  salt,  and  when  two  or  more  salts 
are  present  in  the  solution,  they  will  not  only  have  separate  and  dis- 
tinct saturation  points,  but  the  presence  of  each  is  likely  to  in- 
fluence the  saturation  point  of  the  others,  either  lowering  or  raising 
them.     Complete  separation  of  all  the  salts- will  occur  upon  com- 
plete evaporation  of  the  water.     Such  salts  are  called  evaporation 
products,  or  briefly,  evaporates.1 

2.  Separation  of  salts  from  the  solution  by  the  force  of  attrac- 
tion which  crystals  or  particles  of  mineral  matter  exert  on  material 
of  the  same  kind  in  the  solution  (generally  a  saturated  one)  in  which 
those  crystals  or  particles  are  immersed. 

3.  Separation  by  the  abstracting  of  the  solvent  or  substance 
which  holds  the  salt  in  solution. 

4.  Separation  by  chemical  reaction  between  minerals  in  solution 
in  the  water,  and  other  substances  introduced  from  extraneous 
sources,  either  as  gases  or  as  solutions,  and  the  consequent  forma- 
tion of  new  and  less  soluble  compounds  which  are  then  precipitated. 

5.  Separation  by  electrolysis. 

1  Strictly  speaking,  this  is  also  a  chemical  combination  of  the  dissociated  ions,  which 
occurs  at  the  moment  of  solidification.  Such  salts  are  therefore  also  precipitates,  but 
it  is  well  to  keep  this  type  distinct  from  that  resulting  through  reactions  with  newly 
introduced  substances  as  given  under  4. 


2i6  The  Aqueous  or  Hydrogenic  Rocks 

As  illustrations  of  the  first  class  we  may  cite  the  separation  of 
salt  when  sea-water  is  evaporated  or  when  incrustations  of  salt 
are  formed  from  a  solution  of  that  substance  which  is  allowed  to 
stand  for  a  time  in  a  warm  room.  The  second  method  is  illustrated 
by  the  crystallization  of  the  alum  of  a  saturated  solution  around  a 
crystal  of  alum  introduced  into  that  solution,  or  the  formation 
of  rock  candy  from  a  saturated  solution  of  sugar.  Precipitation 
through  abstraction  of  a  solvent  is  shown  by  the  deposits  of  lime  on 
the  inside  of  boilers  and  tea-kettles  where  "  hard  "  water  is  used, 
the  solvent,  which  is  carbon  dioxide,  being  driven  off  by  the  heat. 
It  is  also  shown  by  the  precipitation  of  lime  around  the  mouths  of 
springs,  in  which  the  water  escaping  from  under  pressure  loses  some 
of  the  carbon  dioxide  which  was  the  solvent  of  the  lime  in  the  water. 
Examples  of  precipitation  of  salts  by  the  addition  of  substances  in 
solution  which  by  chemical  reaction  produce  a  less  soluble  com- 
pound, or  by  the  passing  of  gases  through  a  solution,  are  familiar  to 
all  workers  in  chemical  laboratories,  while  the  electrolytic  method 
is  one  much  practiced  in  the  arts. 

The  first  three  methods  cited  are  most  commonly  observed  in 
nature.  The  precipitation  by  the  addition  of  reagents,  whether 
liquid  or  gaseous,  is  chiefly  found  in  the  formation  of  lime  deposits 
under  the  influence  of  ammoniacal  gases.  Since  these  are,  however, 
in  nearly  all  cases  produced  by  the  direct  activities  or  the  indirect 
influence,  through  decay,  of  organisms,  such  deposits  are  best 
referred  to  the  organic  or  biogenic  group,  and  they  will  be  discussed 
there  in  this  book.  Electrolytic  processes  in  nature  are  still  little 
understood,  but  that  they  are  going  on  cannot  be  doubted.  Some 
of  the  inclusions  of  salt  in  the  pore  spaces  of  marine  sediments  have 
been  explained  in  this  manner. 

Most  precipitates  from  an  aqueous  solution  are  called  salts,  no 
matter  how  they  are  formed  or  what  their  composition.  There 
are,  however,  some  simpler  substances,  such  as  the  oxide  of  silicon 
or  silica  (SiO2)  and  others,  which  cannot  properly  be  called  salts. 
(See  Chapter  IV.)  One  of  the  essential  characteristics  of  a  salt  is 
its  purity  of  composition,  though  it  is  true  that  under  certain  con- 
ditions precipitation  of  several  salts  may  take  place  simultaneously, 
thus  producing  what  is  called  a  eutectic  mixture.  When  pure,  the 
material  is  in  reality  a  single  mineral  mass,  as  for  example,  the  min- 
eral Halite  or  rock  salt ;  but  because  of  its  extensive  development 
it  is  treated  as  a  rock. 


The  Textures  of  Aqueous  Deposits  217 

Classification  of  aqueous  precipitates  according  to  composition.  —  It 
is  evident  that  chemical  composition  is  the  primary  basis  on  which 
aqueous  precipitates  must  be  classified.  Furthermore,  most  salts  look 
very  much  alike  except  when  they  are  crystallized,  or  when,  as  in 
special  cases,  color,  hardness,  and  weight  make  distinction  possible. 
In  the  following  pages  will  be  given  some  of  the  more  important  and 
common  salts  and  oxides,  the  classification  being  made,  for  the  sake 
of  convenience,  upon  the  basic  element  in  the  composition. 

THE  TEXTURES  OF  AQUEOUS  DEPOSITS 

The  texture  of  aqueous  precipitates  is  either  crystalline  or  non- 
crystalline  —  the  latter  also  being  designated  amorphous.  Crystals 
of  all  sizes  and  degrees  of  perfection  may  form,  the  most  perfect 


FIG.  154  a.  —  Oolitic  limestone  silicified.     (Photo  by  B.  Hubbard.) 

being  those  least  interfered  with  by  other  crystals.  The  amor- 
phous texture  may  be  either  in  the  form  of  separate  or  discrete 
particles,  which  may  or  may  not  be  tied  together  subsequently,  as 
in  oolites,  or  it  may  be  a  solid  or  concrete  mass. 

Discrete  particles.  —  These  comprise  the  two  following  textures : 

(a)  Oolitic  texture  (Fig.  154  a,  b),  characteristic  of  oolites.     The 

particles  are  small  spheres,  generally  with  a  nucleus  and  with  radial, 

or  with  concentric  or  zonal,  structure,  the  size  suggesting  the  roe 

of  fish ;  typically  lime  carbonate. 


218 


The  Aqueous  or  Hydrogenic  Rocks 


(b)  Pisolitic  texture  (Fig. 
155),  characteristic  of  piso- 
lites. The  spherules  are  of  the 
size  of  a  pea  or  larger.  These 
are  also  chiefly  lime  carbonate. 

Concrete  Masses.  —  These 
include  the  following  textures : 

(a)  Botryoidal    (Fig.    156), 
with  grape-like  rounded  sur- 
faces. 

(b)  Banded,  in  layers  which 
in    section    show   a  banded 
structure. 

(c)  Laminated,    in    thin 


FIG.  1546.  —  Thin  section  of  Jurassic 
oolite    showing    the    characteristic   zonal 
structure.     Brown  Jura,  Schonberg,  near    layers  or  laminae. 
Freiburg  I.  B.,   Germany.     Enlarged   24 
diameters.     (After  Rosenbusch.) 


(d)  Scaly,    composed   of 
small  scale-like  masses. 
(e)  Fibrous,  in  slender  hairlike  fibers,  often  elongated  crystals. 
(/)   Tufaceous  (Figs.  157,  158),  porous  as  in  calcareous  tufa. 
(g)  Concretionary  (Fig.  1 59) ,  in  large  more  or  less  spherical  masses. 


FIG.  155.  —  Photograph  of  a  specimen  of  pisolite  somewhat  reduced.    (Photo 

by  B.  Hubbard.) 


The  Textures  .of  Aqueous  Deposits  219 


FIG.  156.  —  Botryoidal  structure  in  calcium  carbonate  deposits. 


FIG.  157.  —  A  fragment  of  calcare- 
ous tufa.  Reduced.  (Photo  by  B. 
Hubbard.) 


FIG.  158.  —  Bird's  nest  and  eggs 
"petrified  "  or  covered  with  a  deposit 
of  lime  carbonate,  by  submersion 
in  tufa-depositing  spring-water.  Re- 
duced. (Photo  by  B.  Hubbard.) 


FIG.  159.  —  Concretionary  limestone ;  basin  of  ancient  Lake  Lahonton. 

Russell.) 


(After 


22O  The  Aqueous  or  Hydrogenic  Rocks 

THE  PRINCIPAL  TYPES  OF  AQUEOUS  OR  HYDROGENIC 
DEPOSITS 

Among  the  many  precipitates  or  other  deposits  formed  directly 
from  aqueous  solutions,  a  certain  number  is  found  in  sufficiently 
large  quantities  to  be  treated  as  rock  material,  while  others  are  im- 
portant as  sources  of  valuable  substances  or  are  themselves  of 
economic  value.  Their  essential  mineral  characters  have  already 
been  given  in  the  tables  in  Chapter  IV. 

Non-Metallic  Aqueous  Deposits 

Rock  Salt.  —  Chloride  of  sodium  (NaCl).  — This  is  the  most  important  as 
well  as  the  most  abundant  mineral  substance  obtained  from  ocean  water.  It 
commonly  occurs  in  beds,  which  may  have  a  thickness  of  a  hundred  feet  or  more 
but  generally  are  only  a  few  feet  thick,  forming  a  succession  of  beds  separated 
by  gypsum,  anhydrite,  limestone,  dolomite,  or  clay,  —  or  more  rarely  by  other 
mineral  matter.  Such  deposits  are  formed  by  concentration  of  sea- water  either 
in  basins  cut  off  from  the  sea  by  elevation  or  by  the  formation  of  a  barrier,  or 
in  lagoons  behind  a  bar  with  continued  supply  of  sea-water  through  an  inlet, 
as  more  fully  discussed  in  the  next  chapter.  Salt  is  also  separated  from  lake 
basins  in  arid  regions  by  the  concentration,  through  partial  evaporation,  of  the 
water.  Finally,  salt  deposits  are  formed  in  desert  basins  from  salt  disseminated 
through  the  rocks  in  the  rims  of  those  basins,  as  more  fully  discussed  in  the  next 
chapter. 

Rock-salt  deposits  are  seldom  very  continuous,  having  in  most  cases  a  lens- 
like  form.  Salt  is  also  extensively  manufactured  by  the  evaporation  of  sea- 
water  in  shallow  salt  gardens  or  sea  salinas,  which  are  located  on  nearly  all  shores 
where  the  climatic  and  other  conditions  are  favorable.  Natural  as  well  as  ar- 
tificial brines,  formed  by  the  solution  of  old  salt  beds  in  the  earth's  crust,  or  of 
disseminated  salt,  are  also  extensively  used  in  the  manufacture  of  salt.  Salt  is 
mined  in  central  New  York,  in  southern  Michigan,  in  Louisiana,  and  in  a  few 
other  localities  in  the  United  States.  Extensive  salt  mines  exist  in  Austria, 
Galicia,  Rumania,  and  elsewhere  in  the  Old  World.  The  salt  mines  of  North 
Germany  are  worked  chiefly  for  their  potash  deposits.  Hills  or  mountains  of 
salt  are  found  in  many  of  the  dry  regions  of  the  world,  —  in  northern  Spain 
(Cardona),  Algeria,  Persia,  and  elsewhere. 

The  chief  uses  are  for  domestic  and  dairy  purposes  and  for  chemical  industries. 

Gypsum.  —  Hydrous  sulphate  of  lime  (CaSO4  •  2  H2O). — This  is  a  common 
associate  of  salt  deposits,  being  found  beneath  beds  of  rock-salt  of  marine  origin 
(sometimes  replaced  by  anhydrite)  and  always  separating  from  sea- water,  which 
undergoes  concentration  before  the  rock-salt  separates  out.  Such  rock  gypsum 
is  commonly  massive  and  sometimes  impure,  but  pure  white  gypsum  (alabaster) 
also  occurs.  It  also  occurs  as  crystals  (selenite),  scattered  through  mud  and 
sand  deposits,  especially  in  desert  regions.  A  fibrous  variety  (satin  spar)  occurs 
in  veins.  Gypsum  is  seldom  formed  in  extensive  beds  in  desert  regions  except 


Principal  Types  of  Aqueous  Deposits          221 

where  sea-water  evaporates.  It  is  also  formed  as  an  alteration  product,  gen- 
erally of  limestones,  by  waters  carrying  sulphuric  acid ;  large  beds  of  gypsum 
result  in  this  way,  those  of  New  York  state  being  an  example.  Anhydrite 
deposits  are  also  changed  to  gypsum  when  surface  waters  come  in  contact  with 
them.  Extensive  deposits  of  gypsum  are  found  in  the  "  Red  Beds"  of  the  west- 
ern United  States,  and  in  Kansas,  Oklahoma,  and  Texas.  It  is  found  in  Nova 
Scotia  and  in  the  Paris  Basin,  where  it  has  been  and  still  is  extensively  quarried 
and  burned  into  Plaster  of  Paris ;  also  in  many  other  parts  of  the  world. 

Raw  gypsum  is  ground  and  used  as  a  natural  fertilizer  (land  plaster),  to  re- 
tard the  setting  of  cement,  and  for  many  chemical  purposes.  When  burned 
or  "  calcined  "  at  350°  F.  it  loses  most  of  its  water  (CaSO4  •  |  H2O),  and  is 
ground  into  the  familiar  "  Plaster  of  Paris."  When  this  is  mixed  with  water 
gypsum  is  again  formed.  It  hardens  rapidly  and  is  used  extensively  in  the 
arts  for  molding,  statuary,  etc.,  and  for  stucco  work. 

Anhydrite.  —  Calcium  sulphate  (CaSO4) .  —  This  is  distinguished  from  gyp- 
sum by  its  greater  hardness  and  specific  gravity.  Its  color  is  often  also  more 
grayish.  It  is  formed  as  a  primary  deposit  from  sea-water  in  cut-off  basins, 
especially  when  the  water  contains  an  excess  of  chlorides.  The  largest  known 
deposits  thus  formed  are  in  northern  Germany,  where  they  underlie  the  salt  and 
potash  beds.  Anhydrite  slowly  changes  to  gypsum  by  taking  on  water,  with 
an  expansion  of  the  mass.  It  is  of  little  economic  importance. 

Carbonate  of  Lime.  —  Calcite,  Aragonite,  Limestones,  etc.  (CaCO3).  —  The 
great  bulk  of  the  deposits  of  carbonate  of  lime,  including  the  limestone  beds,  is 
of  organic  or  of  clastic  origin,  but  there  are  a  number  of  lime  deposits  which  be- 
long to  the  hydrogenic  division.  The  most  extensive  of  these  is  probably  cal- 
careous tufa  (Fig.  157),  which  is  formed  by  springs  issuing  in  limestone  regions 
and  depositing  the  excess  of  lime  which  they  hold  in  solution.  Calcareous  tufa 
is  mostly  porous  and  light,  incrusting  not  infrequently  mosses  and  other  plants 
and  also  other  objects.  Some  massive  deposits  of  th'is 
type,  however,  are  known,  constituting  the  so-called 
"  Mexican  Onyx."  Compact  carbonate  of  lime  de- 
posits are  also  formed  in  caves  as  stalactites  and 
stalagmites.  These  usually  have  a  banded  structure, 
showing  the  successive  addition  of  layers.  That  of 
the  stalactites  is  concentric  around  the  longitudinal 
axis,  which  in  the  early  stages  is  formed  by  a  delicate 
tube. 

Beds  of  limestone  are  sometimes  built  up  from  large 
rounded  masses  or  "  concretions,"  which  have  re- 
sulted from  the  deposition  of  carbonate  of  lime,  gen- 
erally around  a  nucleus.  Sometimes  these  are  -  FIG.  160.  —  Concre- 
formed  as  original  masses  of  limestone,  as  in  the  tionary  magnesian  lime- 
limestone  deposits  of  Lake  Lahonton  (Fig.  159),  or  *tone  (Permian)>  Dur~ 
an  older  limestone  of  this  type  which  occurs  in  the  ha,m'  *fg ^  *£"* 
Permian  series  (Magnesian  limestone)  of  northeastern  photo  ) 
England  (Fig.  160).  In  other  cases  these  concretions 

occur  in  shale  beds,  forming  distinct  layers,  which  when  they  become  confluent 
by  ^continued  growth  form  bands  of  limestone.  When  separate  they  often 


222 


The  Aqueous  or  Hydrogenic  Rocks 


FIG.  161  a.  —  Septarium  or 
Turtlestone.  A  concretion  from 
calcareous  shales.  The  fissures 
of  the  interior  were  filled  with 
calcite  veins,  which  became 
exposed  after  erosion  and 
weathering  of  the  surface  of 
the  concretion ;  about  \  natu- 
ral size.  (B.  Hubbard,  photo.) 


take  on  disk-shaped  or  spherical  forms,  while  a  series  of  radial  cracks  are  de- 
veloped in  the  interior  with  the  growth  of  calcite  veins.     Such  concretions  are 

called  septaria    (Figs.    161  a,  b).  .All  car- 
bonate of   lime   deposits   effervesce  readily 
with  dilute  hydrochloric  acid,  this  being  the 
most  distinctive  test. 
L  Dolomite.  —  Carbonate  of  lime  and  mag- 

;  ~"**^        nesia  ((CaMg)CO3).— This  differs  from  the 

m  V  I       pure  carbonate  of  lime  deposits  in  its  greater 

A  *  ^fl        hardness  and  in  the  fact  that  it  effervesces 

^^^^  .          ^jB          only  in  strong  hydrochloric  acid.     It  is  some- 

1^ — ^^Jl^BII       times   a   primary    deposit  in  basins  where 

anhydrite  is  forming,  with  which  it  becomes 
more  or  less  intimately  mixed.  It  may  also 
be  formed  as  an  original  precipitate  in  por- 
tions of  the  sea  in  which  the  solution  has  be- 
come concentrated.  Many  dolomitic  lime- 
stones are,  however,  of  secondary  origin,  the 
original  magnesia  content,  which  was  derived 
for  the  most  part  from  calcareous  algae,  etc., 
being  proportionately  increased  through  the 
solution  of  carbonate  of  lime  by  ground  water.  Secondary  deposition  of  mag- 
nesium carbonate  also  takes  place  from  solutions  in  the  circulating  ground 
water.  Pure  dolomite  contains  54.35%  CaCO3,  and  45.65%  MgCO3. 

Magnesite.  —  Carbonate  of  Magnesia  (MgCO3).  —  This  is  harder  and  more 
compact  than  dolomite,  and  dissolves  with  effervescence  only  in  hot  hydro- 
chloric acid.  It  occurs  as  a  crystalline  mineral,  as  replacement  of  dolomite, 
and  also  as  an  amorphous,  earthy,  hard,  compact  mineral,  probably  a  colloidal 
precipitate.  It  is  often  concretionary  with 
conchoidal  fracture,  appearing  like  un- 
glazed  porcelain,  this  type  being  usually 
derived  from  the  alteration  of  serpentine 
and  other  magnesian  rocks.  Beds  of  mag- 
nesite  are  found  associated  with  gypsum  in 
fresh- water  limestones  of  France. 

Apatite  and  Phosphate  Rock.  —  Al- 
though the  most  extensive  deposits  of  phos- 
phate of  lime  are  of  organic  origin,  there 
are  some  that  must  be  considered  as  purely 
aqueous  or  hydrogenic  deposits.  Among 
these  are  the  apatite  veins  and  the  marine 
concretions  of  phosphate  around  an  or- 
ganic nucleus.  Phosphate  of  lime  occurs 
also  as  a  secondary  replacement  of  lime- 
stone. Apatite  crystallizes  in  hexagonal 
prisms,  etc.,  and  is  readily  recognized  by 

its  form,  hardness,  and  color  (see  Table  in  Chapter  IV).     Rock  phosphates 
are  generally  amorphous  and  compact.    The  pure  mineral  (tricalcium  phos- 


FIG.  161  b.  —  A  weathered  sep- 
tarium,  showing  the  mineral, 
which  filled  the  fissures,  left  in 
relief,  thus  producing  the  typical 
"septarium"  structure.  (Photo 
of  a  specimen  in  Columbia  Uni- 
versity, by  B.  Hubbard.) 


Principal  Types  of  Aqueous  Deposits  223 

phate)    contains   45.8%   of  phosphoric  acid   (P2O6).      The  principal  use  of 
phosphate  is  as  a  fertilizer. 

Potash  Salts.  —  There  are  a  number  of  evaporation  products  which  are  pri- 
marily salts  of  potash  and  are  important  sources  of  this  substance.  Among  the 
more  common  of  these  are  Sylvite,  the  chloride  of  potassium  (KC1),  a  very 
soluble,  soft,  transparent,  milky  reddish  or  yellowish  mineral,  commonly  mixed 
with  rock  salt;  Carnallile,  the  hydrous  double  chloride  of  potassium  and 
magnesium  (KC1  •  MgCl2  •  6  H2O),  a  colorless  or  snow-white  or  variously 
colored  salt,  easily  soluble,  and  an  important  source  of  potash ;  and  Kainite, 
the  compound  chloride  of  potassium  and  sulphate  of  magnesium,  with  water 
(KC1  •  MgSO4  •  3  H2O),  colorless  to  deep  blood-red,  and  chiefly  an  alteration 
product.  The  chief  use  of  potash  is  for  agricultural  purposes  and  in  chemical 
works. 

Trona.  —  Sodium  carbonate  (Na2CO3  •  NaHCO3  •  2  H2O).  —  This  is  a  com- 
mon evaporation  product  of  alkaline  lakes.  It  is  glassy  or  transparent 
when  crystallized,  but  usually  forms  a  white  salt.  Important  American 
localities  are  Searle's  Marsh,  Owens  and  Mono  Lakes,  Cal.,  and  Soda 
Lake,  Nevada,  while  some  Russian  and  Hungarian  lakes,  and  the  Natron 
lakes  of  Egypt  west  of  Cairo,  represent  foreign  localities.  It  is  used  for 
domestic  and  chemical  purposes,  but  much  of  the  commercial  trona  is  arti- 
ficially produced.  On  account  of  its  solubility  it  is  not  generally  found 
in  the  older  rocks. 

Mirabilite.  —  (Glauber  salt.)  —  Hydrous  sulphate  of  sodium  (Na2SO4  • 
10  H2O).  — This  is  a  crystalline,  granular  salt,  usually  colorless,  and  readily 
soluble  in  water.  It  is  formed,  especially  in  winter,  by  certain  saline  bodies, 
such  as  Great  Salt  Lake,  Utah,  and  the  Kara  Bugas  Gulf  on  the  east  coast  of 
the  Caspian  Sea.  On  the  floor  of  the  Kara  Bugas  a  bed  of  mirabilite,  estimated 
to  contain  1000  million  metric  tons,  has  formed  but  it  is  not  yet  exploited.  In 
Wyoming  and  New  Mexico  occur  beds  of  this  salt  mixed  with  epsomite,  natron, 
and  common  salt  (halite).  In  some  cases  the  deposit  is  15  feet  thick  and  covers 
an  area  of  100  acres. 

Glauberite.  —  Double  sulphate  of  sodium  and  calcium  (Na2SO4  •  CaSO4).  — 
This  is  a  white,  gray,  yellow,  or  red  mineral,  a  little  harder  than  common  salt 
(2.5-3).  It  occurs  in  many  playa  lakes  (Borax  Lake,  Searle's  Marsh,  Death 
Valley),  and  in  the  salt  deposits  of  Germany,  Spain,  Austria,  Sicily,  etc.  It  also 
occurs  in  a  number  of  Tertiary  deposits  in  Spain. 

Soda  Niter,  Chile  Saltpeter.  —  Nitrate  of  sodium  (NaNO3).  —  This  is  a 
white  to  reddish  brown,  gray,  or  lemon-yellow  salt,  commonly  impure,  when  it 
is  called  caliche.  It  is  abundant  in  the  desert  tracts  of  western  Chile  and  else- 
where in  South  America  and  other  parts  of  the  world. 

Borax  Salts.  —  These  include  Borax  or  Tinkal,  sodium  borate  (Na2B4O7  • 
10  H2O),  a  colorless  or  yellowish  or  green  to  gray,  soluble  mineral,  somewhat 
harder  than  common  salt ;  Colemanite,  the  hydrous  borate  of  calcium  (CaBeOu 
•  5  H2O),  generally  a  massive,  glassy  or  colorless,  more  or  less  transparent  min- 
eral, found  as  a  bed  from  five  to  20  feet  thick  in  San  Bernardino  Co.,  Cal.,  and 
elsewhere ;  and  Ulexite,  the  double  borate  of  sodium  and  calcium  with  water 
(NaCaB5O9  •  8  H2O),  which  occurs  as  white  balls  of  fibrous  material  and  satiny 
luster  (cotton  balls).  Borax  salts  are  found  in  volcanic  regions,  such  as  the 


224  The  Aqueous  or  Hydrogenic  Rocks 

famous  "  saffioni,"  i.e.,  fumaroles,  in  the  volcanic  region  of  Tuscany;  in  hot 
spring  and  lake  deposits  of  volcanic  districts,  as  in  Tibet,  etc.,  and  the  Coast 
Ranges  of  California,  and  in  playa  deposits,  as  in  Death  Valley  and  elsewhere 

in  the  western  United  States.     Colemanite 
is  the  chief  American  source  of  borax. 

Silica.  —  Oxide  of  silicon  (SiO2)  frequently 
with  water.  —  The  most  familiar  form  of  this 
is  quartz  in  crystalline  or  amorphous  form, 
which  occurs  in  veins,  geodes,  etc",  colorless 
to  variously  colored;    also  as   addition   to 
quartz  grains  in  sandstones,  enlarging  them 
to  fill  all  the  interstices.     It  occurs  exten- 
sively as  a  porous,  white,  and  light  spongy 
FIG.    162.  —  Concretion    of       deposit  of  hydrous  quartz,  around  geysers 
flint   from  the   chalk  beds  of       (gee  r  Ig  }   and  .   hence  k 

England.    About  one  fifth  nat-  '.,.  .          .   ,          A     .     .. 

ural  size.  (Photo  by  B.  geysente  or  Sllicious  smter'  Ag^m,  it  occurs 
Hubbard  )  as  concretions  of  flwt  m  chalk  beds  (Fig. 

162),  or  as  chert  nodules  or  layers  in  lime- 
stone. These  types  are  compact  and  have  a  conchoidal  fracture,  with  a  black 
or  dark  brown  color  on  the  freshly  fractured  surface.  Flint  and  chert  are 
secondary  concentrations  through  solution  and  redeposition  of  disseminated 
silica  particles  of  organic  origin.  (Fig.  163). 

Glauconite.  —  Greensand.  —  This  is  a  complex  silicate  of  potassium  and  iron 
(K-FeSi2Oe  •  H2O).  It  consists  mainly  of  dark  green  grains,  generally  mixed 
with  impurities,  found  in  strata  of  Cretaceous  and  other  formations,  and  is  form- 
ing at  the  present  time  along  the  margin  of  the  continental  shelf  in  the  Atlantic 
Ocean  and  elsewhere.  It  promises  to  be  an  important  source  of  potassium,  in 
which  the  New  Jersey  deposits  are  especially  rich.  They  also  contain  much 
phosphorus. 


Metallic  Aqueous  Deposits 

Manganese  Ores.  —  Nodules  of  oxide  of  manganese  occur  in  many  parts 
of  the  deep  sea  in  the  region  of  red  clay  deposits  (Fig.  174).  They  are 
more  or  less  rounded  or  irregular  masses  of  black  color.  Oolitic  pyrolu- 
site  (MnO2),  an  iron-black  mineral  of  metallic  luster,  occurs  interstrati- 
fied  with  other  beds  in  certain  localities.  A  hydrous  oxide  of  manganese 
(wad)  occurs  as  an  amorphous  earthy  deposit  in  bogs,  generally  associated 
with  iron  ores. 

Limonite.  —  Bog  iron  ore,  hydrous  oxide  of  iron  (2  Fe2O3  •  3  H2O).  —  This 
occurs  in  massive,  botryoidal,  earthy,  or  porous  masses  of  yellow  or  ochery  color 
and  yellow  streak.  It  forms  deposits  in  bogs  and  swamps,  in  shallow  water,  in 
depths  above  12  feet,  from  the  oxidation  of  carbonate  of  iron  carried  into  the 
bog  in  solution.  The  ores  are  always  mixed  with  sand  or  earthy  impurities. 
There  are  several  other  ferric  hydrates,  some  with  less  and  some  with  more 
water;  e.g.  Gothite  (Fe2O3 .  2  H2O).  Oolitic  limonites  occur  in  older  strata, 
where  they  form  an  important  source  of  iron,  though  of  low  percentage.  Such 


Principal  Types  of  Aqueous  Deposits  225 

are  the  minette  ores  of  Lorraine  and  Luxemburg,  formerly  one  of  the  chief  domes- 
tic sources  of  iron  in  Germany.  These  are  believed  by  many  to  have  resulted 
from  the  oxidation  of  oolitic  siderite  or  glauconite. 

Hematite.  —  Red  oxide  of  iron  (Fe2O3).  —  This  ranges  in  color  from  deep 
red  to  reddish  brown  and  black  with  red  streak.  It  occurs  interbedded  with 
shales  and  limestones  in  the  Palaeozoic  formations  of  Germany,  France,  Bohemia, 


FIG.  163.  —  Quarry-wall  of  Cummings  Cement  Mine,  Akron,  Erie  Co.,  N.  Y. 
The  upper  bed  (a)  is  a  very  cherty  limestone  (Corniferous)  the  chert  nod- 
ules standing  out  in  relief  as  the  result  of  weathering;  (b)  Onondaga  lime- 
stone without  chert;  (c)  Akron  dolomite,  7  feet  thick,  with  fossils  of  upper 
Silurian  Age.  Between  it  and  the  Onondaga  limestone  (Middle  Devonian) 
is  a  hiatus  or  break  in  succession  involving  the  whole  of  the  lower  Devonian, 
which  is  absent.  The  two  formations  are  disconformable ;  (d)  Bertie  water-lime 
mined  for  natural  cement.  (Courtesy  N.  Y.  State  Museum.) 


and  the  United  States,  where  the  (Clinton)  iron  ores  of  New  York  and  the  Appa- 
lachian region  form  a  characteristic  example.  Many  of  these  beds  appear  to 
be  replacement  of  limestone  by  the  iron ;  in  other  cases  the  iron  ore  seems  to 
be  a  primary  deposit. 

Siderite.  —  Carbonate  of  iron  (FeCO3).  —  This  important  iron  ore  occurs 
in  crystallized  form  in  veins  cutting  limestone  and  other  rocks,  and  is  readily 
recognized  by  its  perfect  rhombohedral  cleavage,  vitreous  to  pearly  luster, 
greenish  to  brownish  color,  and  translucent  to  subtranslucent  character.  It  is 
more  common,  however,  as  an  interbedded  rock  in  the  older  sedimentary  series, 


226 


The  Aqueous  or  Hydrogenic  Rocks 


being  known  as  clay-iron-stone,  spherosiderite,  or  black  band.     Clay-iron-stone 
has  a  dense  or  fine-grained  structure,  forming  concretions,  which  often  include 


FIG.    164.  —  Clay-iron-stone  concretion,  Connecticut  valley.     (After 
Gratacap.) 


organic  remains  (Fig.  164).     The  "black  band"  forms  continuous  layers  in 
the  formations  which  carry  coal. 


CHAPTER  XI 

MODE   OF   OCCURRENCE  AND   ORIGIN   OF  THE 
AQUEOUS   OR  HYDROGENIC   ROCKS 

TYPES  OF  DEPOSITS* 

EACH  of  the  several  water  bodies  of  the  earth  may  form  precipi- 
tates of  mineral  matter,  and  he'nce  we  may  classify  these  deposits 
under  the  following  heads : 

A.  Marine  deposits,  or  those  formed  in  the  sea  and  its  depend- 
ent water  bodies. 

B.  Lacustrine  deposits,  or  those  formed  in  lakes,  ponds,  fresh- 
water marshes,  salinas,  playas,  etc. 

C.  Flumatile  deposits,  or  those  formed  by  rivers  in  their  beds,  or 
on  the  flood-plain  or  delta  surfaces,  except  those  formed  in  lakes 
along  the  river  course,  or  at  its  mouth. 

D.  Terrestrial  deposits,  or  those  formed  by  springs  and  by  the 
ground  water  in  fissures,  caverns,  cavities  in  the  rock,  etc.     To 
these  belong  many  important  deposits  of  mineral  matter. 

The  several  types  may  grade  one  into  the  other,  but  their  main 
characteristics  are  quite  distinct. 

SEA- WATER  AND  THE  EVAPORATION  PRODUCTS  AND  CHEMICAL 
PRECIPITATES  FORMED  FROM  IT 

Amount  of  Salt  in  Sea- Water.  —  As  has  been  noted  in  the  pre- 
ceding chapter,  the  oceans,  which  are  the  large  bodies  of  sea-water 
lying  between  the  continents,  contain  the  normal  salt  water,  in 
which  about  35  grams  of  salt  occur  in  every  liter  of  water.  Since 
a  liter  of  pure  water  weighs  1000  grams,  the  quantity  of  salt  is 
essentially  35  per  thousand  by  weight,  which  is  expressed  by  the 
formula  35  per  mille  (or  35^).  This  corresponds,  of  course,  to 
3.5  per  hundred  or  3.5  per  cent  (3.5%),  but  since  the  difference  in 
salinity  between  different  water  bodies  Is  often  very  slight,  and 

227 


228  Aqueous  or  Hydrogenic  Rocks 

because  in  brackish  and  fresh  waters  the  actual  quantity  of  mineral 
matter  in  solution  is  very  small,  it  is  more  satisfactory  to  express 
the  quantity  in  permillages  than  in  percentages. 

The  quantity  of  salt  in  solution  determines  the  salinity  of  the 
water.  Thus  the  average  salinity  of  the  ocean  water  is  35  per  mille, 
(3.5  per  cent),  varying  somewhat  for  the  different  oceans,  for  differ- 
ent parts  of  the  same  ocean,  and  for  different  depths.  On  the  other 
hand,  the  salinity  of  the  Red  Sea  surface  waters  is  38.8  per  mille, 
while  that  of  the  surface  waters  of  the  Black  Sea  is  only  18.3  per 
mille,  which  is  due  to  the  fact  that  this  water  body  is  almost  entirely 
cut  off  from  the  rest  of  the  sea,  and  that  it  receives  many  fresh- 
water rivers.  Finally,  the  average  surface  salinity  of  the  Baltic 
Sea  is  only  7.8  per  mille,  whereas  if  the  water  of  this  enclosed  basin 
is  taken  as  a  whole,  it  is  somewhat  more  saline  because  of  the 
greater  salinity  of  the  deeper  layers.  Even  then,  however,  it  is 
only  10  per  mille.  The  Baltic  Sea,  moreover,  shows  a  remarkable 
gradation  in  the  salinity  of  its  waters  from  west  to  east.  Where  it 
joins  the  North  Sea  at  the  Skager  Rack,  the  salinity  is  34  per  mille, 
but  in  the  Kattegat  it  is  only  22  per  mille.  Thence  it  gradually 
decreases  eastward  and  northward  until  the  waters  near  the  heads 
of  the  Gulf  of  Finland  and  that  of  Bothnia  are  essentially  fresh. 

Composition  of  the  Sea-Salts 

Although,  strictly  speaking,  the  material  held  in  solution  by  the 
water  of  the  sea  is  not  in  combination  as  salts,  such  as  are  produced 
on  evaporation,  but  rather  in  the  form  of  ions,  the  basic  elements 
and  the  acid  radicals  being  separated,  nevertheless  it  is  customary 
and  convenient  to  consider  them  as  combined  into  the  form  of 
salts.  Among  these,  common  salt  or  sodium  chloride  makes  up  the 
bulk  of  the  material,  being  nearly  78  per  cent  of  the  total  mass  of 
salt,  or  over  27  per  mille  of  the  salinity  (which  is  taken  as  35  in  round 
numbers) .  In  the  following  table  the  composition  of  the  sea- water 
salts  is  given  in  the  form  of  such  combinations,  together  with  the 
permillage  of  each  in  normal  sea-water,  and  the  number  of  short 
tons  in  a  cubic  mile  of  sea-water. 

With  the  calcium  carbonate  are  included  the  small  quantities 
of  other  salts  present,  such  as  the  iodine,  lithium,  manganese, 
and  phosphorus  salts,  and  the  silver,  gold,  nickel,  and  other  metals 
which  are  present  in  minute  quantities  in  the  solution. 


Sea- Water  and  Its  Products 

TABLE  OF  THE  COMPOSITION  OF  SEA-SALTS 


229 


PERMTLLAGE 

PERCENTAGE 

OR  ACTUAL 

TONS  OF 

SALT 

SYMBOL 

OF  TOTAL 
SALTS  TAKEN 

WEIGHT  IN 
GRAMS  PER 

2000  LBS.  EACH 
PER  CUBIC  MILE 

AS  100 

LITER  OF  SEA 

OF  SEA  WATER 

WATER 

i.  Sodium  Chloride 

NaCl 

77-758 

27.213 

131,526,080 

2.   Magnesium  Chloride 

MgCl2 

10.878 

3.807 

18,399,360 

3.   Magnesium  Sulphate 

MgSO* 

4-737 

1.658 

8,012,480 

4.   Calcium  Sulphate     . 

CaS04 

3.600 

1.260 

6,089,440 

5.   Potassium  Sulphate  . 

K2S04 

2.465 

0.863 

4,169,760 

6.   Calcium  Carbonate  . 

CaCO3 

o-345 

0.123 

583,520 

7.  Magnesium  Bromide 

MgBr2 

0.217 

0.076 

367,360 

IOO.OOO 

35.000 

169,148,000 

Common  Salts  Produced  by  Evaporation  of  Sea-Water 

The  two  principal  salts  which  are  produced  by  the  evaporation 
of  sea- water  are  the  common  salt,  sodium  chloride  (NaCl),  and 
gypsum  or  calcium  sulphate,  with  two  molecules  of  water  (CaSC>4 
+  2H.2O).  By  local  oversaturation  with  lime,  calcium  carbonate 
(CaCO3)  may  also  separate  out,  but  this  is  more  commonly  pro- 
duced by  chemical  reaction.  When  the  evaporation  has  gone  very 
far,  other  salts,  such  as  those  of  magnesium,  and  finally  potash  salts, 
separate  out.  Common  salt  and  gypsum  are  obtained  on  many 
sea-coasts,  either  by  evaporation  of  the  water  by  artificial  heat,  or 
by  conducting  the  sea-water  into  large  shallow  "  pans,"  that  is, 
fields  surrounded  with  dams  and  having  a  hard,  flat  bottom.  When 
the  outlet  of  the  pan  is  closed,  evaporation  takes  place  under  the 
influence  of  the  sun  and  drying  winds,  and  after  a  while  gypsum 
separates  out.  When  most  of  this  has  been  deposited,  the  water 
is  conducted  to  another  pan,  where  evaporation  continues  until  a 
large  part  of  the  common  salt  (sodium  chloride)  has  separated  out. 
Then  the  remaining  dense  brine,  which  is  called  the  mother  liquor, 
is  drawn  off  and  either  returned  to  the  sea  or  further  evaporated 
for  the  rarer  salts.  In  this  manner  a  large  part  of  the  world  is 
supplied  with  its  domestic  requirements  of  salt,  though  vast  quan- 
tities of  this  commodity  are  also  obtained  from  inland  salt  deposits, 
which  are  the  product  of  evaporation  either  of  sea-water  or  of  in- 
land salt-bearing  waters,  in  former  geological  periods. 


230  Aqueous  or  Hydrogenic  Rocks 

Experiments  in  Evaporation  of  Sea-Water 

In  1849,  the  Italian  chemist,  J.  Usiglio,  published  the  results  of 
experiments  which  he  had  made  at  Cette  on  the  south  coast  of 
France.  He  had  taken  5  liters  of  the  sea-water  from  the  Medi- 
terranean and  evaporated  this,  keeping  an  exact  record  of  the  point 
which  the  evaporation  had  reached  when  separation  of  the  several 
salts  took  place,  and  determining  the  amount  of  the  various  salts 
separated  at  the  successive  stages. 

No  separation  of  salts  occurred  until  the  water  was  evaporated  to  nearly 
one  half  its  volume,  when  the  iron  oxide  and  a  part  of  the  carbonate  of  lime  of 
the  sea- water  separated  out.  Later  still,  when  the  original  5  liters  of  the  water  had 
been  reduced  by  evaporation  to  about  one  liter,  the  remainder  of  the  carbonate 
of  lime,  together  with  the  hydrous  sulphate  of  lime,  or  gypsum,  was  precipitated. 
More  than  84  per  cent  of  the  total  amount  of  gypsum  contained  in  the  sea- water 
was  deposited  before  the  water  became  dense  enough  to  allow  separation  of  the 
common  salt  (NaCl),  the  remainder  of  the  gypsum  being  thrown  down  with 
that  salt  and  with  various  amounts  of  magnesium  salts  (MgSC>4  and  MgCla), 
and  finally  with  sodium  bromide.  A  significant  fact  was  that  no  sodium  sepa- 
rated out  until  the  evaporation  had  reduced  the  original  5  liters  to  less  than 
half  a  liter,  or  to  less  than  one  tenth  the  original  volume.  At  that  tune  the 
amount  of  solid  matter  still  held  in  solution  in  the  half  liter  of  water  remaining 
was  about  184.4  grams,  which  would  correspond  to  368.8  grams  per  liter,  or  a 
salinity  of  368.8  per  mille,  whereas  the  salinity  of  the  original  sea-water  was 
about  38.5  per  mille.1  From  this  we  must  conclude  that  a  water  body  must 
reach  this  high  degree  of  salinity  by  evaporation  before  salt  can  be  deposited 
in  nature.  Since  there  are  to-day  no  known  large  bodies  of  water  with  such  a 
salinity,  it  follows  that  extensive  salt  deposition  is  not  going  on  to-day  by  simple 
evaporation  of  large  bodies  of  sea- water.  To  be  sure,  there  are  many  small  and 
shallow  marginal  lagoons  on  the  sea-coast,  and  especially  on  the  shores  of  more 
or  less  enclosed  salt-water  bodies,  such  as  the  Black  and  Caspian  Seas,  and  Great 
Salt  Lake,  where  evaporation  goes  far  enough,. to  precipitate  salt  —  but  this  is, 
as  a  rule,  only  in  comparatively  small  amounts,  though  commercially  important. 
Complete  evaporation  of  the  5  liters  of  sea  water  was  not  achieved  by  Usiglio, 
for  his  experiments  ceased  when  the  volume  had  been  reduced  to  about  81  cubic 
centimeters.  This  remaining  dense  "  mother  liquor  "  retained  all  of  the  potash 
salt  of  the  original  sea- water  in  solution,  together  with  some  of  the  sodium  and 
magnesium  salts.  The  amounts  present  were  as  follows : 

NaCl  12.9425  grams 

MgSO4  9. 2  7  25  grams 

MgCU  .  15. 8200  grams 

NaB  r  i .  6500  grams 

KC1  2.6695  grams 

Total  42.3545  grams 

1  It  is  probable  that  actual  separation  of  salt  (NaCl)  begins  at  a  somewhat  lower 
salinity,  for  at  the  stage  here  noted  something  over  3  grams  of  salt  had  already 
separated  out. 


Conditions  Favoring  Deposition  of  Sea-Salts     231 

This  corresponds  to  a  salinity  of  522.9  grams  per  liter  or  522.9  per  mille  (52.29 
per  cent) . 

The  salts  of  the  mother  liquor  are  precipitated  only  at  very  high  or  very 
low  temperatures,  and  from  this  we  must  conclude  that  potash  deposits  in  nature 
are  formed  only  under  exceptional  conditions. 


SPECIAL  CONDITIONS  FAVORING  DEPOSITION  OF  SEA-SALTS 
Modern  Examples 

From  the  foregoing  it  becomes  apparent  that  the  first  requisite 
for  the  deposition  of  sea-salts  in  nature  is  the  concentration  of  the 
sea-water  under  the  influence  of  conditions  which  favor  evapora- 
tion, such  as  the  heat  of  the  sun  and,  above  all,  drying  winds.  That 
such  evaporation  cannot  go  on  in  the  open  ocean  is  apparent,  and 
so  we  must  look  to  bodies  of  sea-water  cut  off  from  the  main  oceans. 

The  Caspian  Sea  (Fig.  165).  —  This,  the  largest  isolated  salt- 
water body  of  the  earth,  may  in  many  respects  be  considered  typical. 
It  lies  within  the  region  of  drying  winds,  and  is  partly  surrounded 
by  deserts.  Evaporation  has  gone  so  far  that  its  surface  is  85  feet 
below  sea-level,  yet  the  salinity  of  its  water  is  only  12.94  per  mille, 
or  a  little  over  one  third  that  of  normal  ocean  water.1  This  is  due 
to  the  fact  that  a  large  amount  of  fresh  water  is  brought  in  by  the 
Volga  and  other  rivers  tributary  to  it,  so  that  in  spite  of  the  evapo- 
ration, the  salt  content  is  very  low.  As  we  shall  see  later,  much  of 
the  original  salt  has  been  specially  concentrated  in  the  Kara  Bugas 
Gulf  and  other  dependent  bodies,  and  some  salt  has  no  doubt  been 
deposited  on  the  bottom  of  the  lake  and  then  preserved  by  a  cover- 
ing layer  of  impervious  material  (gypsum,  clay,  etc.). 

The  Black  Sea. —  This  nearly,  but  not  quite,  isolated  mediter- 
ranean water  body  has  a  surface  salinity  of  only  18.3  per  mille,  but 
the  lower  layers  are  denser,  so  that  the  average  salinity  of  the  water 
as  a  whole  is  22.04  per  mille.  Obviously  no  salt  can  be  deposited 
in  the  open  parts  of  this  sea. 

There  can  be  no  question,  however,  that  were  it  not  for  the  supply 
of  fresh  water,  both  the  Black  and  the  Caspian  seas  would  have  a 
high  salinity,  while  the  Baltic  would  be  nearer  to  sea- water  in  that 
respect.  Indeed,  should  the  fresh-water  supply  be  entirely  cut  off 
from  the  Caspian,  continued  evaporation  would  result  in  the  sepa- 
ration of  most  of  its  salt  upon  the  bottom  of  its  basin,  and  eventu- 

1  Average  of  five  analyses  made  in  1878. 


232 


Aqueous  or  Hydrogenic  Rocks 


ally  only  a  layer  of  mother  liquor  would  remain  to  cover  these  de- 
posits, and  under  special  conditions  this  mother  liquor  might  also 
be  forced  to  part  with  its  salts.  That  such  evaporation  of  large 


FIG.  165.  —  Map  of  the  Caspian  Sea  and  the  salt  lake  region  north  of  it. 

enclosed  bodies  of  sea-water  has  occurred  in  the  past  history  of  the 
earth  is  indicated  by  the  nature  of  the  salt  deposits  now  found  en- 
closed in  the  rocks  of  the  earth's  crust,  as  will  be  seen  presently. 
To-day  such  deposits  are  formed  in  local  "  salt  pans  "  along  the 


Conditions  Favoring  Deposition  of  Sea-Salts    233 


margin  of  the  Black  Sea,  the  Red  Sea,  and  especially  in  the  Ran  of 
Cutch  on  the  west  coast  of  India,  deposits  which  for  centuries  have 
supplied  the  natives  with  salt. 


FIG.  166.  —  Map  of  the  Colorado  Desert  with  the  Salton  Sink  and  Pattie 
Basin.      (After  Sykes,  from  McDougal,  Am.  Geog.  Soc.  Bulletin.) 

The  Salton  Sink  (Fig.  166).  —  This  is  a  great  depression  at  the 
head  of  the  Gulf  of  California,  surrounded  on  three  sides  by  high 
mountains  which  shut  out  the  moisture-bearing  winds,  especially 
on  the  Pacific  side.  The  valley  is  separated  from  the  Gulf  of  Cali- 


234  Aqueous  or  Hydrogenic  Rocks 

fornia  by  the  delta  of  the  Colorado  River.  The  lowest  part  of  its 
floor  lies  273.5  feet  below  sea-level,  and  is  occupied  by  a  small  lake, 
which  is  surrounded  by  extensive  salt  deposits.  It  is  generally  held 
that  this  depression  was  formerly  a  part  of  the  Gulf  of  California 
and  was  cut  off  from  it  when  the  Colorado  built  its  delta.  Under  the 
influence  of  the  drying  winds  which  descend  from  the  Coast  Ranges, 
the  cut-off  portion  of  the  sea-water  evaporated,  and  much  of  the 
salt  was  deposited  on  the  floor  of  the  basin,  which  was  converted 
into  a  desert.  Some  of  the  salt  may  have  been  previously  removed, 
when  the  Colorado  drained  into  this  basin  and  converted  it  into 
a  fresh-water  lake,  which  stood  40  feet  or  more  above  sea-level,  as 
shown  by  old  shore  lines.  Recently  the  Colorado  has  several  times 
reentered  this  basin  and  enlarged  the  central  lake. 

Effects  of  Condensation  of  Sea-Water  on  Its  Animal  Life 

A  moment's  reflection  will  show  that  in  the  process  of  evapora- 
tion and  concentration  of  the  cut-off  portion  of  the  sea-water,  all 
the  animals  which  lived  in  that  water  would  be  killed,  and  their 
remains  would  sink  to  the  bottom  or  be  cast  upon  the  shores.  The 
shells  of  the  Mollusca,  the  horny  coverings  of  Crustacea,  and  the 
bones  and  teeth  of  fish  and  other  vertebrates  would  be  embedded 
in  the  layer  upon  which  the  salt  later  comes  to  lie.  Thus  a  very 
definite  and  restricted  fossiliferous  substratum  is  produced  for 
salt  deposits  of  this  type,  and  this  will  furnish  a  criterion  by  which 
ancient  salt  deposits  can  be  interpreted.  If  the  change  in  salinity 
is  gradual,  because  the  water  body  subject  to  evaporation  is 
large,  extensive  fossiliferous  deposits  may  be  formed,  including 
important  beds  of  limestone,  before  the  water  is  dense  enough  to 
kill  the  organisms.  After  that  the  water  will  remain  essentially 
lifeless  (though  there  are  certain  forms  of  animals  which  live  only 
in  strong  brines),  and  the  deposits  formed  in  it  will  be  barren  of 
organic  remains.  An  exception  to  this  may,  however,  occur  if  the 
sea  should  break  into  the  basin  again,  flooding  it  with  normal  sea- 
water,  and  bringing  in  with  it  the  normal  sea-fauna.  Then,  if  the 
basin  is  again  cut  off  from  the  sea,  evaporation  will  set  in  with  the 
repetition  of  the  series  of  evaporation  deposits. 

Order  of  the  Deposition  of  Sediments  and  Salts 

If  the  basin  whose  waters  become  subject  to  evaporation  is 
large,  the  waters,  as  they  shrink,  will  leave  a  succession  of  de- 


An  Ancient  Rock  Salt  Deposit  235 

posits  of  various  kinds  around  the  margin.  Along  the  shores  would 
be  found  sands  and  clays  which  farther  out  might  merge  into  or- 
ganic limestone.  As  the  sea-water  becomes  concentrated,  the  or- 
ganically formed  limestones  would  come  to  an  end,  and  chemically 
formed  limestones  and  dolomites  would  take  their  place.  Gypsum 
or  anhydrite  deposits  follow  next,  and  finally,  as  the  water  becomes 
very  .saline  and  the  area  much  contracted,  salt  is  deposited.  Last 


ESSLlSajxUndCUy 

meatone 


FIG.  167.  —  Diagrammatic  cross-section  of  a  cut-off  basin,  and  the  deposits 
formed  in  it  after  complete  evaporation. 

of  all,  the  mother  liquor  salts  are  precipitated  under  favorable  con- 
ditions, but  only  in  the  central  area  of  the  original  basin,  where 
the  last  of  the  water  lingers.  The  relationships  of  these  several 
deposits  are  shown  in  the  preceding  diagram  (Fig.  167),  which  is 
entirely  schematic.  It  must  be  emphasized  that  normal  salt  de- 
posits formed  from  evaporating  sea-water  must  always  be  under- 
lain by  a  layer  of  gypsum  or  its  anhydrous  equivalent,  the  mineral 
anhydrite. 

AN  ANCIENT  ROCK-SALT  DEPOSIT  FORMED  BY  EVAPORATION 
OF  SEA-WATER 

Among  the  many  salt  deposits  within  the  earth's  crust  which 
were  formed  during  earlier  geological  periods  and  preserved  through 
burial  by  later  deposits,  a  certain  number  can  best  be  explained  as 
formed  by  the  evaporation  of  cut-off  portions  of  the  sea  in  the 
manner  above  outlined.  This  means,  of  course,  that  they  have  the 
essential  characteristics  which  we  have  seen  are  the  normal  accom- 
paniments of  salt  deposits  formed  from  complete  evaporation  of  a 
cut-off  body  of  sea-water.  The  most  notable  example  is  found  in 
the  great  salt  deposits  of  North  Germany  (Magdeburg-Halber- 
stadt  region) ,  most  widely  known  as  the  Stassfurt  deposits,  though 
this  is  only  one  of  the  localities  where  these  salts  are  mined.  Their 
peculiar  interest  lies  in  the  fact  that  they  have  associated  with 
them  the  rare  potash  salts  which,  we  have  seen,  are  precipitated 


236  Aqueous  or  Hydrogenic  Rocks 

from  the  mother  liquor  on  complete  evaporation  of  the  sea-water. 
Before  the  World  War  these  deposits  furnished  by  far  the  largest 
amount  of  potash  to  the  commerce  of  the  world,  and  their  abun- 
dance is  such  that  they  can  supply  the  entire  world  at  the  present 
rate  of  consumption  for  perhaps  2000  years  to  come. 

If  we  take  a  section  through  these  deposits  from  top  to  bottom,  as  revealed 
by  the  numerous  boreholes  and  by  mines  in  operation,  we  find  the  following  char- 
acteristic succession : 

At  the  base  lies  a  variable  thickness  of  limestones  and  dolomites,  which  in 
some  sections  contain  old  reefs  formed  by  organisms  which  inhabited  these 
waters  while  they  were  still  connected  with  the  sea,  and  for  some  time  after. 
This  limestone  is  known  as  the  Zechstein,  and  it  takes  its  definite  place  with  the 
salt  deposits  and  with  the  underlying  red  sandstones  (Rothliegendes),  in  the 
series  of  successive  formations  which  were  made  during  the  later  portion  of 
the  Palaeozoic  era  of  the  earth's  history. 

Above  the  Zechstein  limestones  and  dolomites  lies  a  formation  of  anhydrite 
and  gypsum  100  meters  in  thickness,  and  this  is  followed  by  the  salt  beds.  These 
average  245  meters  in  thickness  and  are  subdivided  by  about  3000  layers 
of  anhydrite,  the  so-called  annual  rings.  Above  this  follow  the  mother  liquor 
salts  which  have  given  these  deposits  their  great  value.  They  include  more 
than  30  rare  minerals,  the  more  abundant,  as  a  rule,  in  layers  or  strata,  the  whole 
averaging  from  about  60  to  over  90  meters  in  thickness.  The  most  important 
of  them  from  a  commercial  point  of  view  are,  of  course,  the  potash  salts. 
Above  these  mother  liquor  salts  occurs  a  second  series,  beginning  generally  with 
a  salt  clay  containing  remains  of  marine  animals,  followed  by  anhydrite  (30-80 
meters)  and  rock-salt,  with  about  400  annual  rings  of  polyhalite,  which  is  a  com- 
plex hydrous  sulphate  of  calcium,  magnesium,  and  potash.  The  series  is  closed 
by  a  succession  of  minor  layers  of  red  clay,  anhydrite,  rock-salt  (about  40  meters), 
anhydrite,  and  red  clay,  the  last  forming  the  top  of  the  deposits.  Through- 
out the  entire  series,  except  in  the  basal  Zechstein  limestones  and  the  salt  clay 
at  the  middle,  remains  of  organisms  are  wanting,  except  a  few  plant  fragments, 
which  were  blown  into  the  water  body.  It  appears  then  that  both  series  of 
deposits,  the  lower  as  well  as  the  upper,  represent  the  succession  which  we  have 
seen  is  characteristic  of  a  progressively  drying  up  cut-off  portion  of  the  sea. 
Moreover,  as  we  shall  see  later,  they  differ  from  the  succession  of  salt  deposits 
formed  in  other  ways,  and  they  may,  therefore,  unhesitatingly  be  interpreted  as 
accumulations  of  the  first  type.  The  regular  intercalation  of  the  anhydrite 
layers  in  the  lower  salt  suggests  that  they  are  due  to  seasonal  fluctuations  in  the 
density  of  the  water — a  periodic  slight  reduction  in  the  salinity  during  a  moister 
period  putting  a  temporary  end  to  salt  deposition  and  permitting  the  forma- 
tion of  anhydrite,  which  is  separated  from  less  concentrated  waters  rich  in  salt. 
If  these  changes  were  of  annual  recurrence,  as  seems  likely,  it  would  appear 
that  the  formation  of  this  salt  mass  took  three  thousand  years.  Each  annual 
ring  is  on  the  average  about  7  millimeters  thick,  while  the  salt  layers  with  which 
they  alternate  are  from  8"  to  9  millimeters  in  thickness. 

After  the  deposition  of  most  of  the  mother  liquor  salts,  a  period  of  fluctua- 


An  Ancient  Rock  Salt  Deposit  237 

tion  occurred,  and  salt  with  polyhalite  layers  was  formed.  This  alternation, 
too,  is  probably  seasonal,  but  shows  that  the  brine  was  now  more  highly  con- 
centrated and  of  changed  composition,  and  it  is  probable  that  regular  changes 
in  temperature  were  most  influential  in  producing  the  succession  of  deposits. 
The  higher  anhydrite  and  salt  series  indicates  that  the  sea- water  again  filled 
the  basin,  then  it  was  cut  off  anew,  and  again  underwent  complete  evaporation. 
It  is  also  quite  probable  that  the  waters  of  these  basins  were  enriched  by  salt 
brought  in  solution  by  intermittent  streams  from  the  surrounding  hills,  as  is 
the  case  in  many  modern  desert  basins,  and  that,  because  of  this  excess  of  salt, 
anhydrite,  rather  than  gypsum,  was  deposited. 

We  cannot,  however,  settle  the  question  of  origin  from  the  de- 
posits of  North  Germany  alone.  If  they  were  formed  by  the  dry- 
ing up  of  an  enclosed  sea,  there  should  be  neighboring  salts  and 
sediments  which  should  show  such  variations  as  we  would  expect 
nearer  the  shores  of  this  water  body,  and  these  deposits  should 
be  of  the  same  age.  The  method  of  searching  out  and  determining 
formations  of  like  geological  age  in  different  parts  of  the  world  will 
be  dealt  with  later.  Suffice  it  to  say  here  that  such  age  determina- 
tion and  correlation  of  formations  in  different  countries  is  quite 
possible  by  the  aid  of  fossils,  by  the  relationship  of  the  beds  to  one 
another,  and  by  other  criteria. 

Taking,  then,  such  deposits  of  the  same  age  in  other  parts  of 
Europe,  we  find  first  that  the  mother  liquor  salts  fail  as  we  proceed 
away  from  the  North  German  region,  where  apparently  was  the 
center  of  concentration.  Salt  is  still  found  in  a  number  of  sections 
even  in  eastern  England.  Here,  however,  magnesian  limestones 
prevail,  some  of  them  showing  a  peculiar  structure  suggestive  of 
chemical  deposition  (Fig.  160,  p.  221).  Limestone  formed  of  a 
restricted  number  of  organic  remains  elsewhere  makes  up  the  de- 
posit of  Permian  age  in  England.  Finally,  in  the  north  and  we&t  of 
England,  only  red  sandstone  deposits  represent  the  Permian,  these 
being  formed  near  the  old  shore-line  of  the  Permian  basin,  and  in 
part  above  it,  by  rivers  which  brought  sands  from  the  uplands  and 
dropped  them  in  their  lower  shallow  courses  (Fig.  168). 

In  the  other  direction,  toward  the  Ural  Mountains  of  Russia,  a 
change  in  the  character  of  the  Permian  formations  may  also  be  ob- 
served. At  first  limestones  predominate,  but  farther  east  these 
are  largely  replaced  by  sandy  and  clayey  beds,  among  which  coal 
seams  are  found,  which  indicate  a  swamp-land  condition. 

It  seems  practically  demonstrated,  then,  that  these  ancient  salt 
deposits  with  their  valuable  beds  of  potash  salts  were  formed  by 


238 


Aqueous  or  Hydrogenic  Rocks 


the  drying  up  of  a  large  body  of  sea-water  which  had  become  sepa- 
rated from  the  main  ocean  by  the  formation  of  a  land  barrier.  This 
water  body  appears  to  have  extended  from  central  England  on  the 
west  nearly  to  the  Ural  Mountains  on  the  east.  Its  southern 


'  FIG.  1 68.  —  Map  of  the  Zechstein  Sea  of  Permian  time  in  northern  Europe, 
showing  the  approximate  outline  of  that  water  body,  and  the  mountains  of  that 
period.  B=Berlin;  Br.  =  Breslau;  ~Dr.  =  Dresden;  Ha,.  =  Halberstadt;  Hb.= 
Heidelberg;  HI  =  Halle;  L  =  Leipzig;  M  =  Magdeburg;  Mb  =  Marburg;  Th  = 
Thorn;  Vf  =  Wesel. 

boundary  was  north  of  the  Danube,  and  its  northern  in  the  region 
of  the  Baltic  Sea  of  to-day.  A  significant  corollary  is  that  the 
climate  of  North  Europe  was  very  much  dryer  at  that  time  than 
it  is  to-day,  for  at  present  no  such  complete  evaporation  would  take 
place.  This  will  be  referred  to  again  in  a  later  part  of  this  book. 


DEPOSITS  OF  SALT  BY  CONCENTRATION  IN  LAGOONS. 
THEORY  or  OCHSENIUS 


BAR 


Complete  evaporation  of  a  salt  lake  is  only  one  way  —  though 
a  most  effective  way  —  of  producing  salt  deposits.  A  second 
method  consists  in  the  concentration  of  the  sea-water  in  a  nearly 
shut-off  lagoon  in  regions  of  arid  climate.  As  a  typical  example  of 
this  we  may  select  the  Kara  Bugas  Gulf  (Fig.  169),  a  bay  which 
lies  on  the  eastern  coast  of  the  Caspian  Sea,  from  which  it  is  sepa- 
rated only  by  a  sand  bar,  across  which  a  narrow  strait  maintains  con- 
nection with  the  Caspian.  On  the  other  sides  the  gulf  is  surrounded 
by  deserts,  and  there  are  no  streams  entering  it.  Since  the  Caspian 
Sea  is  a  large  salt-water  body,  though  of  lower  salinity  than  the 
ocean,  the  Kara  Bugas  Gulf  is  nearly  as  satisfactory  an  illustration 
as  a  similar  bay  on  the  open  sea  would  be. 


Deposits  of  Salt  by  Concentration 


239 


Because  of  the  narrow  inlet  from  the  Caspian,  sufficient  water 
is  not  supplied  to  counterbalance  the  evaporation  over  the  surface 
of  the  Kara  Bugas  Lagoon,  and  so  a  slight  difference  of  level  is 


FIG.  169.  —  Map  of  the  Kara  Bugas  (Karabugas)    Gulf  or   Adji-darja  (salt 
water),  on  the  eastern  border  of  the  Caspian  Sea. 

produced,  the  Kara  Bugas  surface  being  sufficiently  lower  to  cause 
a  constant  inflowing  current  of  water  from  the  Caspian  (Fig.  i6ga). 
Since  the  salt  thus  carried  in  solution  is  not  removed,  the  waters  of 


240 


Aqueous  or  Hydrogenic  Rocks 


the  lagoon  become  more  and  more  saline.  A  determination  made 
some  time  ago  showed  a  salinity  of  285  per  mille  (28.5%),  while 
that  of  the  Caspian,  from  which  the  salt  was  abstracted,  was 
only  12. 94  per  mille  (z.294%).1  This  is  a  sufficient  concentration 
for  the  deposition  of  some  salts,  but  not  of  the  common  salt,  which 
is  not  forming  at  the  present  time  on  the  bottom  of  the  gulf.  An 


20  ZO         50    IOO          ZOO  ZOO  100 


FIG.  169  a.  —  Cross-section  of  the  Caspian  Sea  and  the  Kara  Bugas  Gulf, 
from  Baku  across  the  inlet  to  the  northeastern  shore  of  the  Gulf.  Vertical  scale 
greatly  enlarged.  Depths  given  in  meters.  The  level  of  the  Kara  Bugas  is 
slightly  lower  than  that  of  the  Caspian. 

extensive  bed  of  sodium  sulphate  or  glauber  salt  has,  however, 
formed  on  the  bottom  of  this  bay,  and  much  gypsum  is  being  de- 
posited. 

It  is  evident  that  as  concentration  progresses,  ordinary  salt  will 
be  deposited,  if,  indeed,  beds  of  salt  do  not  actually  underlie  the  layer 
of  glauber  salt,  having  been  formed  during  a  period  of  former  greater 
concentration.  It  is  also  evident  that  so  long  as  the  connection 
with  the  Caspian  is  maintained,  no  mother  liquor  salts  will  be  pre- 
cipitated, since  that  requires  nearly  complete  evaporation.  It  is 
not  likely  that  such  separation  and  complete  evaporation  of  the 
Kara  Bugas  waters  can  take  place  so  long  as  it  remains  separated 
from  the  Caspian  by  only  a  sand  bar,  for  the  lowering  of  the  water 
in  the  Gulf  would  create  a  sufficient  inward  current  to  keep  the 
inlet  open,  and  even  enlarge  it.  Therefore,  while  salt  deposits  of 
considerable  thickness  may  accumulate  on  the  bottom  of  such  a 
gulf  as  the  Kara  Bugas,  no  potash  salts  can  be  formed  in  it. 

The  Caspian,  like  the  ocean,  abounds  in  animal  life.  Thousands 
of  fish,  many  seals,  and  other  animals  are  carried  through  the  nar- 
rows across  the  bar  and  into  the  Kara  Bugas,  where  they  are  killed 
by  the  high  salinity  of  the  water.  Their  carcasses  float  about  and 
later  sink  to  the  bottom,  or  are  cast  upon  the  shore,  portions  of 
which  are  literally  covefed  with  dead  fish  which  furnish  food  for 
migratory  birds.  Shells  of  dead  mollusks,  especially  the  cockle 
(Cardium  edule),  which  lived  in  these  waters  before  they  became 

1  It  must  be  remembered  that  the  salinity  of  the  ocean  water  is  35.0  per  mille  or  3.5%. 


Deposits  of  Salt  by  Concentration  241 

concentrated  to  their  present  degree,  occur  in  enormous  numbers  on 
the  shore  of  the  Kara  Bugas,  and  equally  large  numbers  are  buried 
with  the  fish  remains  in  the  mechanical  sediments  on  the  bottom  of 
the  gulf.  These  sediments  are,  therefore,  highly  fossiliferous,  and 
should  they  harden  into  rock,  they  would  constitute  beds  of  fossil- 
iferous clays  and  sandstones  in  close  association  with  the  salt  de- 
posits. Moreover,  while  salt  deposits  are  forming  in  the  Kara 
Bugas,  normal  sediments  free  from  salts  are  forming  in  the  Caspian 
in  close  juxtaposition  to  the  Kara  Bugas,  and  these,  too,  are  fossil- 
iferous. All  of  these  factors  must  be  kept  in  mind  when  we  at- 
tempt to  use  this  example,  or  others  like  it,  in  the  interpretation 
of  the  history  of  older  salt  deposits. 

From  its  simplicity  this  example  has  gained  wide  currency  as  an  explanation 
of  the  origin  of  rock-salt  deposits  in  all  parts  of  the  world.  The  theory  was 
first  developed,  though  not  originated,  by  the  German  chemist,  Professor  Karl 
Ochsenius,  and  it  is  commonly  spoken  of  as  the  "  Bar  Theory  of  Ochsenius." 
While  it  explains  many  an  ancient  salt  deposit,  especially  the  great  series  of 
Tertiary  salts  in  the  region  of  the  present  Carpathian  Mountains  and  elsewhere, 
it  does  not  satisfy  the  conditions  found  in  other  older  salt  deposits,  especially 
those  of  the  United  States,  for  which  another  mode  of  origin  must  be  postu- 
lated, as  will  be  shown  in  a  later  section. 

An  Older  Salt  Deposit  Formed  According  to  the  Bar  Theory 

Perhaps  the  best  example  of  an  older  rock-salt  deposit  which 
can  be  explained  by  the  Bar  Theory  was  found  in  the  salt  beds  of 
the  Bitter  Seas  on  the  peninsula  of  Suez  (Fig.  170).  Before  the 
Suez  Canal  was  cut,  these  lakes  were  of  very  high  salinity,  and  on 
the  bottom  of  at  least  one  of  them  an  immense  bed  of  rock-salt  had 
formed.  This  has,  however,  been  entirely  redissolved  by  the  fresher 
waters  of  the  canal.  A  characteristic  feature  of  this  salt  bed  was 
not  only  the  presence  of  many  layers  of  gypsum,  but  also  of  innu- 
merable layers  of  clay  in  which  were  embedded  the  shells  of  mollusks 
such  as  now  live  in  the  Red  Sea  near  by,  while  similar  shells  were 
entombed  in  the  sediments  which  formed  on  the  bottom  of  the 
Red  Sea  during  the  period  of  salt  deposition,  though  these  sedi- 
ments are  free  from  salt. 

We  have  here  a  deposit  which  satisfies  all  the  requirements  of 
the  lagoon  and  bar  example  of  the  Kara  Bugas,  namely,  a  circum- 
scribed area,  presence  of  numerous  organic  remains  in  the  salt- 
bearing  series,  and  the  association  in  a  neighboring  area  (the  Red 


242 


Aqueous  or  Hydrogenic  Rocks 


Sea)  of  normal  deposits  with  the  same  fossils  but  with  no  salt.  In 
the  present  case  it  is  known  that  these  Bitter  Lakes  formerly  con- 
stituted an  extension  of  the  Gulf  of  Suez  and  the  Red  Sea,  forming 

the  ancient  Heroopolitan  Gulf.  The 
silting  up  of  the  mouth  of  this  gulf, 
which  was  not  yet  complete  during  the 
sixth  century  before  Christ,  formed 
the  bar  which  cut  off  the  lagoon  from 
the  remainder  of  the  gulf.  According 
to  some  authorities,  this  bar  appears  to 
have  been  the  site  of  the  crossing  of 
the  Red  Sea  by  the  Israelites  in  their. 
exodus  from  Egypt.  With  the  forma- 
tion of  this  bar,  which  now  constitutes 
the  southern  margin  of  the  isthmus  of 
Suez,  the  conditions  favorable  to  the 
deposition  of  the  gypsum  and  salt  were 
produced.  Repeated  overflow  from  the 
Red  Sea  supplied  the  waters  and  the 
mud  in  which  were  buried  the  or- 
ganisms which  could  live  here  until 
the  water  became  too  salty.  Then 
their  shells  and  other  remains  sank  to 
the  bottom ;  a  layer  of  gypsum  was 
formed  over  them,  and  then  a  layer  of 
salt,  covered  by  a  more  or  less  imper- 
vious layer  which  prevented  re-solution 

when  next  the  waters  of  the  Red  Sea  poured  in  again.  In  the 
waters  thus  freshened,  new  organisms  developed  from  the  larval 
stages  brought  by  these  waters,  and  a  new  cycle  of  deposition  was 
inaugurated.  The  mother-liquor,  however,  never  evaporated  in 
this  lagoon,  but  remained  behind,  forming  the  bitter  waters  of  the 
lakes,  which,  however,  have  lost  much  of  their  character  since  the 
letting  in  of  the  sea-water  by  the  canal. 

DEPOSITION  OF  SALT  IN  INLAND  DESERT  BASINS 

Salt  deposits  are  forming  in  many  portions  of  the  world  to-day, 
where  no  direct  connection  with  the  sea  exists.  The  inland  salt 
lakes  and  salinas  are  in  some  cases  shrunken  bodies  of  fresh  water 
formerly  of  greater  extent.  This  is  the  history  of  Great  Salt  Lake 


FIG.  170.  —  Map  of  the 
Bitter  Lake  of  Suez  and  the 
Suez  Canal. 


Deposition  of  Salt  in  Inland  Desert  Basins     243 

of  Utah,  which  is  only  a  remnant  of  a  much  larger  fresh-water  lake 
—  Lake  Bonneville.  By  evaporation  and  concentration  of  the 
water  in  the  deeper  part  of  the  basin,  the  salinity  was  increased, 
and  finally  in  some  cases  reached  the  concentration  necessary  for 
the  deposition  of  salt.  Much  additional  salt  is  constantly  brought 
into  the  lake  by  the  streams  which  feed  it.  Naturally  the  question 
as  to  the  origin  of  the  salt  in  these  waters  arises.  The  answer  is  that 
it  is  leached  out  of  the  rocks  within  the  drainage  area  of  the  basin. 
Not  all  rocks  contain  sodium  chloride,  but  this  is  generally  present 
in  clastic  and  other  rocks  which  have  been  formed  on  the  bottom 
of  the  sea  in  former  geological  periods.  These  rocks  include  an- 
cient sea-water,  often  highly  concentrated,  within  their  pores,  where 
it  is  hermetically  sealed 
up  and  is  set  free  only 
when  the  rocks  are  sub- 
sequently exposed  to 
atmospheric  decay  and 
erosion.  Then  the  con- 
nate water,  as  it  is 
called,  evaporates,  but 
the  salt  remains  behind 
to  be  redissolved  by  the 
surface  waters  and 

carried  away.     If  it  is 

•   j    •        '  •   i      j  FIG.  1 71  a.  —  The  irregular  salt  surface  of 

earned  into  an  inland  tfae  Salt'plain  of  Lop,  Eastern  Turkestan, 
drainage  basin  from  (After  a  photograph  by  Ellsworth  Huntington.) 
which  there  is  no  escape, 

except  by  evaporation,  salinas  and  even  extensive  salt  deposits 
are  formed.  Such  areas,  covered  with  glistening  white  rock- 
salt  or  with  irregular  salt  masses,  are  found  in  the  desert  basins  of 
central  Asia  (Figs.  171  a,  b),  and  indeed  are  not  uncommon  in  many 
other  desert  areas.  So  long  as  the  supply  of  salt  lasts,  such  a  de- 
posit will  continue  to  grow  by  periodic  additions,  and  thus  beds  of 
salt  of  great  thickness  may  form.  If  silt  is  brought  in  during  a 
period  of  flooding,  or  if  sand  or  dust  is  blown  across  the  salt  plain, 
a  layer  of  clastic  sediment  may  form  over  the  salt,  and  this  in  turn 
may  again  be  covered  by  pure  salt  deposits.  As  a  rule,  however, 
gypsum  is  not  deposited  in  such  basins,  not  because  the  connate 
waters  imprisoned  in  the  rock  did  not  contain  it,  but  because  after 
it  is  set  free  it  is  less  readily  redissolved  by  the  surface  waters  than 


244 


Aqueous  or  Hydrogenic  Rocks 


is  the  common  salt,  and  that  which  is  dissolved  is  likely  to  separate 
out  again  from  the  waters  before  they  reach  the  central  basin.  In 
conformity  with  this  we  often  find  the  sands  which  surround  such 

basins  filled  with  gyp- 
sum crystals  which  have 
grown  from  the  ground 
water  as  it  passed 
through  the  sands, 
carrying  the  more 
soluble  sodium  chloride 
to  the  central  basin. 

In  like  manner  the 
potash  salts  will  not,  as 
a  rule,  reach  the  central 
basin,  for  though  they 
are  very  soluble,  the 


FIG.  1716.  —  The  southern  edge  of  the  Salt 
Plain  of  Lop,  in  Turkestan.  (From  a  photo- 
graph by  Ellsworth  Huntington.) 


fine  particles  of  the  soil 
of  the  desert  among 
which  the  water  must  find  its  way  to  the  central  basin  have  a 
great  affinity  for  the  potash  and  will,  by  some  still  little  under- 
stood process,  separate  this  substance  from  the  water  during  its 
passage.  Hence  the  water  which  reaches  the  central  basin  will 
contain  little  else  than  pure  sodium  chloride,  and  therefore  only 


FIG.  172.  —  Silver  Peak  Marsh,  Nevada,  a  typical  playa  (Photo  F.  R.  Porter 
from  U.  S.  G.  S.). 


pure  rock-salt  deposits  are  formed.  Gypsum  may,  however,  result 
from  the  alteration  of  limestones  formed  of  lime-mud  and  dust 
which  is  washed  or  blown  over  the  salt.  In  this  manner  gypsum 
beds  may  be  formed  above  the  salt  beds,  whereas  in  normal 
marine  deposits  of  salt,  or  in  those  due  to  the  evaporation  of  cut- 


Deposition  of  Salt  in  Inland  Desert  Basins     245 


FIG.  1 73.  —  Professor  Johannes 
Walther,  widely  known  for  his 
investigations  of  desert  phe- 
nomena. 


oils,  the  gypsum  will  underlie  the  salt.  The  importance  of  these 
facts  in  the  determination  of  the  origin  of  older  salt  beds  is  very 
great.  Another  fact  that  must  not 
be  overlooked  in  desert  salt  de- 
posits is  that  the  parting  and  en- 
closing layers  of  sediment  will  con- 
tain no  marine  organisms.  They 
will,  indeed,  be  for  the  most  part 
entirely  free  from  organic  remains. 
A  few  desert  organisms  and  mi- 
gratory birds  may,  however,  become 
entombed  in  these  deposits  or  even 
in  the  salt  itself,  but  their  terres- 
trial character  is  readily  recognized 
by  the  expert.  Huntington  reports 
finding,  in  the  salt  of  Lop  Nor  in 
eastern  Turkestan,  a  dead  plover 
which  had  been  preserved  in  the  salt 
for  centuries.  Around  the  border 
of  some  of  the  Persian  salinas  a 
zone  of  mud  is  sometimes  found, 

in  which  are  entombed  the  bones  of  animals  which  came  to  drink 
of  the  salty  water  and  perished  there. 

Beyond  these  salinas  in  all  directions  we  pass  into  the  region  of 
desert  sands  and  dust  deposits,  and  these  have  very  definite  char- 
acteristics which  can  be  recognized  even  after  they  have  hardened 
into  rock  beneath  a  cover  of  other  deposits.  Thus  the  geologist 
will  generally  be  able  to  recognize  in  the  rocks  associated  with  the 
ancient  desert  salts  the  structures  which  clearly  indicate  that 
origin.  The  appearance  of  a  typical  American  salt-marsh  or 
playa  is  shown  in  Fig.  172. 

Among  the  geologists  who  have  made  extensive  investigations 
into  the  origin  and  mode  of  deposition  of  desert  salts,  the  foremost 
rank  must  be  assigned  to  Professor  Johannes  Walther  of  the  Uni- 
versity of  Halle.  (Portrait,  Fig.  173.) 

An  Ancient  Example  of  a  Desert  Salt  Deposit 

In  the  central  part  of  the  state  of  New  York,  in  western  Ontario, 
and  in  southern  Michigan,  occur  ancient  rock  salt  deposits,  all  of 
which  were  formerly  and  some  of  which  are  still  buried  under  thou- 


246  Aqueous  or  Hydrogenic  Rocks 

sands  of  feet  of  rock,  a  considerable  portion  of  the  latter  being  of 
marine  origin.  The  rock-salts  themselves,  however,  which  rest 
in  some  places  upon  marine  beds  of  Lower  Silurian  (Niagaran)  age 
and  in  others  upon  non-marine  sediments,  are  in  some  places  covered 
by  marine  beds  of  Upper  Silurian  age  (Monroan)  and  are  associated 
laterally  with  deposits  of  clastic  material  which  shows  all  of  the 
features  of  sediments  found  in  modern  deserts.  Moreover,  no 
fossils  are  found  in  these  deposits  nor  in  those  which  separate  the 
several  salt  beds  of  the  series  from  one  another,  nor  are  there  any 
Middle  Silurian  beds  of  marine  origin  to  be  found  within  hundreds 
of  miles  of  the  salt  deposits.  Thus  it  appears  that  these  very  an- 
cient salt  beds  of  Middle  Silurian  age  were  formed  in  a  desert  which 
then  occupied  much  of  the  area  now  covered  by  the  Great  Lakes 
and  adjoining  territory  (see  further,  Chapter  XXXIV). 

No  potash  deposits  are  found  associated  with  these  salts,  and 
from  what  he  has  learned  so  far,  the  student  will  realize  that  there 
is  little  likelihood  of  the  finding  of  these  salts  unless  it  can  be  shown 
that  the  basal  salt  beds  of  some  sections  are  not  of  desert  but  of 
marine  origin,  resulting  from  the  drying  up  of  the  last  remnant  of 
the  Niagaran  sea  which  preceded  the  desert  period. 

There  is,  however,  one  important  fact  which  seems  to  argue 
against  such  a  marine  interpretation  of  the  basal  beds,  and  that  is 
the  absence  of  gypsum  or  anhydrite  beds  beneath  the  salt.  Indeed, 
the  entire  series  of  Salina  salt  deposits  lacks  the  foundation  layers 
of  gypsum  or  anhydrite,  though  gypsum  overlies  the  salt  in  a 
number  of  localities.  Much  of  this  is,  however,  known  to  have 
been  produced  by  the  alteration  of  former  limestone  beds  which 
were  invaded  by  sulphur-bearing  waters. 

CARBONATE  OF  LIME  DEPOSITS 

Although  some  carbonate  of  lime  separates  out  on  evaporation 
of  the  sea-water,  this  is  of  such  small  amount  that  it  practically 
disappears  in  the  evaporation  series  produced  along  the  seacoast 
and  from  cut-off  bodies.  Nevertheless,  carbonate  of  lime  deposits 
are  formed  in  the  sea,  not  so  much  by  evaporation,  —  though 
such  an  origin  may  be  ascribed  to  some  deposits,  —  as  by  the 
force  of  attraction  of  other  particles  of  lime  in  sea-water  saturated 
with  lime  carbonate,  or  by  the  abstraction  of  the  solvent  carbon 
dioxide  through  agitation  of  the  water.  Chemical  precipitation 


Carbonate  of  Lime  Deposits  247 

of  lime  also  takes  place  and  is  perhaps  the  most  common  mode  of 
lime  deposition  in  some  places  aside  from  that  due  to  organic 
action.  This  precipitation,  however,  is  due  largely  to  the  forma- 
tion of  ammonium  carbonate  by  the  decay  of  organic  matter,  and 
this  ammonium  carbonate  reacts  with  the  lime  sulphate  or  other 
lime  salts  in  the  sea-water,  producing  calcium  carbonate,  which 
is  precipitated,  and  an  ammonium  salt  which  remains  in  solution. 
The  reactions  may  be  written  in  the  following  way : 

CaSO4     +     (NH4)2CO3     =     CaCO3     +     (NH4)2SO4 

Calcium  Ammonium  Calcium  Ammonium 

Sulphate  Carbonate  Carbonate  Sulphate 

Or  again 

CaCl2      +     (NH4)2CO3     =     CaCO3     +     2  NI^Cl 

Calcium  Ammonium  Calcium  Ammonium 

Chloride  Carbonate  Carbonate  Chloride 

Where  lime  is  precipitated  by  such  reactions,  it  often  forms  more  or 
less  spherical  or  irregular  masses  or  nodules,  to  which  the  name 
concretions  is  applied.  Such  concretions  have  been  dredged  from 
many  portions  of  the  sea,  and  they  appear  to  be  especially  common 
where  organic  matter  which  has  reached  the  floor  of  the  ocean 
undergoes  a  process  of  decay.  Such  areas  are,  however,  not  uni- 
versal because  the  organic  matter  on  the  ocean  bottom  is  generally 
devoured  by  bottom-feeding  animals  before  the  decay  progresses 
far.  It  is  only  where  the  character  of  the  water,  or  the  tempera- 
ture, is  such  that  bottom-feeders  are  scarce  or  absent,  that  decay 
of  stray  organic  matter  can  take  place. 

Deposition  of  lime  due  to  the  attraction  of  other  lime  particles 
is  illustrated  by  the  hardening  on  the  ocean  bottom  of  the  loose  lime- 
sands  and  muds  worn  from  the  coral  and  other  organic  limestone 
masses  in  the  sea.  It  is  a  general  fact  that  wherever  lime-mud,  or 
sand,  forms  upon  the  sea-floor,  this  is  soon  bound  together  by  the 
filling  in,  between  the  particles,  of  lime  derived  from  the  sea-water. 
This  seems  to  take  place  most  actively  in  warm  regions,  where  the 
amount  of  lime  in  the  sea-water  is  above  the  average.  As  a  result, 
the  floor  of  the  ocean  in  such  regions  is  a  hard  surface  to  which 
various  stationary  marine  animals  attach  themselves,  while  others, 
such  as  certain  worms  or  sponges,  bore  into  this  rock  to  a  certain 
depth. 

On  the  surface  of  ancient  limestone  beds  we  often  find  the 
marks  of  animals  which  had  become  cemented  to  it.  Such  cemen- 


248  Aqueous  or  Hydrogenic  Rocks 

tation  could  of  course  take  place  only  if  the  surface  were  of 
sufficient  firmness,  and  this  indicates  that  the  old  deposits  of 
lime-sand,  or  mud,  from  which  these  beds  were  formed  in  the  sea, 
hardened  by  further  separation  of  lime,  so  that  the  animals  living 
there  could  become  attached  to  it. 

Another  illustration  of  such  lime  deposition  is  seen  in  the  coating  of  lime 
around  grains  of  quartz,  basalt,  or  other  sand,  or  around  fragments  of  shell, 
etc.,  which  are  found  on  the  shores  of  the  Island  of  Gran  Canaria  in  the  Canary 
Islands.  These  coated  grains  are  more  or  less  spherical  and  of  the  texture 
called  oolitic  (see  p.  217).  By  further  deposition  of  lime  between  the  grains 
they  are  bound  together  into  a  solid  rock,  an  oolite,  which  is  quarried  at  low  tide. 
The  water  here  is  warm,  having  throughout  much  of  the  year  a  temperature 
above  20°  C.,  and  there  is  much  lime  in  solution.  Still  another  interesting 
example  of  lime  deposition  is  seen  in  certain  Mexican  lagoons  where  insect 
eggs  are  coated  with  lime,  producing  a  series  of  rounded  oolite  grains.  Most 
of  the  grains  of  oolitic  character  are,  however,  produced  by  the  activities  of 
bacteria  or  minute  algae  in  the  sea,  and  on  this  account  must  be  classified  as  of 
organic  origin.  They  will  be  more  fully  discussed  in  a  later  chapter. 

OTHER  CHEMICAL  DEPOSITS  IN  THE  SEA 

Small  quantities  of  other  substances  are  deposited  in  the  sea 
as  the  result  of  certain  chemical  reactions,  or  through  attraction 
by  material  of  like  composition.  The  most  important  of  these 
are  phosphatic  concretions  and  concretions  of  oxide  of  manganese, 
both  of  which  have  a  wide  distribution  over  the  sea-bottom.  A 
third  group  of  such  deposits  forms  grains  of  the  green  mineral 
glauconite,  of  which  "  green-sands  "  are  made. 

Phosphate  of  Lime.  - —  This  is  produced  by  many  marine  animals 
which  take  the  phosphoric  acid  either  directly  from  the  sea-water, 
or  from  their  food.  The  phosphate  of  lime  is  built  into  certain 
hard  tissues,  as  the  shells  of  some  brachiopoda  (Lingula),  the  bones 
and  teeth  of  fish,  etc.,  and  it  is  also  present  in  the  excrements  of 
fish  and  other  animals.  Such  particles  accumulating  on  the  sea- 
bottom  have  the  power  of  attracting  to  them  more  phosphate  and 
precipitating  it  upon  their  surfaces,  which  thus  become  a  nucleus 
around  which  phosphate  concretions  are  built.  Such  phosphate 
concretions  are  found  on  the  ocean  floor  in  many  localities  at  mod- 
erate depths.  They  are  also  found  in  many  old  limestones,  in 
which  they  are  generally  scattered.  By  the  weathering  of  this 
limestone  the  nodules  which  are  left  behind  are  concentrated  into 
beds  which  are  rich  enough  to  be  worked.  By  solution  of  the 


Other  Chemical  Deposits  in  the  Sea 


249 


phosphate  and  by  redeposition  in  veins  or  on  the  walls  of  cavities, 
or  by  replacing  limestones  upon  which  they  rest,  rich  deposits  of 
phosphate  are  produced. 

Manganese  Concretions.  —  Concretions  of  oxide  of  manganese 
with  oxide  of  iron,  clay,  and  other  substances  are  also  found  on  the 
floor  of  the  deep  sea  in 
many  localities  (Fig.  174). 
The  manganese  and  iron 
form  concentric  layers 
about  some  nucleus,  which 
may  be  the  tooth  of  a 
shark,  or  some  other  sub- 
stance. It  is  not  fully 
known  whether  the  man- 
ganese is  derived  from  the 
sea-water  in  which  it  is 
present  in  very  small 
quantities,  or  whether  it  is 
derived  from  the  decom- 
position of  basic  volcanic 
rocks  on  the  floor  of  the 
ocean.  Manganese  con- 
cretions are  also  found  in 

ancient  marine  deposits,  and  in  some  cases  at  least  may  represent 
concentration  of  scattered  nodules  by  the  weathering  of  the  rock 
which  contained  them. 

Glauconite.  —  Still  another  deposit  formed  by  chemical  means 
on  the  sea-floor,  is  the  mineral  glauconite,  which,  when  abundant, 
forms  beds  chiefly  composed  of  small  grains  of  this  mineral.  On 
account  of  their  green  color  such  beds  of  glauconite  grains  are 
commonly  called  "  green-sands."  Chemically  the  mineral  is  an 
impure  hydrous  silicate  of  iron  and  potassium,  and  it  is  commonly 
formed  from  fine  mud  which  fills  the  interior  of  small  shells  of 
Foraminifera  (Fig.  196,  p.  275),  partly  by  the  reaction  with  the 
products  of  decay  of  the  organic  matter  in  these  shells,  and  partly 
by  reaction  with  the  substances  in  the  sea-water.  The  whole 
process  is  too  complex  to  be  further  discussed  here,  and  should  be 
taken  up  again  in  a  more  advanced  course.1 


FIG.  174.  —  Nodule  of  oxide  of  manganese 
from  Red  Clay  of  abyssal  ocean  bottom. 
(After  J.  Murray.) 


1  See  A.  W.  Grabau,  Principles  of  Stratigraphy,  pp.  670-673,  and  the  literature  there 
cited. 


250  Aqueous  or  Hydrogenic  Rocks 

Beds  of  green-sands  are  not  uncommon  in  older  marine  and  other 
deposits.  The  most  characteristic  examples  are  found  in  the  strata 
of  Cretaceous  age  which  crop  out  in  New  Jersey  and  Maryland. 
Sometimes  by  exposure  the  iron  of  the  green-sand  is  changed  to 
an  oxide,  and  ochery  or  red  beds  will  be  produced.  Such  is  the 
vividly  red  sand  bed  which  is  so  prominent  in  the  section  at  Atlantic 
Highlands,  N.  J.,  and  which  underlies  the  town  of  Red  Bank,  to 
which  it  has  given  the  name.  Beds  of  green-sands  are  also  common 
in  the  Cretaceous  strata  of  southern  England,  where  they  are 
generally  spoken  of  as  the  Green  Sands.  They  also  occur  in  France 
and  elsewhere.  Some  beds  of  green-sands  are,  however,  found  in 
deposits  which  accumulated  elsewhere  than  on  the  sea-floor. 


CHEMICAL  DEPOSITS  AND  EVAPORATION  PRODUCTS  or  LAKES 
Lacustrine  Deposits 

Lakes  may  be  classed  as  fresh-water,  alkaline-water,  salt-water, 
and  brine  lakes.  The  salt-water  and  brine  lakes  have  already  been 
referred  to,  and  it  has  been  shown  that  the  deposits  in  these  are 
mainly  pure  salt  (sodium  chloride),  and  in  some  of  the  larger  ones, 
like  Great  Salt  Lake,  also  sodium  sulphate  or  mirabilite.  The  Cas- 
pian must  be  differentiated  from  salt  lakes  of  smaller  size,  as  it  is 
more  properly  a  portion  of  the  sea  which  has  been  cut  off.  Hence 
the  deposits  there  are  generally  like  those  formed  on  the  sea-coast. 

Composition  of  Lake  Water 

The  composition  of  lake  water  varies  of  course  in  an  almost  endless  manner, 
no  two  lakes  having  water  of  exactly  the  same  composition.  Nevertheless, 
it  is  possible  to  select  certain  types  or  averages  of  groups,  which  represent  in  a 
general  way  the  mineral  substances  present  in  such  waters.  Fresh- water 
lakes  are  of  course  the  most  abundant,  and  as  their  composition  varies  to  a  less 
degree  than  that  of  the  other  lakes,  it  is  possible  to  give  a  general  average. 
This  is  shown  in  the  first  column  of  the  annexed  table,  in  which  the  average  also 
includes  that  of  river  waters,  which  are  not  essentially  different  from  those  of 
lakes.  In  the  other  columns  the  composition  of  a  typical  alkaline  water  (Owen's 
Lake,  California),  saline  water  (Lake  Corongamite,  Victoria,  Australia),  and 
brine  (Dead  Sea  at  200  meters  depth)  are  given,  and  the  composition  of  ocean 
water  is  again  given  for  comparison.  In  this  table  the  composition  is  expressed 
in  terms  of  ions  rather  than  of  salts,  which  is  the  more  accurate  way  of  state- 
ment. 


Evaporation  Products  of  Lakes 


251 


TABLE  OF  THE  COMPOSITION  OF  LAKE  WATERS 


FRESH 

ALKALINE 

SALINE 

OCEAN 

WATER 

WATER 

WATER 

WATER 

NAME  OF  ION 

SYMBOL 

Average 

Salinity 

0-1  7-5,, 
per  mule 

Salinity 

2I3-7 
per  mule 

Salinity 
46 
per  mille 

Salinity 
251.1 
per  mille 

Average 
Salinity 

per  mille 

Carbonic  oxide  .... 

C03 

3S-JS 

24-55 



Trace 

0.207 

Sulphuric  oxide      .     .     . 

S04 

12.14 

9-93 

I.65 

0.22 

7.692 

Phosphoric  oxide    .     .     . 

P04 

— 

O.II 

— 



Trace 

Boron  oxide       .... 

B407 

— 

0.14 

— 



Trace 

Chlorine  

Cl 

5.68 

24.82 

59-32 

67.84 

55-292 

Bromine  

Br 

— 

— 

0.22 

i-75 

0.188 

Nitrogen  oxide  .... 

N03 

0.90 

0.45 



— 

Lithium  

Li 



O  O3 





Trace 

Calcium  

Ca 

IO  OO 

o  02 

O  It 

i  68 

1   107 

Magnesium  

Mg 

3-41 

O.OI 

2.77 

16.72 

3-725 

Sodium 

Na 

38  oo 

•3  c  O7 

IO  OO 

30  SO3 

•/v 

Potassium 

K 

212 

I  62 

o  84. 

I   7O 

1.106 

Iron  oxide    
Aluminum  oxide     .     .     . 

Fe203  1 
A1203/ 

2-75 

0.04 

Trace 

Silica 

SiO2 

1  1  67 

O  Id 



Trace 

Trace 

Arsenic  oxide    .... 

AsA 

0.05 



Trace 

IOO.OO 

IOO.OO 

IOO.OO 

IOO.OO 

Deposits  of  Fresh-Water  Lakes 

The  three  most  important  types  of  deposits  formed  from  the 
waters  of  fresh-water  lakes  and  ponds  are  those  of  carbonate  of 
lime  and  carbonate  and  oxide  of  iron. 

Carbonate  of  Lime.  —  This  is  by  far  the  most  important  con- 
stituent of  lake  waters,  and  for  that  matter,  of  fresh-water  bodies  of 
all  kinds.  Nevertheless,  the  actual  quantity  is  smaller  in  fresh  than 
in  sea-water.  As  will  be  seen  from  the  analysis  a  cubic  mile  of  fresh- 
water contains  on  the  average  only  360,915  short  tons  of  calcium  car- 
bonate (CaCO3),  whereas  a  cubic  mile  of  sea-water  contains  583,520 
short  tons  of  this  mineral  in  solution.  It  must  be  remembered,  how- 
ever, that  sea-water  contains  a  vastly  larger  quantity  of  other  sub- 
stances, of  which  common  salt  forms  131,526,000  tons,  while  in 
average  fresh- water  it  forms  only  about  19,656  short  tons  per  cubic 
mile.  Moreover,  the  total  quantity  of  mineral  substances  dissolved 
in  the  sea  is  169,148,000  short  tons  per  cubic  mile,  in  fresh- water  only 
854,100  short  tons;  that  is,  the  sea  contains  about  200  times  as 


252 


Aqueous  or  Hydrogenic  Rocks 


much  mineral  matter  in  solution.  The  mineral  matter  of  fresh- 
water bodies  is  precipitated  by  three  methods:  evaporation, 
chemical  reaction,  and  organic  secretion.  The  last  belongs  to  the 
topic  of  organic  deposits. 

Evaporation.  —  Lake  waters  are,  as  a  rule,  very  far  from  being 
saturated  with  carbonate  of  lime,  and  for  this  reason  such  a  sub- 
stance will  not  be  de- 
posited as  an  evaporation 
product  until  much  or  all 
of  the  lake  water  is 
evaporated.  An  excep- 
tion to  this  is  the  depo- 
sition of  carbonate  of 
lime  in  the  form  of  cal- 
careous tufa  in  the  mar- 
ginal pools  or  on  beaches 
of  great  fresh-water  lakes 
situated  in  dry  climates, 
but  constantly  supplied 
with  water  by  a  large 
river.  Such  tufa  forms 
by  local  complete  evapo- 
ration of  the  water  which 
lies  in  shallow  marginal 
pools  above  the  ordinary 
water-level  and  is  re- 
plenished at  intervals  by 
spray  and  the  waves. 
The  spray  which  satu- 
rates the  sands  of  the 
beaches  may  also,  on 
rapid  evaporation,  leave 
behind  carbonate  of  lime 
to  bind  the  sand  grains 

together.  Such  deposits  of  calcareous  tufa  are  found  in  the  old  ' 
lake  beaches  on  the  slopes  of  the  Colorado  or  Salton  desert  up 
to  40  feet  above  sea-level.  These  were  formed  when  the  Colorado 
River  drained  into  the  basin  and  kept  it  full  of  water  up  to  that 
level,  in  spite  of  the  rapid  evaporation  which  was  taking  place  in 
the  dry  climate  of  the  region,  and  which  has  completely  dried 


FIG.  175.  —  Map  of  Lake  Bonneville  and  its 
present  remnant,  Great  Salt  Lake  of  Utah. 


Evaporation  Products  of  Lakes 


253 


FIG.  176. — Terraces  and  shore-lines  of  Lake  Bonneville,  near  Wellsville, 
Utah,  showing  contrast  between  littoral  and  subaerial  topography.  (After 
Gilbert.) 


FIG.  177.  —  Abandoned  shore-lines  of  Lake  Bonneville.     North  end  of  Oquirrh 
Mountains,  Utah.     (Photo  by  F.  J.  Pack.) 


254 


Aqueous  or  Hydrogenic  Rocks 


out  that  basin  since  the  Colorado  has  become  diverted  to  the 
Gulf  of  California.  The  nearly  complete  evaporation  of  lake 
water  is  illustrated  by  the  history  of  lakes  Bonneville  and  Lahonton, 
which  in  an  earlier  period  of  the  earth's  history  occupied  large 


0    5    iO   i5  20  " 


jca,le    in    Miles 

FIG.  178.  —  Map  of  ancient  Lake  Lahonton  and  some  of  the  present  residual 

water  bodies. 

areas  in  the  western  United  States  (Figs.  175,  178),  and  the  old 
shore  lines  of  which  are  still  traceable  (Figs.  1 76, 177).  The  product 
of  evaporation  of  the  water  of  these  lakes  was  chiefly  carbonate 
of  lime  in  the  form  of  calcareous  tufa,  of  which  several  types  were 
formed.  This  covers  the  older  rock  surfaces  over  wide  areas 
(Fig.  176)  and  forms  layers  of  tufaceous  limestone,  alternating  with 


Evaporation  Products  of  Lakes 


255 


deposits  of  sands  and  gravels,  which  it  sometimes  cements.  In 
places  these  deposits  form  a  mass  over  50  feet  in  thickness,  though 
elsewhere  they  are  represented  only  by  an  average  thickness  of 
20  to  25  feet. 

Three  types  of  calcareous  tufa  are  found  within  the  basin  of  old  Lake  Lahon- 
ton,  each  type  belonging  to  a  separate  period  of  formation.  The  first  and  oldest 
is  called  a  lithoidal  tufa  because  it  is  rock-like,  cementing  the  old  gravels  of  the 


I 

FIG.  179.  — Thinolithic  tufa  or  Thinolite,  from  Lake  Lahonton  Basin.     (After 

Russell.) 

lake  floor,  and  it  not  infrequently  contains  shells  of  fresh- water  Mollusca.  The 
next  type,  formed  after  an  interval  of  exposure,  is  known  as  thinolithic  tufa 
(Fig.  179),  and  consists  of  a  series  of  large  prismatic  crystals  six  to  eight  inches 
long  and  almost  half  an  inch  in  thickness.  These  form  a  layer  from  six  to 
eight  feet  thick  where  best  developed.  The  final  type  of  tufa  is  called  dendritic 
(Figs.  159,  180),  from  its  branching  structure,  and  this  is  the  most  abundant, 


256 


Aqueous  or  Hydrogenic  Rocks 


covering  the  old  rocks  of  the  lake  with  a  deposit  from  twenty  to  fifty  feet  thick. 
Sometimes  this  formed  dome-shaped  or  mushroom-like  masses  up  to  five  or 
six  feet  in  diameter  (Fig.  159,  p.  2.19),  and  where  these  are  crowded  they  often 

assume  a  polygonal  outline 
resembling  paving  blocks. 
Internally  these  masses 
have  a  more  or  less  radiate 
structure. 

Significance  of  lime  de- 
posits of  this  type.  —  Lime- 
stones like  the  above,  due 
to  evaporation,  indicate  a 
dry  climate  during  the 
period  of  their  formation. 
If  such  limestones  are  cov- 
ered by  other  deposits, 
which  later  harden  to  rocks, 
they  will  form  a  member  of 
a  series  of  stratified  rocks 
similar  in  general  appear- 
ance to  many  that  are  found 
in  the  older  parts  of  the 
earth's  crust.  If  in  such 
an  older  series  the  limestone 
member  can  be  determined 
by  its  peculiar  character- 
istics to  have  originated  as 
an  evaporation  product  of 
an  old  lake  basin,  a  definite 
conception  of  the  physical 
conditions  of  that  region  at 
the  time  of  the  formation  of 
the  limestone  bed  is  gained.  It  is  therefore  important  that  the  particular 
characteristics  of  such  limestone  beds  should  be  understood.  At  the  same 
time  it  must  be  remembered  that  with  the  passage  of  time,  such  a  limestone 
will  undergo  more  or  less  change,  so  that  the  old  characters  are  to  a  greater 
or  less  degree  obliterated  or  altered.  Nevertheless,  enough  may  remain  to 
indicate  the  origin  of  the  limestone,  and  from  the  adjoining  formations  and 
those  lying  beneath  and  above  it  additional  evidence  for  or  against  the  evapora- 
tional  origin  of  such  a  limestone  may  be  obtained.  What  may  prove  to  be  an 
example  of  this  type  formed  in  a  past  geological  period  (Permian),  is  the  bed  of 
Magnesian  Limestone  exposed  on  and  near  the  coast  of  Durham  in  England, 
which  shows  a  structure  not  unlike  the  spherical  structure  of  the  dendritic  tufa 
of  Lake  Lahonton  (Fig.  160,  p.  221).  The  internal  structure  is,  however,  more 
strongly  crystalline,  which  may  be  due  to  its  greater  age.  (Fig.  18,  p.  34.) 

Chemical  precipitation.  —  This  will  take  place  in  stagnant  lakes 
and  ponds,  the  waters  of  which  contain  much  carbonate  of  lime  in 


FIG.  1 80. — Tufa-domes,  shore  of  Pyramid 
Lake,  a  remnant  of  Lake  Lahonton.  Dendritic 
tufa.  (After  Russell.) 


Evaporation  Products  in  Lakes 


257 


solution,  and  which  moreover  contain  decaying  organic  matter. 
This  decay,  as  in  the  ocean,  will  produce  ammonium  carbonate, 
which  will  tend  toward  precipitating  the  lime.  It  may  be  doubted, 
however,  that  much  lime  is  precipitated  in  this  manner  from  fresh 
water,  since  the  presence  of  carbon  dioxide,  which  is  also  formed 
as  the  product  of  decay,  would  tend  to  keep  the  lime  in  solution. 

Organic  separation.  —  The  most  common  method  of  abstraction 
of  lime  from  fresh  water  is  that  by  the  activities  of  organisms,  both 
animal  and  plant.  But  deposits  so  formed  belong  to  the  group 
of  organic  rocks,  and  their  discussion  will  be  deferred  to  another 
chapter. 

Iron  Carbonate  and  Oxides.  —  Stagnant  swamps  are  often 
covered  by  an  iridescent  film,  which  is  the  result  of  the  oxidation, 
on  contact  with  the  atmosphere,  of  iron  salts  which  were  contained 
in  the  water.  Such  iron  salts  are  originally  in  the  form  of  ferrous 
carbonate  (FeCO3),  which  in  mineral  form  is  the  ore  siderite.  This 
iron  carbonate  is  derived  from  iron  salts  in  the  soil  and  has  resulted 
from  the  decomposition  of  iron-bearing 
rocks  (the  ferro-magnesian  silicates  of 
igneous  rocks),  the  leaching  being  per- 
formed by  rain  and  ground  waters  which 
have  taken  up  carbon  dioxide  (CO-j)  from 
decaying  vegetation.  When  such  a  so- 
lution of  iron  carbonate  is  brought  into 
a  lake  or  swamp,  one  of  two  things  will 
happen.  When  there  is  much  decaying 
organic  matter  in  the  swamp,  this  will 
appropriate  all  the  available  oxygen, 
and  then  the  iron  is  deposited  in  the 
form  of  the  carbonate.  This  is  gen- 
erally impure,  being  mixed  with  the  mud 
held  in  suspension  and  carried  down  with 
the  iron  carbonate.  This  mixture  is  most 
frequently  deposited  around  some  object 
which  forms  the  nucleus  of  an  iron-stone 
concretion,  as  such  structures  are  called 
(Fig.  181).  Such  iron-stone  or  siderite 
concretions  when  sufficiently  pure  form  ores  of  iron,  and  they  are 
commonly  found  in  formations  in  which  coal  (the  product  of  the 
partial  decay  of  the  vegetation)  is  also  found. 


Hy, 
•I 


FIG.  181. —  Clay-iron- 
stone concretion  split  in 
two,  showing  the  impression 
of  a  fossil  fern  (Neurop- 
teris).  Mazon  Creek,  111. 
(Photo  by  B.  Hubbard. 
Specimen  in  Columbia  Uni- 
versity.) 


258  Aqueous  or  Hydrogenic  Rocks 

If,  however,  vegetation  is  not  abundant,  the  carbonate  will  be 
changed  to  the  oxide,  by  contact  with  the  air  and  with  the  oxygen 
dissolved  in  the  water,  and  in  such  cases  an  iron  'oxide  will 
be  deposited.  This  is  commonly  in  the  form  of  limonite,  the 
yellow  iron  ore,  in  which  water  is  present,  and  the  loose,  porous 
masses  of  this  mineral  which  form  on  the  bottom  of  such  ponds  are 
called  bog-iron-ore.  Such  bog-ores  are  generally  most  abundant 
near  the  margins  of  the  ponds  and  swamps  and  are  often  wanting 
near  their  centers.  Sometimes  such  bog-ore  forms  very  rapidly, 
some  Swedish  lakes  having  deposited  layers  several  inches  thick 
in  twenty-six  years. 

The  decomposition  of  the  iron  carbonate  in  the  solution  and  its 
oxidation  to  insoluble  iron  oxide  is  aided  by,  and  in  some  cases 
largely  due  to,  the  work  of  minute  organisms  in  the  water,  the 
so-called  "  iron  bacteria."  Such  deposits  might  be  referred  to  the 
organic  group,  were  it  not  for  the  difficulty  of  distinguishing  them 
from  purely  chemical  deposits  such  as  those  described. 

Alkaline  Lakes  and  Their  Deposits 

Under  this  heading  are  classed  lakes  in  which  carbonate  of  soda 
plays  a  more  important  part  than  carbonate  of  lime,  which  is 
generally  present  in  small  quantities  only.  The  relatively  great 
preponderance  of  the  CO2  radical,  and  the  much  reduced  chlorine 
content,  further  differentiate  typical  alkaline  lake  waters  from  those 
of  saline  lakes,  in  which  sodium  chloride  is  the  dominant  salt  in  solu- 
tion and  the  carbonates  are  insignificant  in  quantity.  Sulphates 
and  chlorides,  however,  are  generally  present,  and  one  or  the  other 
or  both  may  be  abundant,  making  complex  alkaline  waters.  In 
some  cases,  too,  potassium  may  be  an  important  element,  exceeding 
even  the  sodium,  as  in  Albert  Lake,  Oregon. 

Some  of  the  principal  water  bodies  in  which  deposits  of  this  and 
similar  types  are  formed  are : 

Sodium  Sulphate.  — Laramie  Lakes,  Wyoming 
Soda  Lake,  Cal. 
Sevier  Lake,  Utah 
Mono  Lake,  Cal. 
Owen's  Lake,  Cal. 
Searle's  Lake  or  Marsh,  Cal. 
Albert  Lake,  Oregon 
Altai  and  Domoshakovo  Lakes,  Russia 


Evaporation  Products  of  Rivers  259 

Sodium  Carbonate.  —  Soda  Lake,  etc.,  Nev 

Owen's  Lake,  Cal. 

Searle's  Lake,  Cal. 

Natron  Lakes,  Egypt 

Lakes  in  Hungary,  Armenia,  Venezuela,  etc. 
Borax.  —  Searle's  Lake,  Cal. 
Death  Valley,  Cal. 
Soda  Niter  (Chile  Saltpeter).  —Desert  Lakes  of  Chile 

Searle's  Lake 
Potassium  Nitrate  (Saltpeter). — Cochabamba,  Bolivia 

Different  salts  may  be  deposited  at  different  times  by  these 
lakes.  Thus  Searle's  Lake  or  Marsh  deposits  both  carbonate  and 
sulphate  of  sodium  and  borax  and  niter  as  well.  From  Owen's 
Lake,  California,  both  carbonate  and  sulphate  of  sodium  are  ob- 
tained. Common  salt  (sodium  chloride)  is  also  an  accompaniment 
of  the  deposits  in  many  of  these  lakes. 

The  greatest  deposits  of  niter  known  in  the  world  are  found  in 
the  Atacama  and  Tarapaca  deserts  of  Chile.  The  amount  has 
been  estimated  at  254,760,000  tons  and  is  found  at  elevations 
exceeding  2000  feet  above  the  sea  and  from  50  to  100  miles  from 
the  coast.  The  crude  sodium  nitrate  is  known  as  caliche  and  is 
associated  with  anhydrite,  gypsum,  epsomite,  halite,  and  other 
minerals.  The  origin  of  these  and  of  the  potash  nitrates  of  Bolivia 
is  not  fully  understood,  but  they  were  deposited  from  solution  in 
drying  basins.1 

Deposits  of  Saline  Lakes  and  Brines 

These  lakes  deposit  chiefly  sodium  chloride  or  common  salt, 
especially  those  in  which  the  waters  are  a  brine,  as  is  the  case*  in 
most  inland  salt  lakes.  Great  Salt  Lake,  Utah,  the  Dead  Sea, 
and  numerous  Russian  and  Siberian  lakes  serve  as  examples. 
They  supply  vast  quantities  of  common  salt  for  domestic  and  other 
purposes. 

CHEMICAL  DEPOSITS  AND  EVAPORATION  PRODUCTS  OF  RIVERS 
(Flumatile  Chemical  Deposits) 

These  are  of  comparatively  rare  occurrence  and  are  confined 
chiefly  to  regions  of  arid  climate.  As  carbonate  of  lime  is  the 

1  For  fuller  discussion,  see  A.  W.  Grabau,  Geology  of  the  Non-metallic  Mineral  De- 
posits, etc.  Vol.  I,  Chapter  XIII.  McGraw-Hill  Book  Co. 


260  Aqueous  or  Hydrogenic  Rocks 

main  mineral  constituent  of  the  river  water  and  of  that  of  fresh- 
water lakes,  it  forms  the  chief,  indeed  practically  the  only,  im- 
portant chemical  deposit  of  fresh  water. 

In  Bahia,  Brazil,  the  rivers  which  flow  in  and  over  the  older 
limestones  are  highly  charged  with  lime  in  solution,  and  under  the 
influence  of  the  tropical  sun,  partial  evaporation  of  the  water  and 
precipitation  of  the  lime  takes  place.  Thus  deposits  of  lime  rang- 
ing up  to  100  feet  in  thickness  have  been  formed,  and  they  often 
contain  plant  remains  as  well  as  shells  of  river  and  land  mollusks 
of  species  still  living  in  the  region.  Angular  as  well  as  water-worn 
fragments  of  other  rock  are  also  included,  and  at  times  the  mass 
becomes  brecciated  through  local  disruption  and  cementation  of 
the  fragments.  Similar  deposits  are  also  formed  around  waterfalls 
of  rivers  in  limestone  regions  in  various  parts  of  the  world. 

On  the  broad  flood-plains  of  many  tropical  rivers  and  on  their 
deltas  are  formed  crusts  of  limestone,  which  are  precipitated  from 
the  over-charged  river  water  from  the  limestone  hills.  In  Mexico 
and  the  southern  United  States  these  are  known  as  tepetate,  while 
the  nodular  limestone  masses  embedded  with  the  sediments  of  the 
Indus  and  Ganges,  in  northern  India,  are  known  as  kankar.  Such 
limestone  nodules  of  chemical  origin  are  very  characteristic  of 
river  sediments  in  arid  regions,  and  their  occurrence  in  older  sand- 
stones leads  us  to  ascribe  a  similar  origin  to  these  deposits. 

DEPOSITS  BY  SPRINGS  AND  UNDERGROUND  WATERS 

Lime  Deposits  of  Springs.  —  In  limestone  regions,  the  under- 
ground water  is  generally  strongly  charged  with  carbonate  of  lime 
in  solution.  Where  this  water  reaches  the  surface  in  springs, 
the  relief  of  pressure  and  the  escape  of  carbon  dioxide  combine  to 
cause  the  precipitation  of  some  of  the  carbonate  of  lime  as  a 
flocculent  material,  which  later  hardens  to  solid  stone.  Where 
leaves,  mosses,  or  other  organisms  are  bathed  by  this  spring  water, 
they  are  covered  by  a  deposit  of  lime  or  are  included  as  fossils  in 
a  mass  of  calcareous  tufa  or  travertine.  In  ordinary  spring  water, 
the  deposition  of  lime  goes  on  rather  slowly,  but  sometimes  the 
growth  of  the  travertine  deposit  is  very  rapid.  At  the  Baths  of 
San  Vignone,  in  Tuscany,  for  example,  travertine  is  deposited  at 
the  rate  of  six  inches  a  year,  while  at  San  Filippo,  in  Sicily,  the  rate 
is  one  foot  in  four  months.  Here  a  hill,  a  mile  and  a  quarter  long 


Deposits  by  Springs  and  Underground  Waters      261 


and  a  third  of  a  mile  broad,  has  been  formed  by  such  deposits, 
the  height  being  at  least  250  feet. 

The  water  of  hot  springs  generally  carries  more  mineral  matter 
in  solution  than  that  of  cold  springs.  The  cooling  of  the  water 
on  reaching  the  surface 
is  also  very  conducive 
to  lime  deposition. 
Hence  we  have  here  ex- 
tensive travertine  de- 
posits, as  shown  in  the 
terraces,  dams,  and 
basins  of  the  Mammoth 
Hot  Springs  of  the  Yel- 
lowstone (Fig.  182). 
The  origin  of  the  ter- 
races is  illustrated  in 
the  subjoined  diagram 
(Fig.  183). 

A  peculiar  form  of 
lime  deposit  from 
springs  is  the  well-known 
onyx  marble,  or  Mexican 

onyx,  which  occurs  interbedded  with  normal  tufas  in  Arizona, 
Mexico,  California,  and  elsewhere  in  America,  and  in  TsTorth 
Africa,  Persia,  and  elsewhere  in  the  Old  World.  This  is  a  com- 
pact, highly  crystalline,  and  often  beautifully  variegated  lime- 
stone of  a  semi-translucent  character,  much  used  for  decorative 

purposes,  the  con- 
struction of  soda- 
water  fountains,  etc. 
It  is  frequently  found 
resting  on  crystalline 
rocks  in  regions  de- 
void of  other  lime- 
stones, and  this  sug- 
gests that  the  lime 
and  the  water  which 
brought  it  to  the  sur- 


FIG.  182.  —  Portion  of  the  Sinter  Terraces 
of  Mammoth  Hot  Springs,  Yellowstone  Na- 
tional Park. 


FIG.  183.  —  Diagrammatic  section  of  Sinter 
Terraces  formed  by  the  water  of  hot  springs. 
The  rim  of  the  terrace  is  built  up  most  rapidly 
because  as  the  water  overflows  it  cools  quickly  at 
this  point  and  deposits  its  mineral  matter.  Series 
of  terrace-basins  are  thus  formed.  (From  Kay- 
ser's  Lehrbuch.) 


face  were  of  deep-seated  or  magmatic  origin,  that  is,  derived  from 
hot  igneous  masses  within  the  crust  of  the  earth. 


262  Aqueous  or  Hydrogenic  Rocks 

No  beds  of  this  deposit  have  actually  been  observed  in  the 
process  of  making,  but  from  the  fact  that  it  is  generally  enclosed  by 
beds  of  normal  tufa,  we  may  assume  that  these  deposits  were 
formed  in  temporary  pools  or  lakes  where  standing  water  prevented 
the  rapid  escape  of  CO2,  the  rock  thus  becoming  compact  instead 
of  porous,  as  is  the  case  in  ordinary  tufa  formed  on  the  land. 

Oolites  and  Pisolites  Deposited  by  Springs.  —  At  the  famous 
springs  of  Carlsbad,  in  Bohemia,  carbonate  of  lime  is  deposited 
in  the  form  of  spheroidal,  discrete  masses  of  the  size  of  a  pea,  and 
hence  forming  an  accumulation  of  particles  which  when  bound 
together  into  a  rock  would  constitute  a  pisolite.  As  the  water 
rises  in  the  springs  it  holds  in  suspension  minute  mineral  fragments 
such  as  quartz  or  feldspar,  which  then  receive  a  coating  of  lime 
precipitated  from  the  water.  As  the  particles  are  turned  over 
and  over  in  all  directions  like  a  pith-ball  in  a  fountain  jet,  the 
coating  will  be  uniform  all  over,  and  spheroidal  masses  are  pro- 
duced. Sometimes  gas  and  air  bubbles  form  the  nuclei  around 
which  the  lime  is  deposited,  thus  forming  spheroids  with  a  hollow 
center.  The  water  of  these  springs  is  probably  of  deep-seated 
volcanic  (magma tic)  origin,  from  which  source  the  lime  is  also 
derived.  This  is  indicated  by  the  fact  that  there  are  no  known 
beds  of  limestone  through  which  these  waters  ascend. 

While  no  doubt  both  oolites  and  pisolites  have  been  formed  in 
the  past  by  springs,  the  great  majority  of  these  deposits  now  found 
in  the  rocks  of  the  earth's  crust  were  formed  in  standing  bodies  of 
water  through  the  influence  of  organic  matter  either  directly,  by 
the  physiological  activities  of  living  organisms,  such  as  bacteria 
or  minute  algae,  or  by  ammonia  generated  by  decaying  organic 
matter.  They  will  be  discussed  in  a  subsequent  chapter. 

Lime  Deposits  in  Caves.  —  The  best-known  types  of  lime 
deposits  from  ground-water  are  found  in  caverns.  Two  types  of 
structures  are  generally  recognized,  the  stalactite  (Fig.  184), 
depending  from  the  roof  of  the  cavern  or  from  some  projecting 
edge,  and  the  stalagmite,  which  forms  on  the  floor  of  the  cavern, 
building  up  a  mound  or  pyramid,  or  forming  a  hummocky  lime- 
stone floor  (Fig.  185). 

The  formation  of  stalactites  may  be  observed  in  tunnels  and 
underground  chambers,  the  roofs  of  which  consist  of  blocks  held 
together  by  lime-mortar,  as  well  as  in  limestone  caverns.  Percolat- 
ing ground-water  will  dissolve  a  part  of  this  lime,  and  when  a  drop 


Deposits  by  Springs  and  Underground  Waters     263 

of  this  lime-charged  water  reaches  the  tunnel  and  is  suspended  from 
the  roof,  rapid  evaporation  will  cause  the  formation,  around  the 
drop,  of  a  thin  shell  of  lime.  The  pressure  of  the  percolating 


FIG.   184.  —  Compound  stalactites  and  stalactic-sheets  in  Luray  Cave, 
Virginia.     (From  U.  S.  G.  S.) 

water  will  cause  the  breaking  of  this  film  of  lime,  leaving  only  a 
small  ring  on  the  roof.  The  original  drop  falls,  and  a  new  one, 
equally  charged  with  lime,  suspends  itself  from  the  bottom  of  the 
lime  ring.  By  constant  repetition,  a  slender  delicate  tube  is  formed 
by  the  successive  additions  of  minute  rings  of  lime,  and  this  is 
the  basis  of  the  stalactite.  Other  water,  running  down  the  outside 
of  this  tube,  will  thicken  and  strengthen  it  by  the  addition  of 
successive  layers  of  lime.  In  this  process  the  lower  end  of  the 


264  Aqueous  or  Hydrogenic  Rocks 

initial  tube  is  soon  closed,  and  after  that  the  stalactite  remains 
a  solid  icicle  of  carbonate  of  lime.  Neighboring  stalactites  may, 
from  close  juxtaposition,  become  confluent  and  form  broad  sheets 
or  curtains  of  lime  which  are  often  beautifully  banded.  Such 
sheets  of  lime  depending  from  an  edge  in  the  roof  or  along  the 
line  of  a  crack  are  shown  in  Luray  Cavern,  Virginia,  where  they 
form  one  of  the  striking  features  of  this  beautiful  cave  (Fig.  184). 
Stalagmites  are  built  up  on  the  floor  of  a  cavern  by  the  evapora- 
tion of  the  water  which  drops  from  the  roof  and  generally  from  the 


FIG.  185. — Stalactites  and  stalagmites  in  Marengo  Cave,  Indiana.  Note 
the  numerous  small  stalactites  which  depend  from  the  roof  of  the  cave,  and 
confluence  of  the  larger  stalactites  with  the  stalagmites  to  form  columns. 

end  of  the  stalactite.  The  continued  evaporation  of  this  water 
leaves  a  minute  quantity  of  lime,  which  is  gradually  built  up  into 
a  mound,  and  this  becomes  steeper  and  steeper  as  it  increases  in 
height,  and  finally  forms  a  conical  or  even  columnar  mass  beneath 
the  stalactite,  with  which  it  may  eventually  become  joined  into  a 
continuous  column.  This  is  well  shown  in  the  above,  view  of 
the  interior  of  Marengo  Cave  in  Indiana  (Fig.  185).  Many  such 
columns  may  result  in  the  cutting  off  of  chambers  and  galleries 
from  the  original  caverns.  On  the  margin  of  the  stalagmite  sec- 
ondary dependent  stalactites  are  often  formed,  wherever  a  higher 
portion  of  the  stalagmite  projects  cap-like  beyond  the  lower. 
Thus  highly  complex  and  picturesque  structures  are  produced. 


Mineral  Veins  265 

Where  the  water  spreads  laterally  before  evaporating,  an  extended 
sheet  of  stalagmite  material  is  formed,  and  the  lateral  confluence 
of  many  such  sheets  may  result  in  the  production  of  a  stalagmite 
floor.  In  many  of  the  limestone  caverns  of  southern  France  and 
other  parts  of  Europe,  which  during  Palaeolithic  time  were  in- 
habited by  the  people  of  the  Old  Stone  Age,  implements  and  even 
the  bones  of  these  prehistoric  people  are  found  embedded  in  the 
clay  of  the  cavern  floor,  over  which  not  infrequently  a  cover  of 
stalagmitic  material  has  been  formed. 

Basins  are  also  formed  in  caverns  where  water,  holding  the  lime 
in  solution,  runs  over  a  ledge.  The  edges  of  such  a  ledge  cause  the 
water  to  break  into  ripples  as  it  overflows,  and  this  permits  the 
escape  of  some  of  the  carbon  dioxide  which  holds  the  lime  in  solu- 
tion. In  consequence,  this  lime  is  deposited  at  the  edges  of  the 
ledge,  and  so  a  rim  is  gradually  built  up,  which  holds  back  more 
and  more  of  the  water  in  a  permanent  pool.  Many  such  pools  are 
found  in  limestone  caves  such  as  Luray,  where  the  conditions  for 
their  formation  are  favorable.  They  are  analogous  to  the  sinter 
terraces  of  the  hot  springs  (Fig.  183). 

MINERAL  VEINS 

Of  all  the  deposits  formed  by  waters  circulating  in  or  rising 
through  the  crust  of  the  earth,  the  mineral  veins  are  the  most 
important  to  man.  The  mineral-bearing  waters  which  form  the 
veins  are  probably  in  most  cases  hot,  and  they  may  even  be  in  the 
form  of  vapors.  These  waters  or  vapors  deposit  their  load  of 
mineral  matter  either  upon  the  walls  of  cavities  or  fissures,  or  by 
replacing  the  rock  material 
alter  it  along  their  passage- 
way, which  may  be  a 
minute  crack  or  other 
avenue  of  escape.  The 
first  group  is  called  fissure 
veins,  the  second  replace- 
ment veins  or  deposits. 

Fissure  Veins  (Fig.  FIG.  i860.. —  Section  of  the  Prinzen  Lode, 
186  a).  —  Deposition  in  Freiberg-  <*,  blende;  &,  quartz;  c,  fluorite; 
,.  .  ,  ",  ,  d,  barite ;  e,  pyrite ;  /,  calcite. 

fissures  is  brought  about 

by  the  cooling  effect  produced  by  the  walls  upon  the  solution,  or 
by  chemical  reaction  with  the  material  of  the  wall  rock.  Thus 


266 


Aqueous  or  Hydrogenic  Rocks 


an  acid  solution  coming  in  contact  with  a  limestone  would  become 
neutralized  and  lime  salts  inclosing  other  minerals  and  metallic 
substances  might  be  deposited.  Fissure  veins  have  the  form  of 

sheets,  or  films,  of  min- 
eral matter  cutting  the 
rock.  If  there  is  a  suc- 
cession of  deposits,  the 
vein  will  be  banded  par- 
allel to  the  wall-rock, 
and  the  central  portion 
may  be  filled  with  crys- 
tals. Veins  of  ore  min- 
erals or  metallic  sub- 
stances contain,  besides 
this  material,  quantities 
of  other  minerals  such 
as  quartz,  calcite,  barite, 
etc.,  and  these  constitute 
the  gangue  material  of 
the  miner.  Thus  native 
gold  is  commonly  found 
in  a  gangue  of  quartz, 
but  quartz  veins  without 
gold  or  other  metal  are 

much  more  common.  So,  too,  are  veins  of  calcite  and  other  min- 
erals of  little  commercial  value. 

In  rocks  containing  many  veins  there  can  often  be  recognized 
several  distinct  series  of  successive  origin.  The  relative  age  of 
veins  can  be  determined  from  the  fact  that  the  younger  veins  are 
continuous  across  the  older  ones  which  they  intersect.  Some- 
times cavities  of  irregular  shape  in  the  country  rock  form  the  site 
of  mineral  and  ore  deposits,  these  differing  from  true  fissure  veins 
mainly  in  their  form  and  extent.  Deposits  of  this  kind  are  called 
cavity-filled  ore  deposits  (Fig.  1866). 

Replacement  Deposits  (Fig.  i86c). — Instead  of  filling  a  pre- 
viously existing  fissure  or  cavity  with  its  deposits,  the  mineral- 
bearing  waters  or  vapors  may  deposit  their  material  as  they  pass 
through  the  rock,  by  dissolving  mineral  particles  of  the  country 
rock  and  filling  their  places  with  new  mineral  matter.  Such  a 
replacement  of  one  mineral  by  another  would  go  on,  molecule  by 


FIG.  1 86  b.  —  Transverse  section  of  the  great 
ore  chamber  in  the  Emma  mine,  Utah, 
i  inch  =  159  feet. 


Mineral  Veins 


267 


molecule,  until  an  area  of  the  rock  is  so  re- 
placed by,  or  impregnated  with,  valuable  min- 
eral matter  as  to  become  an  important  ore 
deposit.  Frequently,  of  course,  the  replace- 
ment and  fissure  type  of  deposit  may  inter- 
grade,  for  the  walls  of  an  open  fissure  may 
also  be  partly  replaced  by  mineral  matter. 
Veins  in  which  ores  are  scattered,  i.e.  lean 
veins,  may  become  locally  enriched  by  the  sub- 
sequent solution  and  local  concentration  of  the 
valuable  mineral  matter.  The  movement  of 
the  ores  is  generally  from  higher  to  lower 
levels.  Concentration  may  also  occur  by  the 
weathering  or  solution  and  removal  of  the 
gangue  material. 

Placer    Deposits.  —  Concentration    of  val- 
uable minerals  may  also  occur  at  points  distant 
from  the  original  vein.     Thus  quartz  veins 
carrying    gold    may   be   broken   up   by    the 
weather  and  the  fragments  washed  away  by     granite  impregnated 
the    streams.     On    account    of    the    greater     with  tin  ore;    D,  D, 
weight  of  the  gold,  this  will  be  concentrated 
in  favorable  areas  and  so  form  the  well-known  placer  deposits 
(Fig.  187). 


FIG.  1 86  c.  —  Plan 
of  a  tin  lode  at  East 
Wheal  Lovell  Mine, 
Cornwall,  England. 
A,  B,  leader;  C,  C, 


FIG.    187.  —  Hydraulic   mining  of  Placer    deposits    (gold-bearing    gravels), 

Colorado. 


268 


Aqueous  or  Hydrogenic  Rocks 


Source  of  the  Vein  Minerals.  —  The  material  in  solution  in  the 
vein-forming  waters  or  vapors  may  be  derived  either  by  solution 
from  the  rocks  with  which  surface  waters,  descending  into  the 
earth,  come  in  contact  in  their  circulation  through  the  deeper 
portions  of  the  earth's  crust,  deriving  their  material  chiefly  from 


FIG.  187  a.  —  Park  City,  Utah,  a  typical  mining  camp.     (Photo  by  F.  J.  Pack.) 

rocks  which  overlie  the  point  of  deposition  (descending  solu- 
tion), or  the  waters  and  vapors  circulating  through  the  rocks 
laterally  may  dissolve  their  ore  material  and  deposit  it  on  reaching 
the  fissure  (lateral  secretion).  Again,  water  derived  as  emana- 
tion from  deep-seated  igneous  masses  (juvenile  or  magmatic 
water)  may  carry  upwards  in  solution  the  mineral  substances 
derived  from  these  masses  and  deposit  them  in  the  higher  fissures 
(ascending  solutions).  This  last  mode  of  formation  is  regarded 
by  many  as  the  most  typical  origin  of  mineral  veins. 


CHAPTER  XII 
THE  ORGANIC  OR  BIOGENIC  ROCKS 

BlOLITHS 

BY  organic  rocks,  using  that  term  in  its  strictly  limited  sense,  we 
understand  those  additions  to  the  lithosphere  which  have  resulted 
from,  or  are  the  product  of,  the  direct  physiological  activities  of 
organisms,  both  animal  and  plant.  Rocks  secondarily  derived  from 
organic  deposits,  such  as  beds  of  limestone  made  of  fragments 
worn  from  coral  reefs,  have  sometimes  been  included  under  this 
heading,  but  they  do  not  belong  here,  being  strictly  of  f ragmen tal 
or  clastic  origin.  Only  those  deposits  which  are  formed  in  place 
by  organisms  or  which  are 'largely  built  up  of  such  material  which 
has  been  transported  and  has  accumulated  without  much  wear, 
can  be  included  here.  Of  course  it  must  be  recognized  that  a  lime- 
stone of  shells  or  corals,  which  is  strictly  an  organic  limestone,  may 
pass  laterally  by  degrees  into  one  of  fragmental  origin.  Gradation 
exists  everywhere  in  nature,  but  we  are  now  concerned  with  the 
study  of  types  which  may  be  readily  recognized.  The  true  organic 
rocks  are  conveniently  termed  bioliths. 

TYPES  OF  ORGANIC  ROCKS  OR  BIOLITHS 

At  the  outset  we  must  distinguish  two  groups  of  material  of  or- 
ganic origin  which  enter  into  the  formation  of  rocks.  The  first 
is  the  stony  material,  either  carbonate  of  lime  (with  some- 
times phosphate  of  lime)  or  silica,  which  animals  and  plants  take 
chiefly  from  the  water  in  which  they  live,  and  in  which  these  min- 
erals were  dissolved.  This  they  precipitate  upon  or  within  their  tis- 
sues to  build  shells,  corals,  bones,  and  other  hard  structures.  Min- 
eral matter  thus  formed  may  be  called  organic  precipitates.  The  sec- 
ond group  consists  of  the  soft  tissues  of  organisms,  such  as  the  flesh 
of  animals  and  the  tissues  of  plants,  the  latter  made  up  in  large  part 
of  the  substance  called  cellulose.  Such  organic  tissues,  as  they 

269 


270  The  Organic  or  Biogenic  Rocks 

may  be  called,  are  much  more  perishable  than  are  the  organic  pre- 
cipitates, which  are  generally  preserved  during  long  periods  of 
geological  time  without  undergoing  much  alteration.  Organic 
tissues,  on  the  other  hand,  undergo  decay  as  soon  as  death  has  en- 
sued, and  if  not  protected,  they  will  quickly  disappear  by  changing 
into  gaseous  and  other  matter.  When  protected,  however,  by 
burial,  the  change  is  commonly  incomplete,  and  a  product,  largely 
composed  of  carbon  and  hydrogen,  remains  behind.  The  least 
altered  of  such  products  may  form  beds  of  coal;  the  more  completely 
altered  products  form  various  bitumens.  Because  these  substances 
are  all  more  or  less  subject  to  consumption  by  fire,  they  have 
also  been  called  caustoliths  or  caustobioliths  (/axvo-TiKo?  =  capable  of 
burning).  In  the  present  chapter  we  shall  consider  the  stony  de- 
posits of  plants  and  animals,  and  in  the  next  one  the  deposits  formed 
by  the  organic  tissues  and  the  materials  resulting  from  their  decay. 

Kinds  of  Rock  Material  Produced  by  Precipitation  of  Mineral 
Matter  from  Solution  by  Organisms 

Here  again  we  may  distinguish  two  main  types  according  to  the 
composition  of  the  material  precipitated,  namely,  the  calcareous  and 
the  silicious  bioliths.  There  are  others,  such  as  certain  iron  oxide 
deposits,  which  are  formed  by  the  agency  of  organisms,  but  they 
are  of  minor  importance.  The  calcareous  group  may  again  be 
subdivided  into  those  in  which  the  material  is  carbonate  of  lime 
(calcite,  aragonite,  etc.),  with  more  or  less  magnesia,  and  those 
in  which  it  is  largely  phosphate  of  lime. 

DEPOSITS  or  CARBONATE  OF  LIME  BY  PLANTS 

Deposits  of  carbonate  of  lime  are  formed  by  plants  as  well  as  by 
animals  and  are  among  the  most  abundant  precipitates  in  the  sea, 
though  important  ones  are  also  formed  in  fresh  water.  Only  two 
groups  of  plants  are  active  in  precipitating  carbonate  of  lime,  the 
Bacteria  and  the  Algae,  and  in  each  group  only  a  limited  number 
are  active  in  this  way. 

Lime  Deposited  by  Bacteria 

Bacteria  are  microscopic  plants  of  extremely  simple  organiza- 
tion, but  of  almost  universal  distribution  and  vast  abundance. 
Certain  bacteria  (called  denitrifying)  which  live  in  the  warmer 


Deposits  of  Carbonate  of  Lime  by  Plants      271 

portions  of  the  sea  effect  the  reduction  of  nitrates  in  the  water 
to  ammonia,  which,  with  carbon  dioxide,  produced  by  other  bacteria, 
forms  ammonium  carbonate.  This  reacts  with  the  calcium  sul- 
phate in  the  sea-water,  and  the  result  is  the  formation  of  calcium 
carbonate.  The  reaction  is: 


CaS04   +  (NH4)2C03  =  CaCO3 

Calcium  Ammonium  Calcium  Ammonium 

Sulphate  Carbonate  Carbonate        Sulphate 

The  calcium  carbonate  separates  out 

in    the   form    of   small   spherical    or 

elongated   grains,  which  accumulate 

as  a  mass  of  such  discrete  particles 

and  form  a  deposit  of  oolite.     Such  ^        \ 

deposits  are  forming  to-day  in  great        FIG.  188.  —  Pseudomonas 

abundance  off  the  Florida  coast  and     calds>   lime-precipitating  bac- 

.  _,         teria;  greatly  enlarged.     (After 

near  the  islands  of  that  region.     The     Kellermann.) 

most  common  form  of  the  denitrify- 

ing bacteria  has  been  named  Pseudomonas  calcis,  in  allusion  to 
the  fact  that  it  precipitates  lime.  It  is  illustrated  in  Fig.  188, 
greatly  enlarged. 

Alga  and,  Algous  Limestones 

The  term  alga  (plural  alga)  is  applied  to  one  of  the  lowest  divi- 
sions of  plants.  Most  algae  inhabit  the  sea,  though  many  also  live 
in  fresh  water.  They  range  in  size  from  microscopic  forms  to  the 
giant  kelps  of  the  Pacific  Ocean,  which  sometimes  grow  several 
hundred  feet  in  length,  and  are  important  because  they  contain 
both  potash  and  iodine.  A  number  of  algae  precipitate  lime-car- 
bonate upon  and  in  their  tissues,  and  so  form  stony  structures, 
either  as  distinct  masses  or  as  incrustations  of  rocks,  shells  or 
other  substances.  To  such  structures  the  general  name  nullipore 
is  applied,  and  while  they  occur  in  cooler  waters  as  well,  they  are 
most  common  in  tropical  seas,  where  they  constitute  an  impor- 
tant agent  in  the  building  of  coral  reefs.  Many  reefs  consist,  to 
the  extent  of  more  than  half  of  their  mass,  of  these  organisms. 
Another  important  fact  to  be  noted  is  that  many,  if  not 
most,  of  these  lime-secreting  algae  also  separate  out  a  considerable 
amount  of  magnesium  and  precipitate  it  as  the  carbonate.  Con- 
sequently, the  rock  resulting  from  such  algous  accumulations  will  be 
a  limestome  rich  in  magnesium  carbonate  and  may  approach  a 


272 


The  Organic  or  Biogenic  Rocks 


dolomite  in  composition,  especially  when,  by  subsequent  leaching, 
the  proportional  amount  of  calcium  carbonate  is  reduced.  An 
example  of  such  a  rock,  formed  in  the  ancient  Triassic  sea,  is  seen 

in  the  peaks  called  the  Dolo- 
mites, from  the  character  of  the 
rock,  and  which  are  situated  in 
the  Alps  of  the  Tyrol  (Fig.  4,  p.  9). 
The  rock  is  known  to  have  been 
largely  built  up  from  lime- 
secreting  algae  of  the  genus  Diplo- 
pora  (Fig.  5,  p.  10). 

Among  the  more  important 
types  of  nullipores,  we  may  men- 
tion only  a  few  in  addition  to 
the  Diplopora. 

Lithothamnium  (Fig.  189).  — 
This  forms  irregular  masses  with 
knobby  and  sometimes  leaf-like 
surface  features.  It  abounds  on 
modern  coral  reefs  and  also  formed 
extensive  limestone  masses  in 
former  periods. 

Halimeda  (Fig.  190).  —  This  is 
a  form  of  much  more  plant-like  appearance,  having  structures 
resembling  a  stem  and  leaves,  covered  with  lime,  and  brittle  when 
dry.  It  grows  in  the  pro- 
tected lagoons  of  coral  reefs 
and  other  regions. 

Corallina  (Fig.  191). — 
This  is  a  pink,  jointed  plant, 
remotely  resembling  a  finely 
branched  coral  or  hydroid  and 
common  below  low  tide  on 
all  our  North  Atlantic  coasts 
(Corallina  zone).  When  the 
plant  is  dead  and  dry,  the 
color  becomes  white. 

Chara  (Fig.  192  a).  —  Be- 
sides these  and  many  other 


FIG.  189. — Two  modern  species 
of  Lithothamnium •,  a  lime-secreting 
alga,  which  plays  an  important  part 
in  the  building  of  modern  coral- 
reefs.  Isle  Maurice.  (After 
Zittel.) 


marine  nullipores,  there  are 


FIG.  190.  —  Halimeda  tuna,  a  lime- 
secreting  green  alga  from  the  modern  sea ; 
attached  to  rock. 


Deposits  of  Carbonate  of  Lime  by  Plants      273 


some  which  live  in  fresh  and  mineral-spring  waters.  The  common 
fresh-water  form  is  the  stonewort  or  Chara,  found  in  fresh-water 
lakes  of  limestone  regions.  It  is  a  green  alga,  but  appears  gray 
from  the  amount  of  lime  precipi- 
tated upon  its  surface.  When  dry, 
it  is  white  and  very  brittle.  When 
abundant,  it  forms  deposits  of  marl 
on  the  lake-bottom.  Some  older 
limestones,  such  as  some  of  those  of 
Tertiary  age  which  underlie  the  city 
of  Paris  and  crop  out  some  distance 
from  it,  are  made  of  the  crushed  and 
more  or  less  compacted  limy  fila- 
ments of  this  alga.  Their  origin 


FIG.  191.  —  Corallina,  sp.  A  modern 
lime-secreting  alga,  i,  entire  plant,  natural 
size;  2,  a  small  branch  enlarged. 


FIG.  192  a.  —  Chara  vulgaris, 
Linn.  A  modern  lime-secreting 
alga,  growing  in  fresh  water.  An 
important  marl  and  limestone  for- 
mer. (From  Haas,  Leitfossilien.) 


from  this  plant  is  recognized  by  the  abundance  in  them  of  the  little 
ridged  globular  vessels,  about  the  size  of  a  pin-head,  which  were  the 
spore-bearing  cases  of  the  plant.  These  are  readily  recognized  by 
the  peculiar  spiral  bands  which  surround  them  (Fig.  192  b).  Such 


274 


The  Organic  or  Biogenic  Rocks 


limestones  also  commonly  inclose  the  shells  of  fresh-water  snails 
Lid  other  mollusca. 

Filamentous  algae  are  also  active  in  hot  springs,  separating  out 
the  lime  carbonate,  which  then  builds  up  the  mounds  and  basins 
often  found  around  these,  as  in  the  Yellowstone  region.  It  is  diffi- 


FIG.  192  b.  —  Chara  vulgaris,  L.  A 
recent  calcareous  alga  (fresh  water) ; 
spore-vessel  with  corona.  Enlarged. 
This  is  frequently  found  in  great  num- 
bers in  fresh-water  limestones,  show- 
ing their  mode  of  origin.  (From 
Haas,  Leitfossilien.) 


FIG.  193.  —  A  coccolithophora. 
A  mass  of  coccoliths;  a  marine 
pelagic  plant  of  low  order  covered 
with  calcareous  plates.  (Greatly  en- 
larged. After  Murray.) 


cult  to  determine  in  any  case  what  part  of  the  lime  of  such  hot- 
spring  basins  is  built  up  by  purely  hydrogenic  means  (see<m/e,p.  261), 
and  to  what  extent  algae  are  responsible.  The  oolites  of  Great 
Salt  Lake  and  of  other  highly  saline  waters  have  also  been  regarded 

by  some  authorities  as  largely 
due  to  the  growth  and  lime- 
secreting  habit  of  microscopic 
algae  (Rothpletz). 

Coccoliths,  etc.  —  Finally, 
we  may  mention  certain  float- 
ing organisms  in  the  sea,  gen- 
erally regarded  as  extremely 
low  types  of  plants  called 
Coccolithophores  (Fig.  193), 
which  are  covered  with  an 
armor  of  plates.  These  plates, 
according  to  their  form,  are 
called  coccoliths,  discoliths, 
cyatholiths,  etc.  When  covered 

with  rods  they  are  called  rhabdoliths,  and  form  a  rhabdosphere 
(Fig.  194).  These  structures  are  found  in  calcareous  oozes  which 


FIG.    194.  —  Rhabdosphere.     Much   en- 
larged.    (After  J.  Murray.) 


Foraminifera  and  Foraminiferal  Oozes 


275 


FIG.  195.  —  Globigerina  bulloides. 
A  modern  pelagic  foraminiferan, 
with  expanded  pseudopodia.  (After 
Wyville  Thompson.) 


remain  on  the  floor  of  the  deeper  parts  of  the  oceans  and  which 
are  largely  composed  of  minute  shells  of  Foraminifera  (Globigeriq^ 
ooze),  to  be  described  next. 

FORAMINIFERA  AND  FORAMINIFERAL  OOZES  AND  LIMESTONES 

The  name  Foraminifera  is  given  to  one  of  the  classes  of  the 

•lowest   group  of   animals,   the   Protozoa,  in  which  each   animal 

secretes  a  small  shell  of  carbonate 

of  lime,  to  which  successive  cham- 
bers are  added  as  the  animal 

grows,   all   the    chambers   being 

occupied   by   the    living    animal 

tissue.     Many  of  the  shells  are 

pierced  by  holes,  as  in  the  modern 

Globigerina    (Fig.    195),    through 

which  delicate  threads  of  living 

matter     (pseudopodia)      project, 

which     serve     to     collect    food. 

There    are    many    varieties    of 

these  shell-bearing  Foraminifera  in  the  modern  ocean  (Fig.  196). 

Globigerina  Ooze. 
—  Globigerina  is  the 
most  common  among 
the  floating  organisms 
in  the  upper  layers 
of  the  sea-water.  Its 
shell  consists  of  a 
number  of  chambers 
of  increasing  size, 
the  whole  forming  a 
globular  mass  (Fig. 
196,  2).  Upon  the 
death  of  the  animal 
these  shells  slowly 
sink  to  the  bottom 
(this  requiring  from 
three  to  six  days) ,  and 
if  they  are  not  dis- 
solved again  in  the 
process,  as  happens  in 


FIG.  196.  —  Modern  Foraminiferal  types.  Much 
enlarged,  i,  Orbulina;  2,  Globigerina;  3,  Rotalia; 
4,  Polystomella;  5,  Calcarina.  (After  Neumayer, 
Erdgeschichte ;  from  Ratzel,  Die  Erde.) 


276 


The  Organic  or  Biogenic  Rocks 


very  deep  water,  they  will  accumulate  upon  the  floor  of  the  ocean 
as  a  Globigerina  ooze  (Fig.  197),  made  up  largely  of  this  shell,  but  of 
others  as  well  and  of  coccoliths  and  other  organisms,  including  non- 
calcareous  types.  This  ooze  is  most  abundant  in  depths  between 
2500  and  4500  meters,  the  percentage  of  lime  carbonate  decreasing 

from  70  per  cent  in  the  lesser  to 
50  per  cent  in  the  greater  depths, 
where  more  of  the  shells  have  been 
dissolved.  Nearly  30  per  cent  of 
the  area  of  the  sea-floor  is  covered 
with  this  Globigerina  ooze,  its 
greatest  distribution  being  in  the 
Atlantic  and  its  least  in  the  Pa- 
cific, with  the  Indian  Ocean  in- 
termediate (Fig.  198). 

An  Older  Globigerina  Limestone. 
—  An  example  of  a  limestone  now 
exposed  above  sea-level,  but  formed 
as  a  Globigerina  ooze  in  deep  water, 
is  found  on  the  Island  of  Malta  in 
the  Mediterranean.  The  age  of 
this  rock  is  older  Tertiary  (Oligo- 
cene),  but  nearly  40  per  cent  of 
the  species  whose  shells  compose 
this  rock  still  live  in  the  neighbor- 
ing waters  of  the  Mediterranean. 
Most  of  the  minute  shells  of  which 
the  rock  is  composed  are  those  of  Globigerina.  Scattered  among 
them  are  nodules  of  phosphate  of  lime  similar  to  those  found  in 
the  deeper  ocean  waters  of  to-day.  Altogether,  this  limestone, 
now  a  solid  rock,  represents  admirably  a  former  deep-sea  deposit 
of  Globigerina  ooze,  which,  in  the  course  of  time,  has  solidified  and 
been  lifted  above  sea-level  by  earth-movements  of  the  kind  to  be 
discussed  in  a  subsequent  chapter.  Such  old  Globigerina  limestones 
occur  in  other  districts  as  well. 

Shallow  Water  and  Terrestrial  Foraminiferal  Deposits 

On  tropical  coasts,  especially  those  of  coral  islands,  shells  of  dead 
Foraminifera  often  accumulate  in  large  quantities,  but  these  are 
only  exceptionally  the  shells  of  Globigerina,  other  forms  which  live 


'  FIG.  197.  —  Globigerina  ooze, 
from  the  deep  sea,  enlarged  about 
thirteen  times.  (After  Murray  and 
Renard.)  Besides  the  foraminif- 
eran  shells  there  are  pteropods, 
ostracods,  and  other  organic  struc- 
tures. 


Foraminifera  and  Foraminiferal  Oozes 


277 


3  8  o     .        a  3 


§'-3 


278 


The  Organic  or  Biogenic  Rocks 


in  shallow  water  predominating.     Owing  to  the  lightness  of  these 
shells,  they  are  often  carried  far  inland  by  the  wind,  forming  dunes 


FIG.  199  a.  —  Foraminiferal  shell, 
Miliola  type.  (Spiroloculina  badensis 
d'Orbigny.  Miocene,  Baden.)  Lateral 
and  top  views.  Important  limestone 
builder.  (From  Haas,  Leitfossilien.) 


FIG.  199  b.  —  A  foraminiferal  shell 
of  the  Miliola  type.  (Biloculina  in- 
ornata  d'Orb.  Miocene,  Baden.)  Note 
that  each  new  chamber  covers  all  pre- 
ceding ones.  Two  views  and  section. 
Important  limestone  builder. 


and  even  extended  deposits  chiefly  composed  of  them.  In  the 
western  part  of  India  (Kathiawar  Peninsula)  such  a  limestone, 
called  the  Junagarh  limestone,  from  the  city  of  that  name  which  is 

built  upon  it,  overlies  the 
••*  Deccan  trap  at  a  distance 
of  thirty  miles  from  the  sea. 
It  has  a  thickness  probably 
exceeding  200  feet,  and  its 
cross-bedded  structure  in- 
dicates wind  transportation 
(see  Chapter  XVI).  It  is 
almost  entirely  made  up 
of  foraminiferal  shells  and 
other  lime  particles,  with 
only  from  6.5  to  12.5  per 
cent  of  silicious  material. 
The  chief  foraminiferan 
shell  of  this  rock  is  known 
as  Miliola  (Figs.  199  a,  b), 
on  which  account  the  rock 
is  called  Miliotic  limestone. 
Such  limestones  are  found 
on  the  Arabian  peninsula 
and  elsewhere,  and  in  Tertiary  deposits  as  well. 

Chalk.  —  This  is  a  white,  soft,  friable  rock,  which  consists  of 
minute  shells  and  fragments  of  shells  of  Foraminifera,  of  coccoliths 


FIG.  200.  —  Thin  sections  of  chalk  as 
seen  under  the  microscope.  A,  chalk  from 
Sussex,  England,  enlarged  60  times; 
B,  chalk  from  Farafrah,  Libyan  desert, 
enlarged  66  times;  C,  dried  residue  of 
milky  chalk- water  with  coccoliths,  enlarged 
700  times;  a,  Textularia  globulosa;  b,  Ro- 
talia  (Discorbina)  marginata.  (After 
Zittel.) 


Foraminifera  and  Foraminiferal  Oozes          279 

and  of  other  calcareous  structures,  all  of  them  exceedingly  minute. 
A  properly  prepared  slide  (Fig.  200)  of  the  material,  from  which 
the  finest  dust  has  been  washed  out,  shows,  under  the  microscope, 
a  number  of  scattered  shells  of  Foraminifera,  of  which  a  form  of 
triangular  outline  and  composed  of  a  double  row  of  constantly 
increasing  chambers  is  the  most  abundant.  This  form,  known  as 
Textularia  globulosa  (Fig.  200  a),  lives  to-day  in  the  estuary  of  the 
Dee  River  near  Chester,  England,  and,  like  the  other  common  species 


FIG  201.  —  Wave-cut  cliff  in  chalk  beds  near  Dover,  England.  From  D.  W. 
Johnson's  Shore  Processes.  By  permission  of  John  Wiley  &  Sons.  The  chalk 
is  in  large  part  composed  of  microscopic  shells  ancl  other  calcareous  organic 
structures  as  shown  in  section  under  the  microscope  (Fig.  200).  See  also  the 
view  of  the  Chalk  Cliffs  on  the  French  coast,  Fig.  713. 

of  the  chalk  (Rotalia  marginata,  Fig.  200  b),  is  therefore  a  shallow- 
water  species  and  not  a  surface  floater  (plankton)  as  is  Globigerina. 
From  this  and  other  facts  it  appears  that  the  chalk  is  not  a  deep- 
water  deposit,  as  is  the  Globigerina  ooze,  but  was  formed  in  shallow 
water,  and  the  absence  in  it  of  quartz  sands  and  of  clays  must  be 
accounted  for  by  assuming  that  the  lands  which  could  supply  such 
material  were  too  low  to  affect  the  deposits. 

The  chalk  forms  extensive  beds  over  northern  France  and  Bel- 
gium and  the  south  and  east  of  England.  These  beds  were  once 
continuous  across  the  Channel  and  over  much  of  the  North  Sea, 


280 


The  Organic  or  Biogenic  Rocks 


while  at  the  same  time  they  extended  as  far  to  the  northwest  as 
northern  Ireland.  The  cliffs  which  they  now  present  to  the  sea 
and  inland  are  the  result  of  subsequent  erosion  (Fig.  201 ;  see  also 

Chalk  Cliffs  of  Fecamp  on  the 

French 

flints 

p.   224) 


in 


FIG.  202.  —  Shell  of  a  Nummulite 
cut  transversely  and  in  part  hori- 
zontally. Enlarged.  (Group  of 
Nummulites  lucasana  Defr.  Eocene, 
Bavaria.  From  Haas,  Leitfossilien.) 


coast,   Fig.    713).     The 
the   chalk    (Fig.    162, 
are   the   result   of   sec- 
ondary segregation  of  silica  which 
originally  was  scattered  through 
it,  and  which  originated  from  the 
silicious  skeletons  or  other  parts 
of  marine  organisms  (Radiolaria, 
sponge  spicules,  etc.) .    At  certain 

levels  in  the  chalk,  beds  of  marine  shells  or  other  organisms  are 
found,  indicating  a  temporary  cessation  of  the  chalk-forming  con- 
ditions and  the  inauguration  for  a  time  of  normal  beach  or  shal- 
low sea  deposition.  The  possibility  that  some  chalk  beds  may  be 
formed  by  the  drifting  inland  of  shells  and  fragments  of  lime  by 
wind,  analogous  to  the  Miliolitic  limestone  of  India,  has  been 
suggested. 

Nummulitic  Limestone.  —  Over  large  areas  of  southern  Europe 
and  northern  Africa,  and  in  parts  of  Asia  as  well,  occur  thick  de- 
posits of  limestones  which  are  largely  or  almost  wholly  composed 
of  disk-like  or  button-like  bodies,  varying  in  size  from  that  of  a 
pinhead  to  an  inch  or  more  in  diameter.  From  their  resemblance 
to  coins,  these  bodies  have  long 
been  known  as  Nummulites. 
When  worn,  broken,  or  cut,  they 
show  a  characteristic  internal 
structure,  with  regular  division 
into  chambers  (Fig.  202),  and  they 
are  recognized  as  belonging  to  the 
class  of  Foraminifera,  of  which 
they  constitute  a  remarkable, 
gigantic,  but  now  wholly  extinct 
type.  These  rocks  all  belong  to 
the  early  Tertiary  period,  and  in 
Egypt  they  have  been  quarried  since  the  days  of  Herodotus  and 
before,  and  they  were  extensively  used  in  the  facing  of  the  Great 
Pyramid  (Fig.  28,  p.  76).  A  section  of  such  limestone  from  the 


FIG.  203.  —  A  fragment  of  Num- 
mulitic limestone^irom  the  Pyrenees. 
The  nummulites  are  shown  in  sec- 
tion, and  of  natural  size.  (After 
Haas,  Leitfossilien.) 


Foraminifera  and  Foraminiferal  Oozes 


281 


Pyrenees  is  shown  in  Fig.  203.     These  large  Foraminifera  probably 
lived  in  shallow  water,  as  do  their  nearest  modern  relatives. 

Some  of  the  limestones  of  our 
Gulf  Coast  States  (Vicksburg 
limestone),  and  those  of  the  West 
Indies  and  elsewhere,  are  largely 
composed  of  related  Foraminifera. 
One  of  these,  on  the  island  of 
Cuba,  is  made  up  entirely  of  the 
shells  of  such  a  form  (Orbitoides), 
somewhat  larger  than  a  pinhead ; 
an  enlarged  photographic  view  of 
one  of  these  is  reproduced  in 
Fig.  204.  Similar  limestones  of 
great  thickness  occur  in  Jamaica. 
It  is  not  impossible  that  these 
were  formed  after  the  manner  of 
the  Miliolitic  limestone  of  India  (Junagarh  Limestone)  described 
above,  the  shells  being  blown  inland  from  the  coast.  This  is  sug- 
gested by  the  almost  total  absence  of  other  organisms. 

Fusulina  Limestone.  —  Another  type  of  limestone,  also  formed 
of  large  foraminiferal  shells,  is  found  in  the  upper  Palaeozoic  series 
(Pennsylvanian  and  Permian)  of  western  North  America,  Europe, 


FIG.  204.  —  Orbitoides  (Lepidocy- 
clina)  kempi,  O'Connell.  Enlarge- 
ment of  a  single  shell  in  section. 
Cuba.  (After  M.  O'Connell.) 


FIG.  205.  —  A  polished  piece  of  Fusulina  limestone  of  the  Carbonic.  En- 
larged nine  times.  On  left  sections  cut  the  Fusulina  transversely,  on  the  right 
obliquely  to  the  longitudinal  axis.  (From  Haas,  Leitfossilien.) 


282 


The  Organic  or  Biogenic  Rocks 


and  Asia.  An  enlarged  section  of  such  a  limestone  is  shown  in 
Fig.  205.  These  foraminiferal  shells  frequently  resemble  a  kernel 
of  rice ;  they  are  elongate  and  spindle-shaped  (Fusulina,  Fig.  206) 


FIG.  206.  —  Fusidina  cylindrica,  a  typical  foraminiferal  shell  forming  rocks 
in  the  later  Palaeozoic.  A  group  natural  size,  and  a  single  shell  much  en- 
larged and  partly  sectioned  to  show  interior.  (After  Kayser.) 

or  more  or  less  like  a  football  in  form  (Schwagerina,  Fig.  207),  but 
seldom  more  than  a  fraction  of  an  inch  in  greatest  diameter.  They 
are  restricted  to  that  part  of  the  geological  series,  becoming  extinct 
with  the  close  of  the  Palaeozoic  era,  though  a  form  of  similar  appear- 


A  B 

FIG.  207. — Schwagerina  verbecki,  Geinitz.     A,  diagrammatic  view;   B,  plan  of 
structure ;  a,  natural  size.     (After  Schwager.) 

ance,  but  very  different  structure,  occurs  in  Tertiary  rocks.  In 
general,  the  Fusulina  is  like  a  Nummulite  with  the  axis  of  coiling 
greatly  elongated.  Since  the  Fusulinae  indicate  the  horizon  of 
the  oldest  extensive  coal  deposit,  their  recognition  is  of  importance. 
This  will  be  more  fully  discussed  in  a  later  chapter. 

CORALS  AND  RELATED  REEF-BUILDING  ANIMALS 

Corals  and  Coral  Polyps.  — The  name  coral  is  applied  to  the  hard 
structures  (usually  of  carbonate  of  lime),  built  by  delicate  and 


Corals  and  Related  Reef-building  Animals      283 


as  a  rule  small  animals,  which  live  only  in  normal  sea- water  and 
mostly  in  regions  of  tropical  or  subtropical  climates.  The  animals 
are  called  polyps,  and  they  are  more  or  less  cylindrical,  fleshy,  but 
very  delicate  organisms,  closed  at  the  bottom,  but  having  a  central 
opening,  the  mouth,  at  the  top,  around  which  there  are  one  or  more 
rings  of  tentacles.  In  the 
simpler  forms,  which  are 
known  as  hydroid  polyps,  the 
entire  interior  of  the  body- 
xylinder  is  hollow  and  con- 
stitutes the  stomach ;  but  in 
the  coral  polyps  proper  (Fig. 
208),  the  interior  is  variously 
modified,  chief  among  the 
modifications  being  a  series 
of  fleshy  plates  which  extend 
from  the  bottom  to  the  top 
of  the  cylinder  and  divide 
the  inner  cavity  into  a  num- 
ber of  radial  chambers. 

Not  all   polyps  secrete  a 
hard  structure,  but  those  that 

FIG.  208.  —  Vertical  section  through  a 
polyp  greatly  enlarged  (Astroides  caly- 
cularis).  (After Lacaze-Duthiers.)  Mouth 
surrounded  by  tentacles  and  beneath  it 
the  "stomatodaeum. "  The  fleshy  mesen- 
teries are  shown,  and  the  calcareous 
septa  which  lie  between  them  in  position. 
In  the  center  of  the  bottom  is  the 
columella  arising  from  the  calcareous 
basal  plate  (Sk).  (From  Hass,  Leit- 
fossilien.} 


do  so  precipitate  the  lime 
from  the  sea-water  in  and 
upon  the  outer  layers  of  their 
body,  especially  at  the  base 
of  the  cylinder.  These  hard 
structures  begin  as  needles  or 
spicules  of  lime,  which  in  some 
groups,  the  gorgonias,  seldom 


or  never  unite  into  a  solid 
mass,  but  remain  scattered  in  the  fleshy  parts  of  the  body  and  are 
left  as  lime  needles  upon  the  decay  of  the  flesh.  In  other  groups, 
however,  these  needles  are  so  numerous  and  crowded  that  they 
become  welded  into  a  solid,  more  or  less  porous,  stony  mass,  the 
true  coral.  Of  this,  two  types  may  be  recognized,  the  solid  rod 
and  the  star  coral.  The  first  of  these  is  built  by  a  colony  of 
polyps  which  are  bound  together  by  a  solid  fleshy  substance  into  a 
cylinder  of  living  matter,  over  the  surface  of  which  the  individual 
polyps  are  scattered.  The  hollow  central  axis  of  the  mass  is 


284  The  Organic  or  Biogenic  Rocks 

filled  by  the  carbonate  of  lime  which  this  fleshy  mass  separates 
from  the  sea-water,  and  when  this  central  calcareous  rod  is  stripped 
of  the  surrounding  fleshy  substance,  it  appears  as  a  much  branch- 
ing solid  structure  of  carbonate  of  lime,  colored  a  deep  pink  or 
red  in  the  most  familiar  types.  This  is  the  precious  coral  of  com- 
merce, from  which  coral  beads  and  ornaments  are  cut.  In  the 
more  common,  but  less  familiar  gorgonias,  the  sea-fans  and  sea- 
whips,  so  abundant  on  most  coral  reefs,  this  central  axis  is  horny 
instead  of  calcareous,  but  is  otherwise  much  of  the  same  charac- 
ter. The  lime  secreted  by  the  gorgonias,  as  already  stated,  is  de- 


FIG.  209.  —  A  simple  cup-coral  (Caryophyllia  cyathus),  attached  to  the 
sea-bottom.  (After  Dana,  Corals  and  Coral  Islands,  by  permission  of  Dodd, 
Mead  &  Co.) 

posited  as  lime  needles  or  spicules  in  the  fleshy  mass  which  sur- 
rounds and  builds  the  horny  axis.  These  spicules  are  sometimes 
of  importance  in  the  formation  of  modern  limestones.  . 

Far  more  abundant  than  the  group  just  described,  and  of  more 
importance  in  the  formation  of  organic  limestones,  are  the  star 
corals.  These  are  so  called  because  they  show  upon  their  surfaces 
one  or  many,  generally  more  or  less  depressed,  circular,  oval,  or 
polygonal  areas  or  cups,  each  of  which  contains  a  radial  series  of 
vertical  plates,  which  converge  toward  the  center  of  the  cup  and  give 
the  appearance  of  rays  from  a  central  star.  These  rays  are  called 
the  septa,  and  they  correspond  to  the  radial  fleshy  plates  within  the 
body  of  the  coral  polyps  by  the  base  of  which  they  are  deposited. 


Corals  and  Related  Reef -building  Animals      285 

We  may  recognize  simple  corals  with  only  one  septa-bearing  cup, 
which  then  is  either  circular  or  oval  (Fig.  209),  and  compound  corals, 
in  which  many  such  cups  occur  side  by  side,  separated  by  inter- 
vening limestone  material,  when  their  outline  is  circular  or  oval, 


FIG.  210.  —  Compound  coral  head  (Astrcea  pallida),  with  polyps  partly  ex- 
panded and  partly  contracted.  The  expanded  polyps  show  the  tentacles 
which  surround  the  mouth ;  the  contracted  polyps  show  the  polygonal  outline 
from  crowding.  (After  Dana,  Corals  and  Coral  Islands,  by  permission  of  Dodd, 
Mead  &  Co.) 

(Fig.  210)  or  closely  crowded,  when  they  assume  more  or  less  polyg- 
onal forms  (Fig.  211).  But  always  the  septa  radiate  from  the 
center  of  each  cup  to  its  margins.  Sometimes  the  cups  are  so  mi- 
nute and  separated  by  such  broad  intervals  of  spongy  lime  matter, 
that  the  mass  has  a  more  or  less  homogeneous  appearance,  the  cups 


FIG.  211.  —  A  compound  coral  head  with  crowded  prismatic  corallites 
(Acervularia  ananas}.  The  specimen  represents  a  worn  pebble,  formerly  a 
part  of  a  larger  head.  (From  Kayser.) 

being  recognized  only  on  careful  examination,  as  they  are  small. 
Often  they  form  closely  crowded  tubes  (Madrepora,  Fig.  212);  at 
other  times  a  massive  branch  is  covered  with  a  closely-set  series  of 
minute  cups  (Forties,  Fig.  213).  In  still  other  coral  heads  the  cups 


286 


The  Organic  or  Biogenic  Rocks 


or  calices  are  confluent,  producing   sinuous  valleys  and   ridges 
(Fig.  214). 

Some  of  the  ancient  corals  which  were  important  as  limestone 
makers  consist  of  a  series  of  tubes  arranged  either  in  a  loose,  more 


FIG.  212.  — Reef-coral,  Madrepora  palmata,  natural  size,  and  a  and  b  slightly 
enlarged  calices.     (After  A.  Agassiz ;   from  Ratzel,  Die  Erde.) 

or  less  chain-like  series  (chain  coral  or  Holy  sites,  Fig.  215)  or  closely 
crowded  and  taking  on  a  columnar  prismatic  form  from  crowding 
(honeycomb-coral  or  Favosites,  Fig.  216,  and  Columnaria).  In 
these  corals  the  septa  are  generally  very  short,  or  they  may  be 


Corals  and  Related  Reef -building  Animals      287 

represented  by  vertical  rows  of  spines,  or  again  they  may  be  absent 
altogether.     Instead,  the  tubes  are  divided  by  numerous  horizon- 


FIG.  213. — A  massive  branching  coral  (Porites  mordax)  with  very  small  calices; 
an  important  reef -builder.     (After  A.  Agassiz ;   from  Ratzel,  Die  Erde.) 

tal  partitions  which  are  often  so  closely  crowded  as  to  give  the  tube, 
when  broken  lengthwise,  a  finely  cellular  structure  (Fig.  216). 


FIG.  214.  —  Two  small  heads  of  a  brain-coral.  (Maandrina.)  a,  with 
soft  parts;  b,  corallum.  Slightly  reduced.  (After  Brehm;  from  Ratzel, 
Die  Erde.) 


288 


The  Organic  or  Biogenic  Rocks 


FIG.  215.  — The  chain  coral.  (Haly- 
sites  catemdaria,  E.  and  H.)  Silurian ; 
with  enlargement  of  a  few  corallites. 
This  is  an  important  index  fossil  of 
the  Silurian,  and  an  important  lime- 
stone builder  as  well.  (From  Haas, 
Die  Leitfossilien.) 


FIG.  216. — A  characteristic  a-sep- 
tate  compound  coral.  (Fawsites  got- 
landica.)  Silurian.  The  columns  are 
prismatic  and  their  walls  pierced  by 
regular  pores.  Internally  the  tubes 
are  divided  by  horizontal  plates  or 
tabulae.  This  is  an  important  lime- 
stone builder  in  the  Palaeozoic.  (From 
Haas,  Leitfossilien.) 


FIG.  217.  —  A  modern  hydrocoralline  (Millepora  alcicornis).  Important  as 
a  reef-builder.  (From  Dana,  Corals  and  Coral  Islands,  by  permission  of  Dodd, 
Mead  &  Co.) 


Corals  and  Related  Reef-building  Animals      289 

Hydroid  Polyps  and  Hydrocorallines.  —  Finally  the  hydroid 
polyps  which,  it  will  be  remembered,  have  no  internal  fleshy  radi- 
ating plates,  sometimes  build  calcareous  coral-like  structures  in 
which,  however,  the  cups  are  merely  holes  in  the  moreX)r  less  solid- 
appearing  limestone  mass  (Millepora,  Fig.  217)  which  forms  the 
structure  secreted  by  them  and  to  which  the  name  hydrocoralline 
is  applied. 

Such  hydrocorallines  often  form  important  and  extensive 
portions  of  coral  reefs,  the  form  known  as  Millepora  (referring 


FIG.  218.  —  Fragments  of  large  masses  of  stromatoporoids,  which  were  im- 
portant reef-builders  in  the  Palaeozoic.  A,  Stromatoporella  tuberculata,  a 
weathered  fragment  showing  the  hummocky  surface,  and  the  successive  layers 
(Devonian) ;  B,  Stromatopora  antiqua,  a  transverse  polished  section  of  a 
fragment  snowing  the  coarse  concentric  layers  (Silurian).  (After  Nicholson; 
from  Grabau  and  Shimer,  North  American  Index  Fossils.} 

to  the  thousands  of  pores  on  the  surface,  i.e.  the  cups) 
abounding  on  certain  modern  coral  reefs,  while  another,  the  Stroma- 
topora, sometimes  makes  up  the  main  portion  of  ancient  (Palaeo- 
zoic) coral  reefs,  growing  in  masses  up  to  ten  feet  in  diameter.  It  is 
readily  recognized  by  the  concentric  arrangement  of  the  succes- 
sive layers  of  which  this  mass  is  composed  (Fig.  218).  Each  layer 
will,  on  microscopic  examination,  show  a  very  definite  structure 
such  as  is  found  only  in  limestone  deposits  of  organic  origin.  With- 
out careful  examination  of  details,  however,  the  student  will  not 
be  able  to  distinguish  the  numerous  kinds  of  Stromatoporas  from 
one  another,  nor  will  he  be  able  readily  to  distinguish  them  from 
ancient  limestone  masses  of  similar  concentric  structure  built  by 
marine  or  even  by  fresh-water  plants  of  low  order  (Algae,  e.g.  Cryp- 
tozoon,  etc.). 


290  The  Organic  or  Biogenic  Rocks 


CHARACTERS  AND  TYPES  OF  MODERN  CORAL  REEFS 

Corals  commonly  grow  associated  in  regions  of  the  sea  where 
minute  food  particles  are  abundant,  where  the  mean  temperature 
of  the  coldest  months  does  not  usually  fall  below  21  degrees  C., 
or  the  minimum  annual  temperature  below  18°  C.  and  where  a  favor- 
able hard  bottom,  free  from  silt,  exists  for  their  attachment.  For 
it  must  be  noted  that  the  coral  polyps  are  not  free-moving  or  float- 
ing organisms  (except  in  their  larval  stages),  but  attach  themselves 
to  the  sea-bottom  and  lead  a  sedentary  existence.  By  such  asso- 
ciation in  growth  of  corals,  together  with  other  lime-secreting  or- 
ganisms, a  reef  is  built  up  which  rises  toward  the  level  of  the  sea, 
and  may  grow  so  high  that  it  is  exposed  at  very  low  tides  for  a  short 
period  of  time.  Coral  reefs  must  be  distinguished  from  coral  islands 
(see  Fig.  220),  which  represent  a  stage  subsequent  to  the  reef  when, 
by  wave  action,  the  broken-off  dead  coral-masses  and  coral-sands 
are  heaped  up  to  such  an  extent  that  they  permanently  project 
above  the  water.  Reefs,  on  the  other  hand,  are  always  submerged 
except  for  the  short  period  of  lowest  tides  already  referred  to.  Mod- 
ern coral  reefs  are  chiefly  confined  to  the  region  limited  by  the 
28th  degree  north  and  south  latitudes,  the  Bermuda  Islands,  bathed 
by  the  warm  Gulf  stream,  being  the^  chief  exception  to  this,  lying 
in  latitude  32°  N.  This  distribution  is  partly  due  to  the  inability 
of  polyps  to  separate  out  much  lime  from  sea- water  in  colder  regions, 
and  also  because  whenever  ice  forms  in  winter  the  coral  polyps, 
always  growing  near  the  surface  of  the  sea,  are  readily  destroyed 
by  such  ice.  On  this  account  we  may  confidently  assume  that 
ancient  coral  reefs  required  similar  warm  temperatures  for  their 
formation,  and  if  we  find  that  rocks  now  exposed  in  the  Arctic  re- 
gions are  formed  from  ancient  coral  reefs,  as  are  those  on  the  North 
Siberian  islands,  we  must  assume  that  at  the  time  of  their  forma- 
tion this  region  had  a  more  tropical  climate. 

In  the  second  place,  reef-building  corals  flourish  best  in  shallow 
water,  usually  in  water  not  over  20  to  25  fathoms  (120  to  150  feet) 
in  depth,1  though  some  occur  at  greater  depths.  Many  of  them 
flourish  so  near  the  surface  of  the  sea  that  they  are  exposed  at  low 
tide,  as  is  so  finely  shown  in  the  wonderful  series  of  photographs 
obtained  by  Saville-Kent  from  the  Great  Barrier  Reef  of  Australia 

1  Vaughan  considers  45  meters  the  maximum. 


Characters  and  Types  of  Modern  Coral  Reefs     291 

(Fig.  2I9).1  Frequent  agitation  of  the  sea-water  is  necessary  to 
the  active  growth  of  coral  polyps,  and  as  a  rule,  strong  light  is  re- 
quired. The  salinity  of  the  water  must  not,  as  a  rule,  fall  much 
below  27  per  mille,  nor  rise  much  above  38  per  mille,  though  coral 
polyps  have  been  found  to  flourish  in  very  brackish  and  very 
saline  waters. 

Reefs  vary  greatly  in  the  types  of  organisms  which  built  them. 
In  nearly  all  of  them  calcareous  seaweeds,  or  nullipores,  play  an 
important  part,  and  some  reefs  are  largely  composed  of  them.  In 


FIG.  219.  —  Portion  of  the  Great  Barrier  Reef  of  Australia  at  very  low  tide, 
showing  the  living  coral  masses  growing  close  together,  and  withstanding  the 
periodic  exposure.  (After  Saville-Kent.) 

others,  hydrocorallines  (Milleporq,  Fig.  217)  abound,  while  in  still 
others  the  gorgonias  form  an  important,  if  not  dominant  element. 
Star-corals  are  always  present,  and  in  many  cases  predominate, 
comprising  both  the  branching  types  such  as  the  staghorn  coral 
(Madrepora,  Fig.  212),  the  fringe  coral  (Forties,  Fig.  213),  etc.,  and 
the  massive  or  head  types,  such  as  the  star  corals  proper  (Astraa, 
etc.,  Fig.  210),  the  brain  coral  (Maandrina,  Fig.  214),  and  many 
others.  Besides  these  reef-builders  proper,  there  are  many  other 
animals  which  live  in  and  about  the  coral  branches,  and  many  of 
them  have  hard  shells  or  other  structures  which,  on  the  death  of 
their  possessors,  add  calcareous  material  to  the  growing  mass. 

1  The  student  should  examine  the  photographs  published  in  Saville-Kent's  book  on 
the  Great  Barrier  Reef. 


292  The  Organic  or  Biogenic  Rocks 

According  to  their  location,  we  may  distinguish  two  groups  of 
reefs:  i,  the  oceanic,  and  2,  the  epicontinental.  The  first  have  no 
direct  relationship  to  the  continents,  but  grow  around  islands  in 
the  open  oceans  or  form  separate  rings  of  coral  islands  or  atolls. 
The  second  group  is  marginal  to  the  land,  being  built  upon  the 
continental  platform  or  in  seas  which  indent  the  continents.  Be- 
tween the  two  there  is  a  gradational  series,  but  on  the  whole  they 
are  quite  distinct. 

Oceanic  Coral  Reefs 

Of  these,  three  types  are  recognized;  namely,  fringing  reefs, 
barrier  reefs,  and  atolls. 

The  Fringing  Reef.  —  This  grows  close  around  volcanic  or  other 
islands,  from  the  submerged  slopes  of  which  it  rises,  and  is 
separated  from  the  island  it  fringes  by  a  very  narrow  channel. 

The  Barrier  Reef.  —  This  grows  at  a  distance  of  several  miles 
from  the  shores  of  the  oceanic  island,  thus  leaving  a  broad  strip  or 
channel  of  water  between  it  and  the  island.  This  water  may  be 
from  one  or  two  to  as  much  as  30  or  40  meters  in  depth.  The  barrier 
reef  of  New  Caledonia  in  the  Pacific  is  400  miles  long  and  about 
ten  miles  distant  from  the  shore.  The  outer  or  seaward  margin 
of  such  a  barrier  slopes  off  into  much  deeper  water  than  is  the  case 
with  that  of  the  fringing  reef,  and  the  inner  channel,  besides  being 
broader,  is  also  much  deeper  than  that  of  the  fringing  reef.  This 
channel  is  connected  with  the  outer  ocean  by  cross  channels  cut 
through  the  reef,  and  these  are  kept  open  by  the  ebb  and  flow  of 
the  tide. 

The  Atoll  (Fig.  220).  — This  consists  merely  of  a  ring  of  coral- 
reef  islands  in  the  open  ocean,  with  no  land  in  the  center  of  the  ring, 
but  instead  a  shallow  lagoon  of  quiet  water.  This  lagoon  is  con- 
nected with  the  open  sea  by  cross  channels,  which  are  located  on  the 
leeward  side  of  the  atoll,  and  so  generally  form  a  protected  entrance 
to  a  quiet  inner  harbor.  The  water  around  the  atoll  is  generally 
very  deep,  and  the  outer  slopes  of  the  reefs  are  steep.  At  the  Kokos 
Keeling  atoll,  in  the  eastern  part  of  the  Indian  Ocean,  for  example, 
the  ocean  has  a  depth  of  1200  fathoms  at  a  distance  of  only  2200 
yards  from  the  edge  of  the  reef.  The  lagoon,  on  the  other  hand, 
is  generally  less  than  50  fathoms  in  depth,  and  in  some  atolls,  as 
in  that  of  the  Kokos  Keeling  group,  it  is  only  from  2  to  7  fathoms 
deep. 


Characters  and  Types  of  Modern  Coral  Reefs     293 

Organisms  of  Oceanic  Reefs.  —  Different  organisms  are  gen- 
erally found  to  grow  in  the  lagoon  and  upon  the  outer  slope  of 
the  reef.  The  latter,  bathed  by  the  cool,  pure  sea-water,  rich  in 
food  particles,  is  the  region  of  active  coral  growth,  though  a  fringe 
of  the  nullipore,  Lithothamnium  (Fig.  189),  generally  occurs  here. 
In  the  lagoon,  lime-secreting  sea- weeds  (Halimeda,  etc.,  Fig.  190) 


FIG.  2  20.  —  Whitsunday  Island,  a  typical  Atoll  in  the  Pacific.     (After  Guyot.) 

are  generally  abundant.  Corals  also  grow  there,  being  commonly 
of  the  more  delicate  branching  forms,  though  others  more  common 
on  the  outside  of  the  atoll  are  also  represented.  Some  lagoons  be- 
come gradually  filled  up  by  the  growth  of  these  organisms,  while 
others  seem  to  retain  their  depth  and  perhaps  increase  it  and  widen 
the  lagoon  by  solution. 

Theories  of  the  Origin  of  Oceanic  Reefs 

No  special  theory  is  needed  to  account  for  the  formation  of  a 
fringing  reef,  for  it  can  be  readily  understood  that  coral  polyps  and 
other  lime-secreting  organisms  which  attach  themselves  to  the  sub- 
merged slopes  of  an  oceanic  island  will  in  time  build  up  a  reef  which 
fringes  the  coast  of  that  island  and  rises  nearly  to  sea-level,  followed 
in  many  cases  by  the  formation  of  islands  upon  it.  The  case  is  dif- 
ferent, however,  with  the  barrier  reefs  and  the  atolls,  for  here  the 
lagoons  must  be  explained,  as  well  as  the  fact,  that  the  reefs  seem  to 
arise  from  depths  too  great  for  normal  reef-building  corals  to  grow. 


294  The  Organic  or  Biogenic  Rocks 

Several  theories  have  been  proposed  for  the  explanation  of  such 
conditions. 

The  Subsidence  Theory.  —  This  theory  was  first  proposed  by 
Charles  Darwin  from  his  study  of  oceanic  coral  islands.  It  was 
later  amplified  and  elaborated  by  the  American  geologist,  James  D. 
Dana,  and  has  received  its  most  recent  support  from  the  studies  of 
the  American  physiographer,  William  Morris  Davis.  In  general, 
this  theory  postulates  that  regions  of  barrier  reef  and  atoll  forma- 
tion, are  regions  of  subsidence.  Beginning  as  fringing  reefs  in  moder- 
ate depths  upon  the  submerged  slopes  of  oceanic  islands  which  are 


FIG.  221.  —  Diagram  illustrating  the  conversion  of  a  fringing  into  a  barrier 
reef  by  subsidence.'  (After  Darwin;  from  Vaughan.)  AA,  Outer  edge  of  the 
fringing  reef  at  the  level  of  the  sea ;  BB,  shore  line  of  the  island  at  this  stage ; 
A' A',  outer  edge  of  the  reef  after  its  upward  growth  during  a  period  of  sub- 
sidence and  the  formation  of  a  new  sea-level  indicated  by  the  dotted  line; 
B'B',  the  new  shore  of  the  encircled  island;  CC,  the  lagoon-channel  between 
the  reef  and  the  island. 

slowly  sinking,  the  reefs  grow  upward  and  to  some  extent  outward, 
the  region  of  the  greatest  growth  being  the  seaward  margin  of  the 
reefs.  Subsidence  diminishes  the  diameter  of  the  island,  and  a  re- 
treat of  its  shore-line  from  the  original  contact  with  the  reef  takes 
place,  so  that  a  constantly  widening  lagoon  is  produced  between 
the  reef  and  the  island  shores  (Fig.  221).  As  Davis  has  pointed 
out,  were  it  not  for  comparatively  rapid  subsidence,  the  mechan- 
ical debris  washed  from  the  island  itself,  during  its  destruction  by 
the  weather,  would  fill  up  the  lagoon,  whereas,  in  the  most  typical 
examples  of  such  lagoons,  the  amount  of  visible  mechanical  sediment 
from  the  island  is  far  less  than  would  be  expected  from  the 
extent  to  which  the  island  has  suffered  erosion.  Most  of  this  sedi- 
ment is  submerged  beneath  the  waters  of  the  constantly  widening 
lagoon.  By  continued  sinking  of  the  island  and  the  corresponding 
upward  building  of  the  reef,  the  circular  lagoon  increases  in  size, 
until,  when  the  last  peak  of  the  island  has  disappeared,  the  ring  of 
water  becomes  closed  into  a  central  continuous  lagoon,  bounded 


Characters  and  Types  of  Modern  Coral  Reefs     295 

only  by  the  circular  reef  upon  which  islands  are  built  by  waves  and 
wind,  and  the  atoll  is  complete  (Fig.  222). 

Such  a  theory  of  origin  postulates  the  existence,  among  the  oceanic 
reef-surrounded  islands,  of  examples  of  all  stages  in  this  process, 
from  those  in  which  the  fringing  reef  has  only  recently  been  built, 
to  those  in  which  the  reef  forms  a  well-defined  barrier  at  a  distance 
from  the  coast,  with  central  islands  of  old  rock  of  all  sizes,  from 
those  of  great  extent  and  height  to  mere  rocky  peaks  at  the  center 
of  an  almost  perfect  atoll.  For  it  is  evident  that  all  the  islands  of 


FIG.  222.  —  Diagram  illustrating  the  conversion  of  a  barrier  reef  into  an  atoll. 
(After  Darwin ;  from  Vaughan.)  The  barrier  reef  is  closely  shaded,  the  letter- 
ing corresponding  to  that  of  Fig.  221.  The  dotted  lines  represent  the  upward 
growth  of  the  reef  as  the  island  subsides,  and  the  new  sea-level  is  represented  by 
the  dotted  horizontal  line.  A" A",  the  outer  edge  of  the  reef  which  forms  the 
atoll;  Cf,  the  lagoon  of  the  atoll,  the  depth  of  which  on  this  scale  is  exag- 
gerated, as  is  also  that  of  the  lagoon  channel  C. 

an  area  did  not  have  the  same  height  or  slope  of  surface  at  the  time 
of  the  formation  of  the  initial  fringing  reef.  Therefore,  even  if 
the  subsidence  were  uniform  over  wide  areas,  which  need  not  at 
all  be  the  case,  some  islands  would  disappear  earlier  than  others, 
and  some  lagoons  would  widen  more  rapidly  than  others,  because 
of  the  gentler  slope  of  the  surface  of  the  rocky  mass.  This  is  illus- 
trated in  the  following  diagram  (Fig.  223).  Similar  conditions 
would  be  produced  in  the  case  of  islands  of  the  same  height  and 
slope,  but  located  in  regions  which  were  undergoing  subsidence 
at  different  rates.  Moreover,  this  theory  requires  that  dead  reef- 
building  corals  should  be  found  in  the  position  of  growth  at  depths 
vastly  greater  than  that  normal  for  the  growth  of  corals,  for  it 
is  evident  that  as  subsidence  goes  on,  the  older  parts  of  the  reef, 
formed  in  waters  of  25  or  50  fathoms,  will  be  carried  downward 
with  the  sinking  of  the  islands  which  support  these  reefs.  These 


296 


The  Organic  or  Biogenic  Rocks 


conditions  are  satisfied,  for  not  only  are  all  gradations  from  fring- 
ing reefs  to  perfect  atolls  found  among  the  oceanic  islands  and  reefs, 
but  a  boring  in  one  of  the  atolls,  that  of  Funafuti  in  the  Ellice  Island 
group  in  the  western  Pacific,  has  shown  the  presence  of  these  corals 
at  a  depth  of  over  a  thousand  feet. 

The  chief  objection  urged  against  this  theory  of  origin  is  the 
necessarily  widespread  subsidence  of  the  ocean  bottoms  to  carry 
down  so  many  of  the  islands  in  such  widely  distant  regions.  Fur- 


FIG.  223.  —  Diagram  of  two  islands,  showing  varying  amounts  of  submer- 
gence with  the  same  rate  of  subsidence,  due  to  variation  in  slope.  The  reef- 
deposits  formed  on  the  submerging  slopes  are  shown  in  black.  Note  the  more 
rapid  increase  in  the  .width  of  the  lagoon  in  the  island  on  the  right. 

ther,  it  is  pointed  out  that  in  some  of  these  island  groups  where 
barrier  reefs  and  atolls  would  indicate  subsidence,  there  is  evidence 
of  actual  elevation  at  some  points  where  the  old  coral  reefs  have 
been  raised  to  a  greater  or  less  extent  above  the  sea,  so  that  in  some 
cases  the  actual  foundation  upon  which  the  reef  was  built  has  be- 
come exposed. 

The  Theory  of  Stationary  Levels  and  Upbuilding  of  Reefs.  - 
The  English  oceanographer,  Sir  John  Murray,  following  an  older 
suggestion  of  the  poet-naturalist,  Adelbert  von  Chamisso,  and  of 
the  naturalist,  Carl  Semper,  proposed  a  different  explanation  of 
the  phenomena,  and  this  has  been  further  amplified  and  extended 
by  the  American  oceanographer,  Alexander  Agassiz.  These  inves- 
tigators started  with  the  fact  that  corals  will  begin  to  grow  wherever 
a  submerged  ridge  rises  to  within  the  proper  distance  of  the  sea- 
level  (25  to  50  fathoms),  whether  any  portion  of  this  is  exposed  above 
the  sea  as  an  old  island  or  not.  Such  a  ridge  may  be  of  volcanic 
origin,  it  may  be  an  old  island  which  has  been  worn  down  by  the 
waves  until  it  has  largely  or  entirely  disappeared,  or  it  may  be  a 
submarine  bank  built  up  by  the  accumulation  of  shells  and  other 
organic  structures  until  it  has  reached  the  proper  elevation.  When 
the  coral  masses  growing  upon  such  a  foundation  approach  the 
surface  of  the  sea,  the  centrally  located  individuals  will  die  for  lack 
of  food  and  proper  water  conditions,  while  on  the  margin  of  the 


Characters  and  Types  of  Modern  Coral  Reefs     297 

mass,  where  the  open  sea-water  bathes  the  corals,  growth  is  vigor- 
ous. Fragments  broken  by  the  waves  from  this  margin  will  roll 
down  the  seaward  slope,  and  in  time  build  the  submerged  platform 
outward,  after  which  the  coral  polyps,  etc.,  will  take  possession, 
and  the  margin  of  the  reef  is  widened  by  outward  growth.  Mean- 
while the  dead  coral  masses  on  the  inside  will  undergo  solution  by 
the  waters  rendered  acid  by  the  decaying  organic  matter,  and  a 
lagoon  is  dissolved  out,  the  latter  also  increasing  in  diameter  as 
the  coral  ring  spreads  on  the  outside.  In  this  manner  an  atoll  of 
any  size  may  be  produced  from  an  originally  solid  reef  of  smaller 
size,  while  a  fringing  reef  around  an  island  would  gradually  move 
away  from  it  by  outward  growth  and  the  space  between  it  and  the 
island  would  widen  by  solution  and  scouring  out  of  the  lagoon 
channel. 

Among  the  objections  to  this  theory  may  be  mentioned  the  fact 
that  many  lagoons  are  very  wide,  up  to  thirty  miles  or  more,  and 
also  very  deep,  some  having  a  depth  of  200  feet.  This  would  re- 
quire an  enormous  amount  of  solution  and  removal  of  lime  carbon- 
ate from  the  lagoon,  were  this  produced  from  originally  solid  reef 
masses  inside  of  the  growing  ring.  Moreover,  the  lagoons  of  atolls 
are  generally  becoming  filled  by  the  growth  of  lime-secreting  sea- 
weeds, by  the  accumulation  of  foraminiferal  shells  and  of  other  or- 
ganic products,  and  by  precipitation  of  lime  through  bacteria  and 
other  agents.  While  this  theory  may  explain  certain  cases,  it  does 
not  seem  applicable  to  the  problem  as  a  whole. 

Theory  of  the  Rise  of  the  Ocean  Surface. — The  German  physi- 
ographer, Albrecht  Penck,  has  proposed  the  theory  of  a  rising  sea- 
Igvel  which  has  been  more  fully  amplified  by  the  American  geologist, 
R.  A.  Daly,  though  suggestions  along  this  same  line  were  made  pre- 
viously by  others  (T.  Belt,  1874;  W.  Upham,  1878,  etc.).  It  is 
known  that  in  a  period  preceding  that  in  which  the  modern  barrier 
reefs  and  atolls  were  formed,  an  enormous  ice-cap  covered  the 
northern  portions  of  America  and  of  Europe  and  a  similar  ice-cap 
covered  the  Antarctic  regions.  Such  ice  masses  would  exert  an 
attraction  upon  the  waters  of  the  ocean  which  would,  in  conse- 
quence, flow  toward  them,  lowering  the  sea-level  in  the  equatorial 
regions  throughout  the  world  to  a  corresponding  degree.  The  nor- 
mal depth  at  which  coral  polyps  could  flourish  at  that  time  in  the 
equatorial  regions  could,  therefore,  be  much  below  that  possible 
for  them  at  the  present  time.  As  the  ice  melted  the  sea-water  would 


298  The  Organic  or  Biogenic  Rocks 

be  returned,  raising  the  level,  and  the  additional  water  from  the 
melting  ice  would  increase  the  volume.  During  this  continued 
rising  of  the  sea-level  the  coral  reefs,  beginning  as  fringes  around 
the  islands,  would  grow  upwards,  while  the  central  islands  would 
become  more  and  more  submerged  and  some  would  finally  disappear. 
Thus  barrier  reefs  and  atolls  would  be  formed,  the  character  and 
size  of  which  would  depend  on  the  area  and  height  of  the  original 
island  or  submerged  platform.  According  to  Daly's  estimates,  the 
amount  of  lowering  of  the  sea-level  at  the  beginning  was  from  200 
to  230  feet,  and  this  would  also  be  the  extent  of  the  subsequent  rise 
of  the  water-level.  Hence  the  maximum  depth  of  lagoons  would  be 
indicated  by  these  figures,  while  the  thickness  of  the  entire  reefs 
built  during  that  time  would  range  from  that  amount  to  that  plus 
150  to  300  feet,  the  original  depth  at  which  the  corals  began  to 
grow.  These  requirements  are  borne  out  by  many  facts  known 
concerning  the  reefs,  but  they  do  not  apparently  account  for  all  of 
them. 

Complex  Origin  of  Reefs.  —  It  seems  likely  that  most  or  all  of 
the  factors  here  indicated  are  operative  in  the  formation  of  coral 
reefs,  and  that  some  barrier  reefs  and  atolls  may  originate  in  one 
way  and  others  in  another.  Each  group  must  be  investigated  by 
itself,  and  probably  no  single  theory  accounts  for  all  the  phenomena 
observed  in  such  reefs  in  different  parts  of  the  oceans. 

Epicontinental  Reefs 

Reefs  which  form  in  shallow  water,  either  on  the  continental 
platform  of  the  open  ocean,  or  in  the  more  or  less  enclosed  bodies 
of  water  which  form  indentations  into  the  land,  are  classed  as  epj^ 
continental  reefs,  or  reefs  built  upon  the  submerged  portions  of 
the  continents.  The  best-known  modern  examples  of  reefs  of  this 
type  are  the  Great  Barrier  Reef  of  Australia  and  the  reefs  of  the 
Florida  coast.  Both  are  built  up  on  the  continental  platform  and 
have  been  extended  nearly  or  quite  to  the  seaward  edge  of  that 
platform. 

Great  Barrier  Reef  of  Australia.  —  This  great  complex  of  reefs 
extends,  with  a  few  interruptions,  for  1250  miles,  from  Torres  Strait 
in  9.5°  S.  latitude  to  Lady  Elliott  Island  in  24°  S.  latitude.  At 
Cape  York  the  seaward  edge  of  this  reef  is  nearly  90  miles  distant 
from  the  coast,  and  it  descends  to  a  depth  often  exceeding  1800 
feet.  This  edge  represents  a  great  submarine  wall  or  terrace  which 


Characters  and  Types  of  Modern  Coral  Reefs     299 

fronts  the  whole  northeast  coast  of  Australia  (Fig.  224).  It  rests 
at  each  end  in  shallow  water,  but  near  the  center  rises  from  great 
depths.  The  surface  of  this  reef-complex  forms  a  great  plateau  or 
platform,  regarded  by  some  as  a  submerged  land  surface,  which  is 
covered  by  from  10  to  30  fathoms  of  water,  and  is  studded  all  over 
with  steep-sided,  block-like  masses,  the  individual  reef  mounds, 
which  rise  up  to  low- water  level  (Jukes-Browne).  These  individual 
reef  mounds  are  especially  abundant  along  the  outer  edge  of  the 
bank  or  platform  where  they  are  bathed  by  the  pure  ocean  water. 


FIG.  224.  —  Diagrammatic  section  across  the  Great  Barrier  Reef  of  Aus- 
tralia. (After  J.  B.  Jukes;  from  Vaughan.)  a,  Sea  outside  the  Barrier,  gen- 
erally unfathomable ;  b,  the  actual  barrier ;  c,  clear  channel  inside  the  barrier, 
generally  about  15  or  20  fathoms  deep;  d,  the  inner  reef;  e,  shoal  channel 
between  the  inner  reef  and  the  shore ;  F,  the  great  buttress  of  calcareous  rock 
formed  of  coral  and  the  detritus  of  corals  and  shells ;  G,  the  mainland  formed 
of  granite  and  other  similar  rocks. 

This  linear  series  of  reefs  is  the  true  barrier,  but  the  submerged 
platform  in  some  places  extends  beyond  it.  It  is  breached  by  nar- 
row passages,  and  at  rare  intervals  by  navigable  ship  canals.  The 
main  part  of  the  reef  consists  of  coral  heads  and  fragments  bound 
together  into  a  solid,  hard  mass,  with  the  living  coral  polyps  covering 
the  outer  surfaces.  Most  common  among  these  are  great  masses 
of  the  irregular  coral,  Forties  (Fig.  213),  and  the  brain  coral  (Fig. 
214).  These,  torn  from  their  anchorage  by  the  waves,  are  rolled 
about  and  worn,  while  at  the  same  time  they  grind  down  the  living 
and  dead  coral  masses  of  the  reef  into  fine  coral  sand  and  powder. 
Rolled  and  worn  fragments  of  such  corals,  six  to  eight  feet  in  diame- 
ter, are  common  on  the  outer  slope  of  the  reef,  and  they  furnish  an 
illustration  of  the  manner  in  which  the  reef  is  worn  away  by  the 
action  of  the  waves.  The  coarser  sand  is  washed  into  the  channels, 
while  the  finer  sand  is  carried  seaward  and  settles  on  the  bottom  in 
deeper  water.  The  material  dredged  here  has  the  appearance  of 
an  impalpable,  pale  olive-green  mud,  which  is  wholly  soluble  in 


300  The  Organic  or  Biogenic  Rocks 

diluted  hydrochloric  acid,  thus  showing  that  it  is  pure  carbonate 
of  lime.  When  dried  it  has  the  character  and  consistency  of  chalk. 

Inside  the  barrier  is  a  clear  and  broad  channel,  generally  from 
15  to  20  fathoms  deep.  The  bottom  is  covered  with  unconsolidated 
lime-sand,  ground  from  the  reef,  or  with  sand  largely  composed  of 
the  shells  of  Foraminifera  (Orbitolites)  which  in  places  constitute 
the  entire  sand  mass  around  the  coral  islands  and  the  neighboring 
shores.  Beyond  the  channel  lie  the  inner  reef  mounds,  which  are 
separated  one  from  the  other  by  narrow  water  ways  through  which 
the  tide  rushes  with  great  force.  Such  tidal  currents  may  con- 
tinue in  the  same  direction,  sometimes  for  two  or  three  days,  es- 
pecially after  great  storms,  and  they  form  important  agencies  in 
the  distribution  of  the  lime-sand  and  mud. 

The  reefs  of  the  inner  series  are  peculiar  in  that  many  of  them 
are  composed  chiefly  of  one  kind  of  coral  or  other  lime-secreting 
organism.  Thus  the  Organ-Pipe  reef  of  Thursday  Island  consists 
largely  of  the  organ-pipe  coral  (Tubipora  musica),  while  another 
reef  is  chiefly  formed  of  the  blue  coral  ( Heliopora  ccerulea) .  There 
are  reefs  largely  made  up  of  the  hydrocoralline  Millepora  (Fig.  217), 
and  others  composed  in  large  part  of  gorgonias.  The  inner  reefs 
are  separated  from  the  mainland  of  Australia,  which  is  formed  of 
older  rock,  by  a  shallow  channel  which  is  mostly  free  from  coral 
growth. 

The  Florida  Reefs.  —  These  arise  from  a  shallow  platform  par- 
allel to  the  southern  margin  of  the  peninsula  and  at  a  variable 
distance  from  it  (Fig.  225).  The  southern  coast  of  the  peninsula 
rises  from  12  to  15  feet  above  the  sea-level,  in  the  form  of  a  curving 
ridge,  and  behind  it  lies  the  great  fresh-water  swamp  of  the  Ever- 
glades, the  surface  of  which  is  only  two  or  three  feet  above  sea- 
level  (see  section,  Fig.  226).  This  rim  has  been  regarded  by  Agassiz 
and  Le  Conte  as  marking  the  line  of  an  older  series  of  reefs  behind 
which,  on  the  site  of  the  present  Everglades,  lay  a  lagoon  which 
has  since  been  converted  into  the  fresh-water  swamp.  More  recent 
investigation,  however,  has  shown  that  this  interpretation  is  prob- 
ably not  correct,  though  there  are  Tertiary  coral  reefs  at  Bainbridge, 
Georgia,  and  Tampa,  Florida  (Oligocene),  and  reef  corals  occur  in 
the  Pliocene  (Caloosahatchie).  From  five  to  fifteen  miles  out- 
side of  the  southern  rim  of  the  peninsula  lies  a  line  of  small  islands, 
the  "  Keys,"  which  vary  from  less  than  four  (Key  West)  to  fifteen 
miles  in  length  in  the  largest.  These  Keys  have  a  gentle  north- 


Characters  and  Types  of  Modern  Coral  Reefs     301 

ward  slope  and  a  steep  southward  or  seaward  face,  and  they  clearly 
represent  a  line  of  extinct  reefs  upon  which  waves  and  winds  have 
built  up  the  islands.  The  channel  between  the  Keys  and  the  pres- 
ent mainland  is  very  shallow;  its  floor  is  covered  with  fine  silt, 
which  at  low  tide  forms  exposed  mud-flats  rich  in  decaying  organic 
matter.  Many  small,  low  mangrove  islands  dot  the  channel,  and 
the  aerial  roots  of  the  mangroves  form  a  tangle  where  they  enter  the 
water,  and  this  is  very  effective  in  checking  the  tidal  currents  and 


FIG.  225.  —  Map  of  Florida,  showing  the  Keys  and  reefs.  (After  Le  Conte.) 
aa,  southern  coast;  a'a',  Keys;  a"a",  living  reef;  e,  Everglades;  e'y  inner 
channel ;  e",  outer  or  ship-channel ;  GSS,  Gulf  Stream. 

forcing  them  to  deposit  their  load  of  silt.  In  this  silt  are  buried 
the  remains  of  marine  animals  which  have  migrated  from  the  open 
sea,  and  terrestrial  and  fresh-water  forms  which  have  come  from 
the  land.  Also  there  are  forms  especially  adapted  to  muddy  bot- 
toms, and  their  remains  are  mingled  with  the  other  types. 

Outside  of  the  line  of  Keys,  and  from  three  to  fifteen  miles  dis- 
tant from  it,  is  the  line  of  living  reefs  which  is  still,  for  the  most 
part,  submerged,  and  consists  of  a  chain  of  reef  mounds  formed  in 
part  by  corals  (Madrepora,  Porites,  etc.)  and  in  part  by  nullipores 
(Corallina,  Lithothamnium,  etc.).  The  channel  between  the  living 
reef  and  the  dead  one  (the  Keys)  is  from  five  to  six  fathoms  deep 


302  The  Organic  or  Biogenic  Rocks 

and  its  floor  is  covered  by  coral-sand,  shells  of  marine  organisms, 
and  oolitic  lime  precipitated  by  the  agency  of  bacteria.  These 
deposits  are  bedded,  and  when  consolidated,  will  form  stratified 
limestones.  In  some  sections  the  delicate  branching  coralline  sea- 
weeds grow  in  abundance,  covering  the  floor  of  the  channel  with  a 
carpet  of  "  country  grass  "  as  it  is  called.  The  dead  portions  of 
these  corallines  disintegrate  into  small  fragments,  and  in  places 
these  form  the  main  deposit  on  the  channel  floor. 

On  the  outer  or  seaward  side,  the  reefs  slope  steeply,  and  the 
sea-bottom  quickly  descends  to  great  depth  (2916  feet).    The  sweep 


s 

FIG.  226.  — Diagrammatic  section  of  Florida  along  the  line  N.  S.  in  Fig.  225, 
showing  the  relative  position  of  the  south  shore,  a ;  Keys,  a' ;  and  living  reef, 
a" ;  with  the  Everglades,  e ;  the  inner  channel,  e' ;  and  the  outer  or  ship- 
channel,  e"  \  rin,  ancient  submarine  platform.  The  dotted  lines  indicate 
hypothetical  former  conditions.  (After  Le  Conte.) 

of  the  Gulf  Stream  here  prevents  any  further  southward  extension 
of  the  deposits.  These  relationships  are  shown  in  the  diagrammatic 
section  along  the  line  N.  S.  of  the  map  (Fig.  225)  given  in  the  preced- 
ing figure  (Fig.  226)  and  the  details  of  the  northern  end  of  the  reefs 
in  the  map  (Fig.  227). 

It  is  not  difficult  to  see  that  we  have  here  a  succession  of  reef  lines 
built  seaward  one  after  another,  and  that  the  older  reefs  became 
extinct  and  were  converted  into  islands  (the  Keys)  after  the  new 
line  of  reef  was  built,  and  so  shut  off  the  food-bearing  currents  from 
the  inner  reefs.  In  the  outer  channel,  between  the  modern  reefs 
and  the  Keys,  lime  deposits  only  are  forming.  This  was  also  the 
case  in  the  inner  channel  between  the  present  Keys  and  the  main- 
land, when  the  Keys  were  the  outermost  living  reefs.  After  they 
became  extinct  by  the  building  of  the  new  reefs,  the  lime  deposits 


Characters  and  Types  of  Modern  Coral  Reefs     303 

of  the  inner  channel  were  covered  by  silts  and  muds,  with  much 
organic  material.  This  would  happen  to  the  lime  deposits  of  the 
outer  channel  if  the  outer  or  modern  reef  became  extinct  and  were 


FIG.  227.  —  Chart  of  the  northern  end  of  the  Floridian  Barrier-reef.  From 
United  States  Coast  and  Geodetic  Survey,  Chart  No.  165.  (After  Vaughan.) 
The  depths  are  given  in  fathoms.  Note  the  sudden  drop  of  the  bottom  out- 
side of  the  lo-fathom  line. 

converted  into  islands.  On  consolidation  of  these  deposits,  the  bed- 
ded limestones  beneath  would  be  covered  with  a  mud  rock  or  shale, 
probably  of  black  color,  on  account  of  the  abundance  of  decayed 
organic  material  in  it.  If  the  southern  rim  of  Florida  were  an  an- 


304  The  Organic  or  Biogenic  Rocks 

cient  line  of  reefs,  it  would  represent  the  later  stage  in  development 
to  which  the  present  Keys  advance,  namely  the  formation  of  a  con- 
tinuous belt  of  land.  The  Everglades  have  been  regarded  as  a 
later  stage  in  the  silting  up  of  the  channel  and  its  conversion  into 
a  fresh-water  swamp  in  which  deposits  of  decaying  plants  form  the 
initial  steps  in  the  formation  of  a  coal  bed.  Although  this  inter- 
pretation of  the  Florida  rim  appears  not  to  be  correct,  it  can  be  re- 
garded as  a  possible  stage  in  the  gradual  modification  of  such  lines 
of  reefs.  The  black  mud  which  covers  the  limestone  may,  in  turn, 
be  covered  by  a  coal  bed,  above  which  other  deposits,  such  as  wind- 
borne  sands,  etc.,  may  accumulate. 

Structures  Common  to  All  Reefs 

It  is  important  that  we  should  understand  the  main  structural  features  which 
distinguish  reefs  of  corals  and  other  lime-secreting  organisms  from  other  types 
of  lime  deposits,  so  that  we  may  have  definite  means  by  which  we  can  recognize 
older  limestone  deposits  as  due  to  reef  growth,  if  such  be  their  origin.  In  the 
first  place,  then,  it  should  be  noted  that  the  main  mass  of  the  reef-mound  is 


FIG.  228. — Diagrammatic  section  of  a  Palaeozoic  coral  reef;  the  black 
masses  represent  coral  heads  in  the  position  of  growth  forming  the  reef  proper ; 
around  the  margin  are  deposits  of  coral-sand  and  mud  with  steep  dips  near  the 
reef  where  the  edges  of  the  reef  and  the  coral-sand  interfinger.  (After  Grabau, 
Principles  of  Stratigraphy.} 

composed  of  coral  or  coralline  structures  in  the  position  of  growth.  That  is, 
as  each  new  coral  head  or  coral  branch  developed,  it  remained  attached  to  the 
older  dead  coral  mass  or  to  the  original  rock-floor  which  served  it  as  a  founda- 
tion. Thus,  in  general,  such  a  mound  represents  a  mass  of  undisturbed  coral 
and  coralline  structures.  As  the  growth  is  not  uniform,  however,  in  all  direct- 
ions, numerous  large  and  small  cavities  exist  among  the  coral  masses,  and  these 
cavities  are  generally  occupied  by  shell-bearing  and  other  animals  whose  hard 
parts  remain  there  on  the  death  of  the  creature.  The  lime-sand  and  lime-mud 
into  which  the  waves  grind  the  exposed  corals  is  washed  into  these  cavities, 
which  may  eventually  be  filled  up  by  such  material.  On  the  margin  of  the  reef, 
especially  on  the  outer  one,  many  coral  heads  and  branching  forms  are  broken 
from  their  anchorage  and  rolled  about  by  the  waves,  grinding  into  sand  and 
mud  the  coral  masses  over  which  they  are  rolled.  When  finally  they  themselves 


Ancient  Coral  and  Coralline  Reefs  305 

become  embedded  in  the  coral  sand,  they  are  no  longer  perfect,  but  are  broken 
and  worn,  and  they  may  come  to  lie  in  all  positions,  being  even  completely  over- 
turned. The  fine  coral-mud  resulting  from  the  grinding  will  be  carried  out  to 
deeper  or  quieter  water,  though  it  may  also  be  caught  in  protected  cavities 
within  the  reef.  The  coral-sand  remains  in  shallow  water  to  form  bedded  de- 
posits. 

Along  the  margins  of  the  reef-mounds  the  bedded  deposits  of  coral  sand 
will  often  lie  at  a  steep  angle,  which  is  sometimes  as  high  as  45°  or  even  more. 
Frequently  a  layer  of  small  corals  will  grow  upon  such  a  bedded  deposit,  and 
this  in  turn  may  be  covered  by  other  lime-sands.  Thus  an  interfingering  of 
the  organic  lime  structures,  the  corals,  etc.,  with  the  clastic  lime,  the  coral-sand, 
will  result,  and  this  is  one  of  the  most  characteristic  features  of  the  margins 
of  the  reef-mound  (Fig.  228). 

ANCIENT  CORAL  AND  CORALLINE  REEFS 

In  the  older  limestone  rocks  of  the  earth's  crust  we  often  meet 
with  structures  which  indicate  that  parts  of  these  limestones 
were  old  reefs  similar  to  those  described,  while  the  remainder  of 
the  limestone  forms  bedded  deposits  of  coral  sand  or  shell  and  nulli- 
pore  fragments,  etc.,  similar  to  those  deposited  to-day  between 
lines  of  reefs.  Not  infrequently  the  corals  of  the  reefs  are  distinctly 
recognizable,  but  in  some  cases  they  have  been  so  altered  in  the 
course  of  time  that  they  form  a  compact,  structureless  mass.  When 
such  a  mass,  devoid  of  bedding  planes,  and  showing,  in  sections, 
more  or  less  of  a  mound-like  form,  is  enclosed  by  bedded  limestones, 
which  near  the  mound  have  a  steeper  inclination  away  from  it  in 
all  directions,  and  when,  furthermore,  these  bedded  lime-deposits 
alternate  with  projections  from  the  mound,  it  is  generally  safe  to 
regard  such  a  mass  as  an  ancient  coral  or  coralline  reef.  As  has 
been  said,  however,  many  of  these  ancient  mounds  still  show  their 
coral  or  other  organisms  in  perfectly  recognizable  condition,  and 
in  that  case  there  can  be  little  doubt  of  the  reef  origin  of  the  mass 
if  the  structural  characters  above  outlined  are  shown. 

Silurian  Reefs  of  Wisconsin.  —  Such  a  line  of  old  reefs,  composed  largely 
of  Stromatopora  with  some  corals,  was  formed  in  ancient  Silurian  time  in  Wis- 
consm  and  adjoining  areas.  It  has  been  traced  for  over  sixty  miles  in  length, 
and  extended  parallel  to  the  old  shore-line  of  an  interior  sea  and  at  some  dis- 
tance from  it.  In  general,  it  was  formed  under  conditions  not  unlike  those 
found  to-day  in  the  Great  Barrier  Reef.  Many  of  the  old  reef-mounds  have 
been  opened  in  quarries  near  the  city  of  Milwaukee  and  elsewhere,  and  though 
the  mounds  are  mostly  massive,  the  presence  of  the  Stromatoporas  can  be  recog- 
nized on  the  weathered  surfaces  of  the  quarry  walls.  The  bedded  lime  deposits 
on  the  flanks  of  these  mounds  commonly  show  steep  dips. 


306  The  Organic  or  Biogenic  Rocks 

Silurian  Reefs  of  Gotland.  —  Another  series  of  such  reefs  was  formed  during 
the  same  period,  parallel  to  the  Swedish  coast,  in  a  sea  which  occupied  the  area 
of  the  present  Baltic,  and  extended  far  east  into  Russia.  Much  of  this  reef  series 
has  been  worn  away,  but  a  part  remains  to  form  the  present  Island  of  Gotland, 
and  around  the  coast  of  this  island,  where  the  modern  sea  has  cut  cliffs,  many 
beautiful  sections  of  these  reefs  are  exposed  and  their  structures  are  well 
shown. 

Devonian  Reefs  of  the  Eastern  United  States.  —  At  a  later  period  (Middle 
Devonian)  a  series  of  reef-lines,  similar  to  those  of  the  Florida  coast,  formed  in 
the  great  interior  sea  which  then  covered  much  of  North  America.  These  reefs 
began  as  a  line  parallel  to  the  eastern  coast,  which  was  formed  by  a  land  mass 
lying  between  this  interior  sea  and  the  Atlantic  Ocean,  approximately  along  the 
line  of  the  Older  Appalachian  Mountains.  After  the  formation  of  the  first 
line  of  reefs,  a  second  one  came  into  existence  some  miles  farther  to  the  north- 
west, and  the  old  channel  between  the  first  reef-line  and  the  mainland  became 
silted  up  with  carbonaceous  muds.  When  a  third  and  a  fourth  line  of  reefs 
appeared,  each  farther  to  the  northwest,  where  the  open  sea  of  that  time  lay, 
the  older  channels  were  progressively  silted  up,  and  as  streams  brought  mud 
and  sand  from  the  mainland  of  that  time  (on  the  southeast),  the  old  extinct 
reefs  themselves  were  covered  by  muds  and  sands.  Several  of  these  lines  of 
reefs  have  been  definitely  located.  One  passes  through  eastern  New  York  and 
Pennsylvania,  another  passes  under  the  city  of  Buffalo,  a  third  under  northern 
Ohio,  and  a  fourth,  the  last  formed  of  the  series,  through  northern  and  western 
Michigan.  While  this  last  line  of  reefs  was  forming,  the  older  reefs  on  the  south- 
east were  being  buried  under  heavy  layers  of  mud  and  sand.  Many  of  these 
reefs  show  the  characteristic  structures  in  a  striking  manner,  as  one  passes  from 
one  quarry-opening  to  another.  The  corals  which  enter  into  the  construction 
of  these  reefs  are,  of  course,  very  different  from  those  found  in  modern  reefs. 
The  chief  types  were  the  honeycomb  coral  (Favosites,  Fig.  216)  and  various 
star  corals  (Prismatophylhtm,  Craspedophyllum,  etc.).  Tube  corals  (Syrin- 
gopora)  and  Stromatoporas  also  abound,  the  latter  often  of  very  large  size. 
Besides  these  there  were  many  simple  horn-shaped  corals  and  other  organisms 
as  well.  On  the  margins  of  the  mounds,  where  the  bedded  lime-sand  deposits 
dip  away  from  the  reef,  many  broken  and  worn  coral  and  Stromatopora  fragments 
are  found  embedded  in  the  sands,  lying  in  appositions. 

Triassic  Reefs  of  the  Dolomites.  — Many  other  reefs  of  this  kind  are  found 
in  various  parts  of  the  world,  and  some  of  these  will  be  again  referred  to  in  the 
later  part  of  this  book.  We  must  note  here  only  one  other  example,  which  ap- 
pears to  represent  an  ancient  reef  of  the  oceanic  type.  This  is  a  mass  of  dolo- 
mitic  limestone,  about  3000  feet  thick,  which  now  forms  the  famous  peaks  of 
the  Dolomites  in  the  Alps  of  the  Tyrol  (Fig.  4,  p.  9).  These  limestones  were 
formed  in  the  Triassic  period  of  the  earth's  history  and  are  largely  composed 
of  the  remains  of  nullipores,  though  other  reef-building  organisms  also  occur. 
The  limestone  is  massive  and  without  structure  except  around  the  margins, 
where  the  interfingering  character,  so  typical  of  reefs,  is  shown.  The  surround- 
ing deposits  of  which  this  reef-like  mass  formed  a  part  are  of  the  bedded  type 
of  clastic  material. 


Other  Lime-depositing  Organisms 
OTHER  LIME -DEPOSITING  ORGANISMS 


307 


Bryozoa  and  Limestones  Formed  by  Them 

The  Bryozoa  are  chiefly  marine  animals  of  a  higher  grade  of 
organization  than  the  coral  polyps,  but  they  secrete  structures  of 
carbonate  of  lime,  which  in  many  cases  are  not  easily  distinguished 


FIG.  229  a,  b.  —  A  modern  Bryozoan  (Membranipora  pilosa).  a,  a  group  of 
cells  or  zocecia  seen  from  above  (enlarged),  and  a  single  cell  seen  from  the 
side  (still  further  enlarged);  b,  a  single  zooid  expanded  (much  enlarged). 
(After  Verrill  and  Smith.) 


FIG.  229  c,  d.  —  Another  modern  Bryozoan  (Crisia  eburnea).  c,  a  cluster  of 
branches  (enlarged) ;  d,  a  single  branch  bearing  ovicells  and  zooid  cells  (zocecia) 
(much  enlarged).  (After  Verrill  and  Smith.) 

from  corals,  except  by  the  trained  student.  Modern  Bryozoa  either 
incrust  seaweeds,  rocks,  or  other  substances,  or  form  delicate  leaf- 
like  but  irregular  expansions  of  carbonate  of  lime  (Figs.  229  a-d). 


The  Organic  or  Biogenic  Rocks 


Many  ancient  Bryozoa,  however,  built  branching,  often  cylindrical 
masses,  which  were  admirably  adapted  to  the  making  of  beds  of 
limestone  (Fig.  230).  They  more  often  grew  in  sheet-like  associa- 


FIG.  230.  —  Group  of  rock-forming  Bryozoa  from  the  Ordovician  (Cincin- 
nati Group),  i,  Halloporaramosa;  i  a,  enlargement  of  surface;  2,H.  rugosa; 
3,  H.  dalei;  4  a,  Dekayia  aspcra,  enlargement  of  surface;  5,  Hallopora  an- 
drewsi;  5  a,  enlargement  of  surface.  (After  Nicholson.) 

tion  on  the  sea-floor,  but  in  some  cases  also  formed  reef -like  mounds. 
Examples  of  the  latter  are  shown  in  the  cliffs  cut  by  the  Black  Sea 
and  the  Sea  of  Azof  on  the  coast  of  the  Peninsula  of  Kertch  (Crimea) . 

Shell-bearing  Animals  and  Shell  Limestones 

There  are  two  important  groups  of  shell-bearing  animals  in  the 
seas,  the  Brachiopoda  and  the  Mollusca.  The  latter  are  represented 
by  several  classes,  of  which  three  are  especially  common  in  the  sea  to- 
day, the  bivalves  or  pelecypods,  the  gastropods,  and  the  pteropods. 
A  fourth  group,  the  cephalopods,  was  extremely  abundant  in 
former  times,  but  is  represented  by  only  a  few  types  to-day. 

Brachiopods.  —  In  this  class  the  shell  is  composed  of  two  prin- 
cipal parts  or  valves,  one  generally  larger  than  the  other,  but  each 
symmetrical  about  a  median  line  drawn  through  the  apex  of  the 


Other  Lime-depositing  Organisms 


309 


FIG.  231.  —  Terebratulina 
septentrionalis.  A  characteris- 
tic modern  Brachiopod  of  the 
northern  Atlantic  coast. 
(From  Binney  and  Gould.) 


FIG.  232.  —  A  Palaeozoic  brachiopod  shell 
partly  broken  to  show  the  internal  spiral 
arm-supports.  (Spirifer  striatus,  Sowerby.) 
Mississippian  limestones.  Note  the  sym- 
metrical character  of  the  shell  with  refer- 
ence to  a  median  line  drawn  through  the 
apex  or  beak. 


valve  (Fig.  231).  These  animals  were  far  more  common  in  the 
older  geological  periods,  especially  those  of  the  Palaeozoic  (Fig. 
232),  than  they  are  to-day,  and  in  the  past  they  often  formed  beds 
of  limestone  largely,  or  almost 
entirely,  composed  of  their  shells. 
Some  brachiopods  (Lingula  Fig. 
233,  Obolus)  carry  a  high  percent- 
age of  phosphoric  acid  and  are  an 
important  source  of  lime  phos- 
phate. 

Bivalves  or  Pelecypods.  —  In 
these  mollusks  (also  called  lamelli- 
branchs)  the  soft  body  of  the  ani- 
mal is  enclosed  by  a  shell  of  two 
valves  which  are  generally  simi- 
lar, except  in  the  oyster  and  some 
other  types,  forming  the  right 
and  left  valve  respectively.  Each 
valve,  however,  shows  an  asym- 
metry of  form,  a  line  drawn 

through  the  apex  not  dividing  it        FlG    2^_Lingula  pyramidata> 
into  two  equal  portions.     In  this     natural  size.     A  deep  sea  brachio- 

respect    the    pelecypod    shell    is     Pod-    (After  Brehm;  from  Ratzel, 
,.•,         -,.  , .        .  ,    j     f  Die  Erde.}     The  shell  of  this  ani- 

readily    distinguished    from    the     mal  conta;ns  about  55  per  cent  of 
brachiopod  shell.      Examples  are     phosphate  of  lime  and  magnesia. 


3io 


The  Organic  or  Biogenic  Rocks 


FIG.  234.  —  Pecten   irradians,  the   common   scallop   of   the   Atlantic  coast. 
(From  Binney  and  Gould.) 


FIG.  235.  — Area  transversa, 

a  common  plicated  shell  of  FIG.  236.  —  Venus  mercenaria,  the  common 

the   Atlantic   coast.      (From  quahaug  or  salt-water  clam,  about  three  fourths 

Binney  and  Gould.)  natural  size.     (From  Binney  and  Gould.) 


FIG.  237.  —  Modiola  plicatula,  a  characteristic  mussel  of  the  tidal  flats  and 
salt-meadow  streams  of  the  Atlantic  coast.     (From  Binney  and  Gould.) 


Other  Lime-depositing  Organisms 


the  clam,  scallop  (Fig.  234),  area  (Fig.  235),  quahaug  (Fig.  236), 
mussel  (Fig.  237),  etc. 

Gastropods.  —  The  second  class,  abundantly  represented  to-day, 
is  that  of  the  gastropods  or  snail-like  mollusks,  in  which  the  shell 
is  coiled  in  a  spiral  (Figs.  238-241).  These  shells  are  often  highly 
colored  and  marked  by  various  features  such  as  ridges,  nodes,  spines, 


FIG.    238.  —  Lunatia     heros,     the  FIG.       239.  —  Neptunea      islandica 

common   salt-water  snail  of  the  At-  (curta).     A  common  fusoid  gastropod 

lantic    coast.      (From    Binney    and  of  the  northern  Atlantic ;  three  fourths 

Gould.)  natural  size.    (From  Binney  and  Gould.) 

and  the  like.  Both  pelecypods  and  gastropods  have  formed  lime- 
stone beds  in  the  past.  On  the  coast  of  Florida  such  shell  limestone 
is  forming  to-day,  where  shells  and  fragments  of  them  are  washed 
into  protected  areas,  remaining  long  enough  so  that  the  percolating 
waters  may  deposit  lime  between  them  and  bind  them  together. 
This  rock  is  locally  called  coquina  (Fig.  242). 

Pteropods.  —  A  third  class  of  shell-bearing  mollusks  is  that  of 
the  pteropods,  so-called  because  a  part  of  their  body  (the  foot) 
develops  into  wing-like  appendages.  These  animals  float  in^vast 
numbers  upon  the  surface  of  the  ocean,  the  shell-less  species  (Fig. 
243)  serving  as  an  important  article  of  food  for  the  whalebone  whales. 


312 


The  Organic  or  Biogenic  Rocks 


FIG.  240.  — Chrysodomus  decemcos- 
tatus.  A  characteristic  fusoid  gas- 
tropod of  the  northern  Atlantic ;  three 
fourths  natural  size.  (From  Binney 
and  Gould.) 


FIG.  241.  —  Fulgur  carica,  a  char- 
acteristic gastropod  of  the  Atlantic 
coast  from  Cape  Cod  to  Florida; 
one  half  natural  size.  (From  Binney 
and  Gould.) 


FIG.  242.  —  A  piece  of  modern 
shell-limestone  or  Coquina  from  the 
Florida  coast ;  somewhat  reduced. 
(Photo  by  B.  Hubbard.) 


FIG.  243.  —  Clione  limacina,  a 
modern  shell-less  pteropod ;  enlarged 
twice.  (From  Binney  and  Gould.) 


Other  Lime-depositing  Organisms 


In  correlation  with  their  floating  habit,  the  shell,  when  present,  is 
thin  and  light,  and  often  quite  transparent  (Figs.  244  a,  b).  Such 
shells  accumulate  in  vast  quantities  upon  the  sea-bottom  in  regions 
where  the  animals  abound  in  the  surface  waters,  and  of  them  is 
formed  a  deep-sea  pterdpod  ooze  (Fig.  245).  Limestones  made 
entirely  of  shells  of  such  animals,  though  not  necessarily  of  deep- 
sea  origin,  are  found  in  our  older  geological  series.  One  of  these, 


FIG.  244.  —  Modern  shell-bearing 
pteropods.  a,  Styliola  vitrea,  about 
two  and  one  half  times  natural 
size;  b,  Cavolinia  tridentata,  approxi- 
mately natural  size.  (After  Verrill 
and  Smith.) 


FIG.  245.  —  Deep-sea  pteropod  ooze, 
enlarged  16  diameters.  (After  Mur- 
ray and  Renard;  from  Grabau's 
Principles  of  Stratigraphy.) 


found  in  New  York  State,  carries  on  the  average  40,000  shells  to 
the  cubic  inch  (Fig.  3,  p.  8).  This  limestone  may  have  been  formed 
by  the  rapid  settling  of  millions  of  these  animals  which  were  killed 
by  being  driven  into  the  mouths  of  estuaries  of  that  period.  Where 
the  remains  of  organisms  of  this  kind  abound  in  rocks  which  are  of 
shallow-water  origin,  they  may  often  be  the  source  of  important 
petroleum  deposits,  as  will  be  more  fully  shown  in  the  next  chapter. 
Cephalopods.  —  This  group  is  to-day  represented  chiefly  by  the 
Nautilus  (Fig.  246)  and  by  a  number  of  shell-less  types  (squids, 
Fig.  247;  cuttle-fish,  Fig.  248;  octopus,  etc.).  During  the  Meso- 
zoic  era,  however,  there  lived  a  great  group  of  such  shelled  cepha- 
lopods,  the  Ammonites  (Fig.  249),  which  in  some  cases  were  so 
abundant  that  they  built  up  beds  of  limestone,  while  in  other  cases 


FIG.  246.  —  The  modern  Pearly 
Nautilus  (N.  pompilius} ;  the  animal 
occupies  the  living  chamber  of  the 
sectioned  shell.  (After  Owen;  from 
Woodworth.)  a,  mantle;  b,  dorsal 
fold;  e,  nidamental  gland;  g,  shell- 
muscle;  Hi,  siphon;  k,  funnel  or  hy- 
ponome;  n,  hood;  ooo,  exterior  digi- 
tations;  p,  tentacles;  s,  eye;  xx, 
septa ;  z,  last  or  living  chamber. 


FIG.  247.  — The  Squid  (Loligo 
vulgaris  Linn.).  A  modern  decapod 
cephalopod  with  remnant  of  internal 
shell  only. 


FIG.  248. —  Cuttle  fish  "bone" 
(Sepia  officinalis  Linn.).  Internal 
shell  (much  reduced).  The  fine  point 
at  the  base  of  the  structure  represents 
the  guard  of  the  Belemnite,  the  main 
mass  corresponds  with  the  pro'os- 
tracum. 


FIG.  249.  —  Ammonite  (AmaUheus 
margaritalus),  Middle  Lias,  Swabia; 
side  view.  Where  the  shell  has  been 
partly  worn  away  near  the  aperture, 
the  complex  "  suture  line"  is  shown. 


Other  Lime-depositing  Organisms 


they  constitute  the  chief  source  of  the  calcareous  substance  of  the 
rock.  In  the  Palaeozoic  era  other  types  similar  to  the  modern 
Nautilus,  and  also  straight,  conical  or 
gently  tapering  shells,  the  orthoceran  type 
(Fig.  250),  occurred  in  great  abundance,  and 


FIG.  250.  —  A  simple 
straight-shelled  ceph- 
alopod.  (Orthoceras 
tumidum,  Barr.)  Silu- 
rian of  Bohemia;  two 
thirds  natural  size. 
Where  the  shell  has  been 
removed,  the  straight 
"sutures"  are  shown. 
The  position  of  the 
central  tube  or 
"siphuncle"  is  seen 
in  the  bottom  view. 
(After  Barrande.) 


FIG.  251.  —  Unio  radiatus.  A  common  species  of 
fresh- water  clam,  of  New  England  ponds  and  streams. 
(From  Binney  and  Gould.) 


FIG.  252.  —  Paludina  decisa  var.  Integra.  A  char- 
acteristic snail  of  fresh-water  ponds.  Female  on 
left,  male  on  right.  (From  Binney  and  Gould.) 


were  an  important  source  of  lime  of  many  rocks,  sometimes  forming 
their  chief  constituent  (Orthoceras  limestone). 


Fresh-Water  and  Land  Mollusks 

In  fresh  water,  too,  deposits  of  limestone  are  formed  from  the 
shells  of  bivalve  and  gastropod  Mollusca.  Among  the  former,  the 
great  fresh-water  clam  (Unio,  Fig.  251)  is  the  most  important,  and 
among  the  latter,  the  pond  snails  (Paludina  or  Vivipara,  Fig.  252) 


316 


The  Organic  or  Biogenic  Rocks 


FIG.  253.  —  Planorbis  trivolvis,  a 
common  pond  and  river  snail.  Side 
and  bottom  views.  (After  Binney 
and  Gould.} 


FIG.  254.  —  Limncea  elodes.  A  com- 
mon snail  of  stagnant  ponds.  (From 
Binney  and  Gould.) 


FIG.    255.  —  Physa     heterostropha,          FIG.    256.  —  Helix  albolabris,    the 
the    common   species  of    left-handed  common   garden  snail.     (From  Bin- 
snail  of  brooks  and   ponds.     (From  ney  and  Gould.) 
Binney  and  Gould.) 


FIG.  257.  — Serpula  contortuplicata,  slightly  reduced.     Two  of  the  tubes  show 
the  expanded  fringe  of  the  animal.     (From  Ratzel,  Die  Erde.} 


Other  Lime-depositing  Organisms 


and  the  river  snails  (Planorbis,  Fig.  253 ;  Limnaa,  Fig.  254 ;  Physa, 
Fig.  255).  More  commonly,  however,  these  form  impure  deposits 
of  marly  rock  mingled  with  much 
mechanical  sediment.  The  common 
snail  (Helix,  Fig.  256)  lives  upon 
moist  land,  but  the  shells  may  be 
washed  into  basins  and  so  become 
an  important  constituent  of  lime- 
stones. 


FIG.  259.  —  The  modern  ship- 
or  goose-barnacle  (Lepas).     After 
removal    of     the     right     valve. 
a,  stem;     C,   Te,   and    Sc,   shell 
pieces;    C,    carina;     Te,    terga; 
FIG.    258. —The    Barnacle    (Balanus).     Sc,  scuta;  M k,  mouth;  F,  furca; 
Type  of  fixed  crustacean,  one   half  nat-     P,  cirrus  (or  penis) ;    M ,  muscle, 
ural  size.     (After  Haug.)  (From  Haas,  Leitfossilien.) 


Worms  and  Crustaceous  Animals  as  Limestone  Formers 

Worms.  —  A  small  number  of  marine  worms  build  calcareous 
tubes  in  which  they  live.  These  tubes  often  form  a  dense  mass 
which  may  become  at  times  important  as  a  limestone  former ;  Ser- 
pula  is  an  example  (Fig.  257). 

Crustacea.  —  This  class  is  best  known  from  its  modern  represen- 
tatives, the  lobsters,  crabs,  crayfish,  and  the  very  aberrant  barnacles 
(Fig.  258),  and  the  goose-barnacle,  Lepas  (Fig.  259).  None  of  these 
ever  occurs  in  such  abundance  as  to  form  rocks,  but  a  peculiar  class, 
the  Ostracods  (Fig.  260),  in  which  the  animal  secretes  a  bivalve  shell, 
in  form  often  like  that  of  a  small  bean,  may  be  a  rock-former.  Some 
of  these  abound  in  the  streams  and  ponds  of  localities  in  western 


318 


The  Organic  or  Biogenic  Rocks 


North  America  and  elsewhere,  and  the  accumulation  of  the  shells 
gives  rise  to  a  calcareous  fresh-water  ooze  which,  from  the  prevalent 
form,  is  called  Cypris  ooze.  Members  of  this  class  were  also  abun- 
dant in  the  ocean  in  various  geological  periods,  and  limestones 
largely  composed  of  os traced  shells  have  been  formed  in  the  past. 


FIG.  260.  —  A  group  of  fossil  Ostracoda. 

Upper  row:  Leper ditia  angulifera  (Silurian). 

Middle  row;  left  to  right:  Cytherideris  impressa,  lateral  and  dorsal  views 
X  20  (fresh-wq.ter  Cretaceous,  Bear  River) ;  Cythere  monticula,  lateral  and 
dorsal  views  X  20  (Bear  River  Cretaceous) ;  jEchmina  abnormis,  right  valve 
side  and  (below)  dorsal  view  Xio  (Rochester  shale,  Silurian). 

Lower  row :  Drepanella  crassinoda,  right  valve,  side  and  (above)  dorsal  view 
Xio  (Ordovician) ;  Primitiopsis  punctulifera,  left  valve,  side  and  (above) 
dorsal  view  Xi8  (Hamilton-Devonian) ;  Primitia  seminulum,  left  valve  Xi8 
(Hamilton,  Devonian).  (From  Grabau  and  Shimer,  North  American  Index 
Fossils.) 

Finally,  the  remarkable  extinct  group  of  the  Trilobites  (Fig.  261), 
so-called  because  the  body  is  divided  longitudinally  into  three  parts 
or  lobes,  often  becomes  an  important  source  of  lime  in  the  Palaeozoic 
era,  and  in  Sweden  some  thin  beds  of  (Cambrian)  limestone  are 
entirely  composed  of  the  calcareous  outer  coverings  of  these 
animals  (Fig.  262).  Crustacean  structures  contain  up  to  26  per 
cent  of  calcium  phosphate  and  may  be  an  important  source  of  this 
substance. 


Other  Lime-depositing  Organisms 


319 


FIG.  261.  —  A  group  of  Trilobites,  the  characteristic  Palaeozoic  crustaceans. 

Upper  row :  Ptychoparia  kingi,   Cambrian ;   Microdiscus  speciosus,  pygidium, 

side  and  surface  views;    (enlarged).     The  same,  entire  specimen,  Cambrian; 

Asaphiscus  wheeleri,  Cambrian. 
.Lower  row :  Zacanthoides  typicalis,  Cambrian,  Paradoxides  harlani,  Cambrian, 

Isoldes  gigas,   Ordovician.     (From   Grabau  and   Shinier,   North  American 

Index  Fossils.) 

.   Crinoids  and  Crinoidal  Limestones 

Among  the  great  division  of  spiny-skinned  marine  animals,  the 
echinoderms,  to  which  the  starfish  (Figs.  263,  a,  b)  and  sea-urchins 
(Figs.  263  c,  d)  belong,  there  is  one  group,  that  of  the  Crinoids  (Fig. 
264)  and  their  relatives  (Cystoids 
and  Blastoids),  which  were  im- 
portant rock-formers  in  the  past 
(Palaeozoic  and  Mesozoic,  Fig. 
265).  Crinoids  live  to-day  in 
water  of  considerable  depth,  but 
in  the  Palaeozoic  era  they  appear 
to  have  been  abundant  in  shallow  FIG.  262.— Fragment  of  limestone 
seas.  The  animal  has  a  cup-  entirely  composed  of  the  remains  of 
.  i  j  .^i  a  small  trilobite,  Agnostus.  Upper 

Shaped   body  provided  with  nu-      Cambrian,  Sweden;  one  third  natu- 
merous    slender    arms,    and    is     ral  size.    (B.  Hubbard,  photo.) 


320 


The  Organic  or  Biogenic  Rocks 


affixed  to  the  sea-bottom  by  a  stem,  often  of  great  length.  This 
stem  is  composed  of  a  series  of  calcareous  plates  or  disks,  set  one 
upon  the  other  and  held  together  by  muscular  tissue.  When  the 
animal  dies,  these  disks  fall  apart  because  of  the  decay  of  the 


FIG.  263  a.  —  A  common  form  of 
star-fish  (Asterias  arenicola),  some- 
what reduced.  (After  Verrill  and 
Smith.) 


FIG.  263  c.  —  The  common  sea- 
urchin  of  the  North  Atlantic  (Strongy- 
locentrotus  drobachiensis).  Side  view, 
about  half  natural  size.  (After  Ver- 
rill and  Smith.) 


FIG.  263  b.  —  A  common  form  of 
brittle  star  (Ophiopholis  aculeata). 
Dorsal  view,  about  one  half  natural 
size.  (After  Verrill  and  Smith.)  ' 


FIG.  263  d.  —  The  common  sand- 
dollar  or  flat  sea-urchin  (Echina- 
rachnius  parma).  Upper  surface  with 
the  spines  partly  removed,  about 
five  sevenths  natural  size ;  a,  ambu- 
lacral  zone ;  i,  interambulacral  zones. 
(After  Verrill  and  Smith.) 


muscular  tissue,  and  they  will  then  accumulate  to  form  a  bedded 
deposit  of  lime  which  may  be  many  feet  in  thickness.  _The  body 
and  arms  of  the  animal  are  also  composed  of  calcareous  plates 
which  are  added  to  the  mass,  but  the  stem  disks  are  the  most  abun- 
dant and  prominent.  They  form  what  is  called  a  crinoidal  lime- 
stone, readily  recognized  from  the  form  of  the  disks  (Fig.  266). 


Other  Lime-depositing  Organisms 


321 


FIG.  265.  —  An  extinct  lily-crinoid. 
(Encrinus  liliiformis,  Lk.)  Muschel- 
kalk  (Triassic).  Reduced,  with  basal 
view  of  calyx  and  stem  joints. 


FIG.  264.  —  A  modern  crinoid 
(Rhizocrinus  lojjotensis)  showing  stem 
with  "root"  or  hold-fast  for  attach- 
ment, and  a  crown  composed  of  the 
calyx  and  branching  arms.  Slightly 
enlarged.  (After  Brehm;  from  Rat- 
zel.) 


FIG.  266.  —  A  fragment  of  crinoidal 
limestone,  or  rock  composed  almost 
entirely  of  the  stem-joints  of  cri- 
noids.  (Palaeozoic.)  About  one  half 
natural  size.  (B.  Hubbard,  photo.) 


322  The  Organic  or  Biogenic  Rocks 

Accumulations  of  Bones  of  Vertebrates 

The  highest  division  of  the  animal  kingdom,  the  vertebrates, 
secretes  an  internal  skeleton  which  is  composed  of  carbonate  and 
phosphate  of  lime,  and  accumulations  of  such  bones,  either  around 
salt  licks  or  in  ponds  and  other  basins  into  which  they  are  washed, 
may  form  important  beds  of  limestone  rich  in  lime  phosphate. 
Where  fish  are  suddenly  killed  in  the  sea,  as  by  an  earthquake  or  the 
sudden  encroachment  of  cold  waters  after  a  storm,  vast  quantities 
of  fish-bones  may  accumulate  on  the  sea-bottom;  or,  where  the  sea 
enters  a  river  estuary,  the  river  fish  may  suddenly  be  killed  by  the 
salt  water,  and  a  deposit  of  their  bones  formed  on  the  floor  of  the 
estuary.  Such  fish-beds  are  often  found  in  the  geological  series, 
and  they  may  be  of  local  importance  as  members  of  the  rock  forma- 
tion. Extensive  deposits  of  bones  of  land  animals  are  also  found 
in  caverns,  and  elsewhere,  forming  bone-breccias.  Some  of  these 
will  be  referred  to  in  later  chapters  of  this  book. 


ORGANIC  DEPOSITS  or  PHOSPHATE  OF  LIME 

Phosphate  of  lime  is  probably,  in  most  cases,  of  organic  origin, 
but  in  the  sea,  where  the  source  is  chiefly  the  bones,  teeth,  and  ex- 
crements of  fish,  and  the  shells  of  some  phosphate-secreting  animals 
(such  as  Lingula  (Fig.  233,  p.  309)  among  the  brachiopods,  and  the 
Crustacea),  secondary  chemical  deposition  seems  to  occur  to  pro- 
duce further  phosphatization  of  the  original  deposits  and  to  form 
the  phosphatic  nodules  which  are  subsequently  concentrated  by 
weathering  of  the  enclosing  rock,  until  they  form  important  de- 
posits. Upon  the  land,  accumulations  of  bones  of  animals  may  form 
a  source  of  phosphate  of  lime,  but  by  far  the  most  abundant  and 
striking  deposits  of  this  character  are  formed  by  the  droppings  of 
birds  on  islands  and  of  bats  in  caves.  These  deposits  are  called 
guano ,  and  they  are  common  on  many  of  the  islands  of  the  Pacific ; 
they  also  abound  in  the  West  Indies  and  on  the  islands  off  the 
African  coast.  Indeed,  such  deposits  of  bird  guano  are  almost  uni- 
versal around  the  ocean  borders  and  on  their  islands.  One  of  the 
most  extensive  occurred  on  the  islands  off  the  Peruvian  coast,  but 
this  has  been  largely  exhausted.  In  the  past,  guano  formed  an 
important  source  of  commercial  phosphate  rock,  but  at  present 
deposits  of  other  origin  (generally  concentrated  marine  deposits) 


Organic  Deposits  of  Silicia  323 

are  more  frequently  used.     Bat  guano  is  much  less  important  and 
much  less  common  than  bird  guano. 

ORGANIC  DEPOSITS  OF  SILICA 

Organic  silica  is  far  more  uncommon  and  less  widespread  than 
is  lime  of  organic  origin.  Nevertheless,  it  becomes  locally  of  much 
importance.  Both  plants  and  animals  form  deposits  of  organic 
silica,  sometimes  in  fresh  water,  but  chiefly  in  salt  water. 

Diatoms  (Fig.  267).  — These  are  plants  of  low  organization  be- 
longing to  the  division  of  the  algae.  They  abound  in  both  fresh  and 
ocean  water,  being  mostly  of  microscopic  dimensions,  and  they 
are  furthermore  remarkable  in  that  they  commonly  possess  the 
power  of  locomotion.  Within  the  body,  which  is  a  single  organic 


FIG.  267.  —  Modern  diatoms  (Diatoma  vulgare,  Bory).     The  individuals  are 
joined  in  a  zigzag  band;  much  enlarged.      (From  Haas,  Leitfossilien.) 

cell,  they  build  up  a  structure  of  silica,  often  of  very  beautiful  form 
and  of  great  variety.  This  structure,  called  the  frustule,  consists 
of  two  pieces  which  fit  together  like  the  body  and  cover  of  a  pill 
box.  In  some  fresh-water  ponds  they  are  so  numerous  that  they 
form  accumulations  on  the  bottom  of  the  pond,  largely  composed 
of  these  minute  silicious  bodies.  A  rock  is  thus  formed  which  re- 
sembles chalk  in  consistency  and  general  aspect,  but  which,  unlike 
chalk,  will  polish  metals  or  other  hard  substances.  Such  rock  is 
called  diatomaceous  earth  when  impure  and  tripolite  when  pure  (from 
a  famous  deposit  of  this  material  at  Tripoli  in  north  Africa). 

A  diatom  ooze  is  also  formed  in  the  sea,  especially  in  the  South- 
ern and  Antarctic  oceans,  on  the  floors  of  which  it  is  estimated  to 
cover  an  area  of  ten  million  square  miles,  at  an  average  depth  of 
1500  fathoms  (see  the  map,  Fig.  198,  p.  277).  In  the  northern  part 
of  the  North  Pacific,  an  area  of  about  forty  thousand  square  miles 
is  also  covered  with  diatom  deposits.  These  deposits  are  generally 
not  pure,  but  other  silicious  organisms  (Radiolaria,  sponge 
spicules)  and  earthy  matter  occur  with  them.  Diatoms  sometimes 


The  Organic  or  Biogenic  Rocks 


form  the  chief  element  of  rocks  in  marine  deposits,  not  always  of 
deep  sea  origin.     If  diatoms  are  carried  by  currents  from  the  sea, 

into  a  more  or  less 
enclosed  basin  in 
which  the  waters  are 
stagnant,  or  if  they 
grow  in  such  stag- 
nant waters,  they 
will  accumulate  in 
quantities  upon  the 
bottom,  but  as  in 
such  cases  there  are 
few  animals  which 
feed  upon  the  or- 
ganic matter  of  the 
diatoms,  this  will 
also  accumulate  and 
by  decay  form  a 
hydro-carbon  which 
\  saturates  the  de- 


FIG.  268  a.  —  A  modern  Radio- 
larian,  showing  the  skeletal  struc- 
ture of  silica  and  the  extended 
fleshy  threads  or  pseudopodia 
(Eucystidium  cranoides} .  Much 
enlarged.  (From  Haas,  Leitfos- 
silien. ) 

posits  of  silicious  diatom 
shells,  and  may  become  an  im- 
portant source  of  petroleum 
and  natural  gas.  Such  de- 
posits occur  in  the  Tertiary 
series  of  rocks  in  California, 
and  the  great  oil  accumula- 
tions of  these  rocks  are  be- 
lieved to  have  been  derived  from  the  organic  matter  of  these 
diatoms  which  was  retained  in  this  deposit  because  of  the  peculiar 
conditions  of  formation. 


FIG.  268  b.  —  Heliosphara  echin aides,  a 
modern  Radiolarian.  Greatly  enlarged. 
(After  J.  Murray ;  from  Haug  Traite.) 


Organic  Deposits  of  Silica 


325 


Radiolaria  (Figs.  268^,6). — These  are  minute,  single-celled 
animals  related  to  the  Foraminifera  and  with  them  forming  the 
mineral-secreting  members  of  the  division  of  Protozoa.  Unlike 
the  shells  of  the  Foraminifera,  however,  the  hard  structure  secreted 


FIG.  269.  —  A  modern   sponge  with    silicious    skeletal  structure,  and   basal 
glass  fibers  for  fixation  (Holtenia  carpentcri  Thomson). 

by  the  Radiolaria  is  internal  and  consists  of  a  network  of  glass 
or  silica  of  wonderful  variety  of  form  in  the  various  species.  These 
animals  live  only  in  the  sea,  and  their  shells  accumulate  in  the 
greatest  purity  on  the  deeper  portions  of  the  sea-bottom,  where 
forarniniferal  shells  are  not  found  because  they  are  dissolved  be- 
fore they  reach  that  depth.  Commonly  there  is,  however,  an 


326  The  Organic  or  Biogenic  Rocks 

admixture  of  red  clay,  a  very  characteristic  deep-sea  deposit. 
When  more  than  20  per  cent  of  the  deposit  consists  of  Radiolaria 
it  is  customary  to  speak  of  it  as  a  radiolarian  ooze.  Radiolarian 
oozes  are  not  known  in  the  Atlantic,  but  occur  in  the  deeper  por- 
tions of  the  Pacific  Ocean  and  in  small  areas  of  the  Indian  Ocean 

OsoQlutn 


. . . .  ostla 


FIG.   270.  —  A  Cretaceous  sponge  with  silicious  skeleton  retaining  its  form. 
(Ventriculites  simplex.     Mantell.) 

(see  map,  Fig.  198,  p.  277).  There  is,  however,  a  deposit  now 
exposed  on  the  island  of  Barbadoes  in  the  Windward  group  of  the 
Antilles,  which  is  a  typical  radiolarian  ooze  with  red  clay,  and 
appears  to  be  an  old  sea-bottom  deposit  now  uplifted. 

Radiolaria  are  also  found  in  shallow  water  deposits,  where  they 
become  included  in  other  sediments.  They  are  not  uncommon  in 
the  lagoons  of  coral  reefs,  and  they  have  been  found  in  the  chert 
bands  of  older  limestones  formed  in  association  with  reefs.  The 
chert  bands  themselves  are  often  the  redeposited  silica  derived  from 
the  solution  of  scattered  Radiolaria  and  other  silicious  organisms. 

Sponge  Spicules.  —  Many  modern  marine  sponges  (Fig.  269) 
secrete  within  their  soft,  horny,  and  fleshy  masses,  minute  needles 
of  silica.  These  are  set  free  on  the  decay  of  the  sponge,  and  some- 


Organic  Deposits  of  Silica 


327 


times  form  an  important  source  of  silica  in  other  deposits.     Many 
of  the  sponges  of  former  geological  periods  built  solid  structures 


FIG.  271  a.  —  Isolated  spicular 
bodies  of  an  extinct  silicious  sponge. 
(Eplstomella  clivosa,  Quenst.  White 
Jura.) 


FIG.  271  b.  —  Part  of  the  skeletal 
structure  of  a  lithisdid  silicious  sponge. 
(Jereiea  polystoma,  Roem.  Mucro- 
naten-Kreide.  Upper  Cretaceous, 
Germany.) 


by  a  union  of  such  spicules  (Figs.  270-271  a,  b)  and  so  became  im- 
portant sources  of  organic  silica.  The  flint  of  the  chalk  (Fig.  162, 
p.  224)  is  believed  to  be  largely  derived  from  such  sponge  spicules. 


CHAPTER  XIII 

THE  ORGANIC  OR  BIOGENIC  ROCKS:  DEPOSITS 
FORMED  FROM  THE  ORGANIC  TISSUES  OF  PLANTS 
AND  ANIMALS 

DEPOSITS  FORMED  FROM  VASCULAR  PLANTS 
Conditions  and  Processes  of  Decay 

UNDER  ordinary  conditions,  when  a  plant,  whether  herb  or  tree, 
dies,  it  soon  decays,  and  practically  nothing  remains  behind  except 
a  minute  quantity  of  mineral  matter.  The  plant  tissues,  which  are 
composed  of  carbon,  hydrogen  and  oxygen,  chiefly  in  the  form  of 
the  material  called  cellulose,  unite  with  the  oxygen  of  the  air, 
partly  through  the  activities  of  micro-organisms  (bacteria),  and 
carbon  dioxide  and  water  are  formed.  The  same  result  is  achieved 
more  rapidly  by  the  burning  of  the  dry  plant  tissues,  when,  how- 
ever, some  unconsumed  carbon  may  pass  off  as  smoke.  Slow 
decay  under  the  atmosphere  is,  in  effect,  a  very  slow  but  complete 
burning  of  the  tissues  without  the  production  of  a  flame  or  the 
development  of  easily  perceptible  heat.  The  reaction  in  either 
case  is  as  follows : 

C6HioO6     +     i2O(=6O2)     =     6CO2     +     sH2O 

Cellulose,  6  molecules  6  molecules        5  molecules 

etc.  of  Oxygen  of  Carbon  of  Water 

Dioxide 

The  carbon  dioxide  is  a  gas,  and  the  water  passes  off  as  invisible 
vapor.  If  plants  are  burned  where  oxygen  does  not  have  free 
access,  as  in  a  charcoal  oven  or  kiln,  or  under  a  covering  of  earth, 
the  oxidation  is  incomplete,  the  hydrogen  and  oxygen  escaping 
as  water,  while  some  of  the  carbon  may  also  be  converted  to  carbon 
dioxide ;  but  the  greater  part  of  the  carbon  remains  behind  in  the 
form  of  charcoal. 

In  like  manner,  when  plants  become  submerged  in  the  waters 
of  a  swamp  or  marsh,  free  access  of  oxygen  is  prevented  and  only 
partial  decay  will  result.  At  first  only  a  part  of  each  of  the  com- 

328 


Deposits  Formed  from  Vascular  Plants         329 

ponent  elements  will  pass  into  the  air  in  the  form  of  gaseous  com- 
binations, the  chief  of  these  being  carbon  dioxide  (062),  water 
(H2O),  and  marsh  gas,  a  combination  of  carbon  and  hydrogen 
(CHi).  As  the  change  progresses,  especially  under  pressure  of 
other  material  which  may  be  spread  over  the  plant  tissues,  more 
of  the  oxygen  and  hydrogen  will  be  eliminated,  and  the  relative 
amount  of  carbon  remaining  will  be  progressively  increased, 
though  there  is,  of  course,  no  actual  increase  in  carbon,  but  rather 
some  decrease.  This  forms  the  several  series  of  coals.  Finally, 
with  the  application  of  both  pressure  and  heat,  most  or  even  all  of 
the  other  substances  may  be  driven  off  and  pure  carbon  alone 
remains,  producing  graphite.  In  special  cases  crystallization  of 
the  carbon  may  result  in  the  formation  of  diamonds. 


Types  of  Vegetal  Deposits  and  Stages  in  Alteration 

Peat.  —  The  first  product  of  partial  decay  of  vegetable  matter 
is  peat.  According  to  the  degree  of  decay  and  the  character  of 
the  vegetable  material,  peat  ranges  in  color  from  brown  to  black, 
and  is  a  loose,  spongy  mass  in 
which  the  structure  of  the  veg- 
etable tissue  is  only  partly  ob- 
literated (Fig.  272).  Peat  forms 
only  in  stagnant  waters,  for  here 
alone  complete  decay  is  arrested. 
This  seems  to  be  due  to  the  fact 
that  the  bacteria  which  are  active 

in  the  decay  produce  certain  by-        Ff-  272. -A  fragment  of  peat 
....  —  about    one    half    natural    size, 

products  which  give  to  the  stag-      (Photo  by  Hubbard.) 

nant  water  an  antiseptic  char- 
acter, with  the  result  that  the  bacteria  are  destroyed  and  the 
process  of  decay  is  arrested.  Peat  bogs  are  proverbial  for  their 
antiseptic  qualities,  and  in  them  bodies  of  animals  and  of  men  are 
often  preserved  for  long  periods  of  time.  If  these  by-products 
are  removed,  as  in  running  or  much  agitated  waters,  the  decay 
continues  until  it  is  complete. 

Types  of  Peat  and  Conditions  of  Formation.  —  The  areas  of 
peat  formation,  or  the  moorlands,  may  be  either  low-lying  or  they 
may  be  high  moors.  The  former  division  comprises  the  shore 
moorlands  or  marine  marshes,  and  the  fresh-water  swamps  and 


330 


The  Organic  or  Biogenic  Rocks 


fenlands,  the  latter  the  upland  bogs.     These  types  will  now  be 
more  fully  considered. 

Marine  marshes.  —  Where  the  waves  break  on  a  gently  sloping 
sandy  coast,  they  commonly  build  up  an  off-shore  or  outer  bar, 

5  B        SP  EG  MF  5P      SM 


HT 


FIG.  273.  —  Diagrammatic  section  of  an  off-shore  bar  and  the  barachois  or 
lagoon  enclosed  by  it.  HT,  high  tide-level;  LT,  low  tide-level;  SB,  sand  bar 
formed  by  waves  from  sea-bottom  sand;  Sp,  marsh  grass  or  Spartina  zone; 
EG,  eel-grass  zone;  MF,  mud-flat,  uncovered  at  low  tide;  SM,  salt  meadow, 
covered  only  at  highest  tides. . 

leaving  a  lagoon  or  barachois  of  protected  water  between  it  and  the 
land  (Fig.  273).  This  lagoon  may  be  many  miles  in  width,  but  is 
never  very  deep,  seldom  over  30  feet,  and  its  waters  are  never 


FIG.  274.  —  Eel-grass  growing  upon  a  muddy  bottom,  Woods  Hole,  Mass. 
(Photograph  of  part  of  foreground  of  reproduction  of  annulate  group ;  by  cour- 
tesy of  American  Museum  of  Natural  History.) 

completely  cut  off  from  the  open  sea,  joining  it  either  around  the 
'end  of  the  bar  or  through  narrow  openings  across  it.  In  the  quiet 
waters  of  the  lagoon,  where  the  depth  is  not  over  1 2  feet,  eel-grass 


Deposits  Formed  from  Vascular  Plants         331 

will  begin  to  grow.  This  is  a  plant  belonging  to  the  pond-lily 
family,  but  adapted  to  live  only  in  salt  water  (Fig.  274).  Mean- 
while the  bar  may  be  converted  into  a  barrier  beach  by  the  forma- 
tion upon  it  of  sand  dunes,  heaped  up  by  the  wind  from  the  sand 
of  the  bar,  exposed  and  dried  at  low  tide,  and  the  deeper  parts  of 
the  lagoon  may  slowly  become  silted  up,  after  which,  when  the 
proper  depth  is  attained,  the  eel-grass  will  spread  over  these  parts. 
At  low  water  the  eel-grass  will  form  a  dense  tangle  which  checks  the 
tidal  and  other  currents  and  forces  them  to  deposit  their  load  of 


FIG.  275.  — View  of  the  salt  meadows  at  Winthrop,  Mass.,  showing  the  rank 
growth  of  salt  thatch  (Spartina)  and  the  tidal  stream  dissecting  the  meadow. 

silt,  and  thus  further  filling  of  the  lagoon  takes  place,  the  silt 
accumulating  around  the  eel-grass  blades.  When  silting-up  has 
progressed  so  far  that  at  low  tide  parts  of  the  bottom  are  exposed, 
the  eel-grass  there  will  die,  and  a  mud-flat  results,  in  which  live 
clams  and  other  marine  organisms,  and  from  which  a  fetid  odor 
of  marsh  gas  is  evolved.  Finally,  the  higher  portions  of  the  mud- 
flat  are  taken  possession  of  by  the  marsh  grasses  (Spartina)  and 
as  these  die  down  year  by  year,  patches  of  peat  are  formed  from 
their  roots  and  decaying  stems.  As  the  growth  of  grasses  spreads 
and  the  surface  rises  through  the  accumulation  of  the  peat,  other 
species  of  marsh  grass,  which  can  stand  less  submergence,  will 
take  possession  and  these  will  continue  to  build  up  the  peat  deposit. 
Finally  the  entire  lagoon,  or  a  large  part  of  it,  will  be  converted  into 


332  The  Organic  or  Biogenic  Rocks 

a  salt  meadow,  which  is  submerged  only  at  the  highest  tides,  but 
which  is  intersected  by  numerous  tidal  channels,  on  the  margins  of 
which  the  peat  can  be  observed  at  low  water  (Figs.  275,  276). 
Meanwhile  the  sand  dunes,  traveling  inland  under  the  influence  of 
the  winds  from  the  sea,  will  begin  to  cover  parts  of  the  salt 
meadow,  and  the  waves,  cutting  back  the  bar  which  they  had 
originally  formed,  will  eventually  remove  it,  exposing  the  peat  beds 


FIG.  276.  —  Map  of  the  salt  marsh  near  Newburyport,  Mass.,  with  Plum 
Island  Sound  and  creek  representing  the  remains  of  the  lagoon;  the  sand  bar 
is  known  as  Plum  Island.  (After  Shaler.) 

upon  the  outer  shore  of  the  barrier  beach,  which  is  now  formed  of 
the  sand  dunes  resting  upon  the  peat  beds  of  the  old  lagoon.  Unless 
a  change  of  level  occurs,  the  cutting  process  of  the  waves  will 
eventually  again  destroy  the  entire  series  of  deposits  which  have 
formed  in  the  lagoon.  Hence  such  accumulations  are  not  generally 
of  a  permanent  character,  though  special  conditions  may  occur 
to  preserve  them.  Where,  however,  a  rock  barrier  separates  an 
old  lagoon  from  the  sea,  as  in  parts  of  the  northern  New  Jersey 
meadows,  the  opportunities  for  preservation  of  the  peat  deposits 
are  more  favorable. 

Deposits  of  peat  formed  in  salt  meadows  are  never  very  thick,  unless  the 
coast  is  slowly  sinking  as  the  peat  grows,  and  at  a  corresponding  rate.  On  the 
Massachusetts  coast,  where  such  subsidence  has  taken  place,  peat  deposits 
composed  of  the  remains  of  high-tide  vegetation  have  been  found  to  have 
a  thickness  of  20  feet.  Upon  the  southern  Atlantic  coast  the  average  thick- 
ness is  probably  not  much  over  four  feet,  though  this  would  vary  with  the 


Deposits  Formed  from  Vascular  Plants         333 

magnitude  of  the  tides.  Many  variations  occur,  dependent  on  the  original 
condition  of  the  coast,  and  it  may  even  happen  that  salt-water  peat,  formed 
only  of  the  taller  marsh  grasses,  will  overlie  a  fresh-water  peat  which  was 
formed  before  the  sea  encroached  over  that  area  owing  to  slow  subsidence  of 
the  land  or  other  causes.  Examples  of  such  complex  deposits  are  found  on  the 
Massachusetts  coast.1  Salt  peat  is  almost  always  rich  in  silt,  which  is  brought 
in  by  the  tides  or  in  fine  sand  and  dust  which  is  blown"  there  by  the  wind. 
Moreover,  the  contact  of  the  salt  water  with  the  decaying  vegetation  favors 
the  activities  of  certain  bacteria  which  will  decompose  the  sulphates  in  solu- 
tion in  the  sea- water,  and  cause  the  formation  of  sulphureted  hydrogen  (H2S), 


7  e 


FIG.  277.  —  Diagram  of  plant  zones  in  small  lake  near  Merryman's  Lake, 
Michigan.  (After  C.  A.  Davis.)  o,  Chara;  i,  floating  bladderworts;  2,  yellow 
pond-lily;  3,  lake  bulrush;  4,  SartwelPs  sedge;  5,  bottle  sedge;  6,  spike  rush; 
7,  cat-tails. 

a  characteristic  product  of  the  salt  meadows,  easily  recognized  by  its  odor. 
This  will  react  upon  the  iron  compounds  in  the  silt,  and  finely  divided  iron 
sulphide  (the  mineral  pyrite)  will  form.  If  then  in  time  such  salt  peat  is  con- 
verted into  coal,  that  coal  will  be  high  in  ash,  because  of  the  silt,  and  will  be 
rich  in  sulphur  minerals,  especially  iron  pyrites.  When  formed  in  the  normal 
way  above  outlined,  such  a  coal  should  rest  upon  mud  and  sand  beds  which 
contain  the  remains  of  marine  organisms,  and  the  coal  itself  may  contain 
such  remains,  since  shell-bearing  Mollusca  and  other  animals,  such  as  crabs, 
are  not  uncommonly  found  in  peat  deposits  of  this  origin.  On  the  Atlantic 
coast  the  shells  of  the  plicated  mussel  (Modiola  plicatida,  Fig.  237,  p.  310)  are 
especially  abundant  in  the  peat  beds.  Some  of  the  plants  found  in  these  salt 
meadows  are  shown  in  Fig.  278  a. 

Swamps.  —  Many  small  lakes  are  converted  into  swamps  by 
becoming  choked  with  vegetation,  or  the  margins  of  larger  ones 
may  become  swampy  through  similar  causes.  In  general,  we  may 
distinguish  two  types  of  plant  deposits  in  lakes,  —  that  formed  by 
algae  and  that  formed  by  the  so-called  vascular  plants,  that  is, 

1  See  further  Grabau,  Principles  of  Stratigraphy,  p.  492.  Johnson,  Shore  Processes, 
P-  385- 


334 


The  Organic  or  Biogenic  Rocks 


those  that  have  a  regular  structure  like  or  resembling  that  of 
wood,  and  which  include  all  the  plants  from  mosses  and  ferns  up. 
Algae  abound  in  fresh  as  in  salt  water,  each  habitat  having  its  own 
species  and  genera.  As  the  algae  may  occupy  all  parts  of  the  lake 
waters,  the  deposits  formed  by 
their  dead  tissues  'will  cover  the 
lake  floor  in  a  more  or  less  con- 
tinuous layer,  provided  there 
are  no  bottom  animals  to  feed 
upon  them.  Mingled  with  these 
algae  are  the  pollen  grains  of 
trees  such  as  the  conifers  which 


FIG.  278  a. —  Salt  marsh  plants; 
enlargements  of  the  flowing  portions, 
by  which  they  are  chiefly  distin- 
guished, i,  a  rush,  Juncus  gerardi, 
northern  salt  marshes;  2,  bulrush, 
sedge  family,  Scirpus  americanus, 
common  on  borders  of  salt  and  fresh 
ponds  and  streams ;  3,  rush  salt  grass, 
Spartina  junca,  salt  marshes.  (From 
drawings  by  Mary  Welleck.) 


FIG.  2786.  —  Characteristic  swamp 
plants  taking  part  in  peat-forming, 
i,  a  common  peat-moss,  Hypnum, 
X  i|;  2,  common  duckweed,  Lemna. 
The  entire  plant  is  reduced  to  a  leaf- 
like  expansion  which  bears  a  long 
slender  root  and  small  flowers ;  forms 
green  floating  scum  on  ponds  and 
stagnant  pools,  X8 ;  3,  a  leaf  of  arrow- 
head (Sagittaria),  a  member  of  the 
water-plantain  family,  growing  abun- 
dantly in  swamps.  One  fourth  nat- 
ural size. 


grow  in  the  neighborhood,  while  fresh  water  diatoms  also  accumu- 
late. This  material  will  form  a  homogeneous,  structureless  mass, 
more  or  less  mingled  with  fine  sediments,  with  the  calcareous  par- 
ticles from  the  stone-worts  (Chara)  (Fig.  192,  p.  273)  which  grow 
under  such  conditions,  or  with  the  silicious  frustules  of  diatoms 
(Fig.  267,  p.  323).  Such  a  mixture  of  decaying  organic  material 
with  mineral  matter  is  called  a  sapropelite,  and  according  to  the 
amount  of  impurities  present,  it  becomes,  on  compacting,  either 
an  oil  shale,  or  when  nearly  pure,  a  cannel  coal.  The  pure 


Deposits  Formed  from  Vascular  Plants         335 


decaying  organic  slime  is  called  sapropel.    (Sapros,  craTrpos  =  rotten, 
pelos,  TrcAos  =  mud.) 

The  peat  of  the  lake-swamps,  on  the  other  hand,  is  formed  from 
vascular  plants  (Figs.  277-279).     In  the  deeper  waters  of  many  of 


FIG.  278  c.  —  Cotton  grass  (Eriopho- 
rum  alpinum) ,  a  member  of  the  sedge 
family,  i,  small  entire  plant;  2,  spike; 
3,  a  single  scale ;  4,  a  flower  from  the 
same;  5,  a  spike  in  fruit,  the  bristles 
forming  a  cottony  tuft;  6,  a  single 
one-seeded  dry  fruit  or  achenium, 
with  bristles  —  much  enlarged.  A 
characteristic  peat  plant  in  cold  bogs, 
northern  United  States  and  Europe. 
(After  Gray.) 


FIG.  2  78  d.—  The  twig-rush  (Cla- 
dium  mariscoides) ,  a  characteristic 
plant  of  the  bogs  of  the  northeastern 
United  States,  i,  summit  of  plant; 
2,  detached  spike;  3,  the  same 
opened,  showing  a  staminate  and  a 
perfect  flower;  4,  the  nut-like  fruit 
or  achenium;  5,  longitudinal  section 
of  the  same.  (2-5  enlarged;  after 
Gray.) 


our  lakes,  pond-lilies  form  the  chief  vegetation,  these  rising  from 
depths  not  exceeding  25  feet.  Next  come  the  reeds,  such  as  bul- 
rushes, cat-tails,  flags,  water  plantain,  arrowheads  (Fig.  278  63),  some 
grasses,  and  the  wild  Indian  rice  (Phragmites,  Fig.  2787),  most  of 
these  not  growing  well  in  water  more  than  two  feet  deep.  Be- 


336 


The  Organic  or  Biogenic  Rocks 


sides  these,  there  are  floating  plants  such  as  the  bladderwort  and 
the  duck- weed  (Lemna,  Fig.  278  &2),  which  may  cover  large  sur- 
faces as  with  a  green  carpet. 
Near  the  shore,  sedges  (Car ex, 
Fig.  278  e)  and  peat-mosses  (Hyp- 
num,  Fig.  278  bi)  appear,  and 
these  may  form  a  floating  mat  of 
entangled  plants  which  extends 
from  the  shore  outward  often  for 
a  considerable  distance,  and  be- 
cause of  the  interwoven  roots 
and  branches  makes  a  buoyant 
structure  capable,  in  its  thicker 
portion,  of  supporting  consider- 
able weight,  but  dangerous  to 
enter  upon  in  its  outer  thinner 
parts  (Fig.  280) .  The  peat  formed 
from  such  a  mat  grows  in  thick- 
ness year  by  year,  as  the  under 
part  suffers  partial  decay  and 
compacting,  and  new  growth 
takes  place  on  top,  and  thus  the 
margins  of  the  lake  gradually 
become  filled  in  by  a  zone  of 
peat  which  progressively  extends 
toward  the  center  of  the  lake. 
Outside  of  the  zone  of  sedges  and 
peat-mosses  appears  the  zone  of 
water-loving  trees,  such  as  the 
alders  and  the  tamarack  in  the 
northern,  and  the  cypress  and 
tupelo  in  the  more  southern 
regions,  and  in  the  shadow  of 
these  grows  a  rich  assemblage  of 
ferns,  sedges,  horsetails,  and  the 
peat-moss,  Sphagnum.  As  the 
mat  thickens,  these  trees  will  ad- 
vance over  it,  growing  now  upon  the  site  of  the  former  lake 
margin,  which  is  converted  into  a  "  quaking  bog."  If  Sphag- 
num is  present,  this  will  often  grow  to  such  an  extent  as  to  cover 


FIG.  278  e. — A  typical  peat- 
bog sedge  (Car ex  pauciflora), 
characteristic  of  the  peat-bogs 
of  the  northeastern  United 
States,  i,  entire  plant;  2, 
staminate  flower  with  its  scale ; 
3,  scale;  4,  mature  pistillate 
flower  in  its  perigynium  or  en- 
velope; 6,  dry  fruit  or  ache- 
nium  on  its  stalk  with  style 
and  stigmas.  (After  Gray.) 


Deposits  Formed  from  Vascular  Plants         337 

the  lower  parts  of  the  trunks  of  the  trees,  and  keeping  them 
moist,  will  cause  their  death.  As  the  trunk  breaks  off  above  the 
covering  of  moss,  the  stump  or  '•'  stool  "  remains  and  is  covered 
by  the  growing  moss,  as  are  also  the  fallen  trunks.  A  mass  of 


FIG.  2 78 /and  g.  —  Flowers  of  Reed  and  Cane:  /.  the  common  reed,  Phrag- 
mites  (Arundo)  communis,  of  the  American  and  European  reed  swamps,  growing 
from  5  to  12  feet  high  with  leaves  2  inches  wide  (see  Fig.  279).  i,  spikelet  en- 
larged; 2,  one  of  the  perfect  flowers  enlarged;  3,  lowest  flower  with  stamens 
only.  g.  The  large  cane  (Arundinaria  macros  perma),  which  forms  the  cane 
brakes  of  the  southern  states.  It  grows  from  10  to  20  feet  in  height  with 
leaves  i  to  2  inches  wide,  i,  a  spikelet;  2,  a  separate  flower  magnified.  (After 
Gray.) 


FIG.  279.  —  Reed  zone  (Arundo  phragmites  zone)  on  the  border  of  the  high 
moors.  The  trees  in  the  background,  especially  Pinus  sifaestris,  are  smaller  here 
than  in  the  intermediate  moor.  (After  Potonie,  Die  Entstehung  der  Steinkohle.) 


338 


The  Organic  or  Biogenic  Rocks 


peat  of  variable  character  is  thus  produced,  carrying  in  its  upper 
part  stools  and  fallen  tree  trunks,  leaves,  ferns,  etc.,  and  resting 
near  the  center  of  the  lake  upon  a  deposit  of  sapropelite.  The 


FIG.  280.  —  Diagram  showing  how  plants  fill  ponds  from  the  sides  and  top. 
i,  zone  of  Chara  and  floating  aquatics;  2,  zone  of  pond  weeds  or  Potamogetons;' 
3,  zone  of  water-lilies;  4,  floating  sedge  mass;  5,  advance  plants  of  conifers 
and  shrubs ;  6,  shrub  and  Sphagnum  zone ;  7,  zone  of  tamarack  and  spruce ;  8, 
marginal  fosse.  (After  C.  A.  Davis.) 

thickness  of  the  deposit  is  determined  in  part  by  the  depth  of  the 
lake  and  by  other  causes.  If  covered  by  sediments,  such  a  peat 
deposit  may  be  preserved  and  subsequently  compacted  into  coals. 
Such  conditions  for  preservation  are  best  found  on  broad  river 


FIG.  281.  —  Southern  margin  of  Dismal  Swamp  12  miles  west  of  Elizabeth 
City,  N.  C.,  showing  general  aspect  of  swamp  in  the  month  of  May.  (Photo 
by  Russell,  from  U.  S.  G.  S.) 

flood-plains  or  on  the  great  flat  alluvial  plains  near  their  mouths. 
Here  not  only  exist  all  the  conditions  which  favor  the  formation  of 
swamps,  but  at  intervals  the  rivers,  charged  with  sand  and  mud, 
may  spread  this  over  and  cover  the  peat  deposit  and  so  preserve  it. 
Borings  in  river  deltas,  such  as  that  of  the  Ganges  and  others,  have 


Deposits  Formed  from  Vascular  Plants         339 

revealed  successive  deposits  of  peat  at  various  depths,  covered 
by  and  alternating  with  sands  and  muds,  and  in  a  condition  suit- 
able for  conversion  into  coal. 


FIG.  282.  —  Cypress  trees  in  the  eastern  part  of  Lake  Drummond,  Va.     (Photo 
by  Russell,  from  U.  S.  G.  S.) 


FIG.  283.  —  A  swamp  in  Florida. 

Examples  of  our  largest  swamps  in  which  peat  is  forming  to-day 
are  Okefenoke  Swamp  in  Georgia,  50  miles  from  the  sea  and  115 


340  The  Organic  or  Biogenic  Rocks 


f> 
i 


Deposits  Formed  from  Vascular  Plants        341 

feet  above  it,  with  peat  ten  feet  thick  filled  with  cypress  stumps, 
and  Dismal  Swamp  in  Virginia  and  North  Carolina,  with  an  area 
of  500  square  miles,  near  the  sea  and  only  a  few  feet  above  it  and 
with  a  peat  deposit  at  least  fifteen  feet  deep  (Fig.  281).  In  this 
swamp,  the  chief  plants  are  canes,  wild  grape,  the  bald  cypress 
(Fig.  282)  and  junipers,  with  but  little  sphagnum.  Similar  swamps 
occur  in  Florida  (Fig.  283),  and  extensive  cedar  swamps  with  peat 
up  to  fifteen  feet  in  thickness  exist  on  the  Atlantic  coastal  plain 


FIG.  285.  —  Section  of  a  Scottish  peat-bog.     The  peat  is  underlain  by  clay, 
which  corresponds  to  the  under-clay  of  a  coal  seam.     (After  Geikie.) 

further  north.  This  peat  is  full  of  tree  trunks  and  is  very  pure, 
containing  only  3.35  per  cent  of  ash.  In  tropical  swamps  the  peat- 
moss, Sphagnum,  is  absent,  the  peat-forming  plants  being  sedges, 
grasses,  myrtle,  etc.,  and  woody  plants,  as  in  Bermuda,  where  the 
peat  is  at  least  50  feet  thick  in  one  swamp,  and  in  the  Amazon  re- 
gion, and  in  the  interior  of  Africa.  Woody  plants  are  the  chief  peat- 
formers  in  the  great  tropical  swamp  of  Sumatra,  where  the  peat 
has  been  sounded  to  a  depth  of  nine  meters.  Here  herbaceous 
plants  are  rare,  and  sedges,  grasses,  and  mosses  are  practically 
wanting.  Thread-like  algae  are,  however,  numerous  in  the  water. 
Where  swamps  occur  along  tropical  seacoasts,  mangrove  trees 
form  the  chief  vegetation  (Fig.  284). 

Bogs.  —  These  form  on  uplands,  often   directly  over  rocky  or 
sandy  bottoms,  where  no  standing  water  exists,  the  water  for  the 


342 


The  Organic  or  Biogenic  Rocks 


formation  of  the  peat  accumulating  as  the  growing  vegetation 
arrests  the  drainage.  Thus  actual  ponds  or  lakes  may  be  formed  on 
gentle  hillsides  or  near  their  tops,  the  water  being  held  entirely 
by  dams  of  vegetable  material.  Such  bogs  cover  the  uplands  of 
many  northern  countries,  as  for  example  Great  Britain  and  Ireland, 
Scandinavia,  parts  of  Canada,  etc.,  being  most  common  in  cool 
and  moist  regions  (Fig.  285). 

In  the  formation  of  these  bogs  the  peat  moss  Sphagnum  is  most  active,  this 
plant  growing  rapidly  upon  moist  surfaces  and  building  up  a  spongy  mass  which 
collects  and  holds  back  the  water.  In  the  Scottish  and  other  uplands,  heather 
is  an  important  peat-maker,  for  it  too  holds  back  the  moisture  and  builds  up 
spongy  masses  from  its  decaying  older  branches  and  roots  (Fig.  301  b,  p.  364). 
The  common  reed  (Phragmites,  Fig.  279)  is  another  important  peat-former  in 
the  uplands,  and  so  is  the  bulrush  (Scirpus,  Fig.  278  02),  the  cotton  grass 

(Eriophorum,  Fig.  278  c), 
and  others.  In  the  Arctic 
Tundras,  which  are  great 
peat-deposits  covering  the 
frozen  ground,  mosses  and 
lichens  are  important  peat- 
formers. 

The  peat  formed  in 
bogs  is  known  in  Great 
Britain  as  hill  peat  and 
is  extensively  used  for 
fuel,  being  cut  into 
cubes,  which  are  dried 
in  the  open  (Fig.  286). 
It  is  usually  a  brownish 
or  nearly  black,  fibrous, 
spongy  substance,  with 
the  vegetable  structure 
still  clearly  visible. 
Tree  stumps  are  com- 
mon in  it,  as  are  also  fallen  trunks  and  branches,  for  whenever  the 
peat  begins  to  grow  in  forested  areas,  the  elevation  of  the  water- 
level  around  the  trunks,  due  to  the  retention  by  the  peat,  causes 
the  death  of  the  trees.  The  generally  monotonous  deforested 
areas  of  these  uplands  are  the  result  of  such  a  process.  In  thick- 
ness, these  upland  peat  deposits  seldom  exceed  50  feet,  and  usually 
they  are  much  thinner. 


FIG.  286.  —  Gathering  peat  in  an  Irish  peat- 
bog. 


Altered  Deposits  of  Older  Vegetal  Material     343 


ALTERED  DEPOSITS  OF  OLDER  VEGETAL  MATERIAL 

Brown-Coal.  —  This  is  a  compact  or  earthy  coal,  of  more  or  less 
homogeneous  character,  and  pale  yellow  to  deep  brown  color,  burning 
with  a  sooty  flame  and  strong  odor.  It  is  an  altered  peat  deposit, 
often  still  showing  evidence  of  organic  origin,  and  contains  from  55 
to  75  per  cent  of  carbon.  Its  specific  gravity  ranges  from  0.5  to 
1.5.  Brown-coal  is  most  common  in  the  Tertiary  formations, 
and  often  reaches  a  great  thickness,  this,  in  some  of  the  north 
German  deposits  (Fig.  287),  being  from  75  to  150  feet,  while  in 
Australia  beds  of 
brown-coal  of  much 
greater  thickness  are 
known.  Old  tree 
stumps  in  the  po- 
sition of  growth  are 
commonly  found  in 
these  deposits,  as 
shown  in  the  illus- 
tration (Fig.  287). 

Lignite.  — This 
name  should  be  re- 
stricted to  altered 
woody  tissue  such  as 
is  found  embedded  in 


FIG.  287.  —  Brown-coal  quarry  near  Senftenberg, 
North  Germany.  Many  stumps  of  large  trees  are 
still  standing  in  the  position  of  growth.  (After 
E.  Haase,  from  Walther.) 


brown-coal,  though 
in  some  cases  the  bulk  of  the  deposit  may  be  of  this  origin.  In 
general,  the  woody  structure  is  still  recognizable  in  lignite.  The 
lignites  which  occur  embedded  in  the  north  German  brown-coal 
deposit  still  retain  the  form  of  the  fallen  tree  trunks  and  standing 
stumps  (Fig.  287),  and  they  are  found  to  be  of  the  same  types 
as  those  still  growing  in  our  Dismal  and  other  modern  swamps, 
but  which  have  become  wholly  extinct  in  northern  Europe.  Lig- 
nites are  also  found  embedded  in  sands  and  clays  of  Tertiary  and 
Mesozoic  age  in  many  regions. 

Bituminous  Coal.  —  This  is  the  common  soft  coal,  of  black  color, 
bright  luster,  and  usually  very  brittle  character.  It  contains 
from  75  to  90  per  cent  of  carbon,  and  generally  some  sulphur.  Its 
specific  gravity  ranges  from  1.2  to  1.35,  and  it  burns  with  a  bright, 
clear  flame,  though  some  varieties  cake,  while  others  are  entirely 


344 


The  Organic  or  Biogenic  Rocks 


consumed  to  ashes.     Under  the  microscope,  traces  of  organic  origin 
are  recognizable^  and  impressions  of  plants  are  commonly  found  in 
the  shales  overlying  these  coals  (Fig.  288).     Most  of  the  younger 
coals  (Mesozoic)  are  of  this  character,  as  is  also 
a  large  proportion  of  the  older  coals  (Pennsyl- 
vaniafl)  of  both  America  and  Europe. 

Anthracite  Coal.  —  This  is  hard  coal,  and  is 
the  purest  of  all,  containing  over  90  per  cent 
of  carbon.  It  is  of  black  color,  submetallic  to 
vitreous  luster,  and  breaks  with  a  conchoidal 
fracture.  Its  specific  gravity  ranges  from  1.35 
to  1.7.  It  kindles  with  difficulty,  but  burns 
in  a  strong  draught  with  great  heat  without 
smoke,  caking,  or  odor.  Many  anthracites 
are  found  where  the  rocks  have  suffered  dis- 
turbance, and  they  may  often  be  regarded  as 
somewhat  metamorphosed  bituminous  coals. 
Other  anthracites,  however,  are  of  primary 
origin,  the  vegetable  accumulations  having  lost 
most  of  their  volatile  material  before  burial. 

Graphite ;  Diamond.  —  Graphite  is  pure 
carbon,  soft,  black,  and  with  the  characteris- 
tics of  a  good  lubricant.  It  occurs  chiefly  in  the  ancient  rocks 
which  have  been  subjected  td  metamorphism  and  changed  from 
bituminous  shales,  sandstones,  and  limestones,  to  graphite-bearing 
mica  schist,  gneisses,  and  marbles.  Some  graphite  may  be  due  to 
purely  inorganic  chemical  processes.  Diamond  is  crystallized  car- 
bon, noted  for  its  hardness  and  brilliancy  of  luster, 

Natural  Coke.  —  This  is  another  alteration  product  resulting 
from  contact  metamorphism  of  a  coal  bed  through  the  heat  of  an 
intruded  dike  or  sill.  It  may  also  be  formed  by  burning  of  under- 
ground coal  beds. 

Occurrence  and  General  Character  of  Coal  Deposits 

There  were  three  periods  in  the  earth's  history  when  the  conditions 
for  the  accumulation  of  extensive  deposits  of  vegetable  material 
were  especially  favorable,  and  these  deposits  have  since  been 
converted  into  brown-coal,  bituminous  coal,  or  into  anthracite. 
By  far  the  most  extensive  accumulations  occurred  toward  the  close 
of  the  Palaeozoic  era,  when  all  of  the  important  coals  of  eastern 


FIG.  288.  —  Frag- 
ment of  roof  shale  of 
an  upper  Palaeozoic 
coal  seam,  showing 
the  impression  of  a 
fern.  About  one 
third  natural  size. 
(B.  Hubbard,  photo.) 


Altered  Deposits  of  Older  Vegetal  Material     345 


North  America  (east  of  the  zooth  meridian)  as  well  as  most  of 
those  of  Europe,  and  of  China,  were  formed.  It  has  been  esti- 
mated that  seven  tenths  of  the  coal  deposits  of  the  world  belong 
to  this  period.  The  second  period  was  the  Cretaceous,  in  which  the 
Rocky  Mountain  coals  were  formed;  while  the  third  period,  the 
Tertiary,  witnessed  the  formation  of  the  least  valuable  of  our  coals, 
which  occur  chiefly  west  of  the  i2oth  meridian,  and  also  that 
of  the  extensive  brown-coal  deposits  of  North  Europe.  Some 
coal  is  also  found  in  the  Triassic  of  eastern  North  America,  and 
in  small  amounts  in  other  horizons  in  various  parts  of  the  earth. 

In  general,  much  of  the  coal  was  formed  from  vegetation  which  grew  and 
was  buried  where  the  coal  is  now  found  (autochthonous  deposits),  but  some  of  it 
appears  to  have  been  transported  vegetation  stranded  in  favorable  localities 
(al loch tho nous  deposits).  Most  coals  "appear  to  have  originated  from  vegeta- 
tion which  grew  in  swamps,  on  broad  river  flood  plains,  or  on  coastal  plain  areas, 
and  most  of  them  were  formed  in  fresh  water.  Beds  of  rock  carrying  marine 
fossils  are  as  a  rule  not  directly  associated  with  the  coal  beds,  though  such 
may  lie  between  successive  -seams.  Coals  vary  greatly  in  the  amount  of  mineral 
matter  or  ash  which  they  contain.  Good  coals  have  only  from  i  to  5  per  cent 
of  ash.  When  the  coal  contains  30  or  40  per  cent  of  ash  it  is  called  bony  coal 
and  is  valueless.  Above  that  amount  it  becomes  a  coal  shale. 

A  typical  coal  seam  (Fig.  289)  rests  upon  a  bed  of  under-day 
from  which  much  or  all  of  the  alkaline  material  has  been  removed  by 
the  growth  of  the  plants 
and  by  solution.  This  clay 
is  therefore  suitable  for  the 
formation  of  "  fire  bricks  " 
for  lining  blast  furnaces, 
etc.,  and  the  name  fire  clay 
is  applied  to  it.  Such  clays 
are,  however,  not  always 
present,  and  they  may  occur 
where  no  coal  seam  overlies 
them.  The  seam  itself  may 
vary  in  thickness  from  a 
mere  film  to  many  feet  .  FIG.  289.  -  Typical  coal  beds  or  seams 
,  .m  country  rock  all  steeply  inclined  by 

*lg.    290),    but    the    contl-      subsequent  deformation;  California, 
nuity  of  the  thicker  seams 

is  generally  interrupted  by  coal  shale  or  layers  of  bony  coal.  Over- 
lying it  is  the  roof  shale,  which  is  consolidated  mud,  and  generally 


346  The  Organic  or  Biogenic  Rocks 

contains  the  well-preserved  impressions  of  the  types  of  plants  which 
grew  in  the  coal  swamps.  In  the  majority  of  cases  the  rocks 
between  the  coal  seams  are  sandstones  and  shales,  but  limestone 
may  also  occur. 


FIG.  290.  —  Coal-mine  in  a  mammoth  vein  or  bed. 

ACCUMULATION  OF  DECAYING  ORGANIC  MATTER  FROM  ANIMAL 
TISSUES,  AND  FROM  NON- VASCULAR  PLANTS 

As  we  have  previously  seen,  there  is  in  every  pond  or  lake  a 
region  where  only  the  soft  tissues  of  algae  will  grow,  and  where  by 
their  accumulation  a  layer  of  much  decomposed,  structureless 
material,  more  or  less  mingled  with  mechanical  sediment,  with  lime 
precipitated  by  some  of  these  algae  (Chara),  or  with  the  silicious 
frustules  of  diatoms,  is  formed,  to  which  the  name  sapropelite  has 
been  applied  (Fig.  291).  In  deposits  of  this  kind  the  pollen 
grains  of  coniferous  trees  and  other  plants  growing  on  the  border 
of  the  swamp  usually  abound,  and  spores  of  the  lower  (non-flowering) 
plants  of  the  neighborhood  are  also  common,  and  may  sometimes 
predominate.  In  the  stagnant  portions  of  the  sea-coast,  such  as 
the  channels  behind  the  dead  coral  reefs  (Keys)  of  Florida,  pre- 
viously described,  and  others  like  them,  there  is  mingled  much 
decaying  animal  matter  with  the  decaying  plant  tissues,  and  the 
muds  are  saturated  with  the  product  of  this  decay.  It  should  be 
clearly  understood  that  such  accumulation  cannot,  as  a  rule,  take 
place  in  unenclosed  portions  of  the  sea,  for  the  agitation  of  the 
waters  there  will  hasten  the  dissipation  of  the  products  of  decay, 


Accumulation  of  Decaying  Organic  Matter      347 

and  the  universally  present  bottom  animals  (worms,  mollusks, 
crustaceans,  echinoderms,  etc.)  will  devour  the  organic  material, 
many  of  -them  passing  the  sand  and  mud  through  their  bodies 
and  extracting  its  organic  contents  in  the  process. 

When,   however,   bottom  feeders   are   scarce   or  absent,   such 
organic  material  will  accumulate  either  in  pure  form,  or  become 


--•''  ".'.:•.<  !   •'-":  .'•.':  "'••:'  •>•'  .     "•".-•'.•''*.:-.•. 


FIG.  291. —  Ideal  section,  showing  the  approximate  relation  (i)  of  the  differ- 
ent types  of  peat,  and  (2)  the  plant  societies  of  Algal  Lake,  northern  Michigan. 
(After  C.  A.  Davis.)  The  succession  of  plant  associations  from  without  inward 
is:  (i)  Tamarack-spruce-cedar  swamp,  with  young  tamarack  at  the  inner 
border ;  (2)  open  sedge  marsh,  with  islands  of  tamarack  wood ;  (3)  swamp  loose- 
strife (Decodon  verticillatuni)  gradually  advancing  lakeward,  and  forming  "  stools" 
on  which  grow  mosses,  ferns,  sedges,  and  shrubs,  finally  killing  the  loose-strife ; 
(4)  cat-tail  flags ;  (5)  Potamogeton.  The  peat  formed  by  these  plants  thickens 
away  from  the  lake,  and  is  humus  peat.  Below  this  and  forming  the  lake  bottom 
is  a  mass  of  sapropelitic  peat,  composed  of  green  algae,  with  diatoms,  and  an 
abundance  of  pollen-grains  of  conifers,  forming  a  structureless  mass. 

mingled  with  muds  and  other  sediments.  The  purest  material  is 
chiefly  confined  to  lake  bottoms,  where  it  is  largely  composed  of 
algae  mingled  with  pollen  and  spores.  In  the  sea  the  material  is 
practically  always  mingled  with  foreign  matter  and  so  in  decay 
forms  various  grades  of  sapropelites.  The  principal  regions  for 
the  accumulation  of  such  matter  are  the  following : 

Fresh-water  Lakes.  —  In  these  accumulations  are  chiefly  vegetable 
material  (algae,  etc.),  though  some  animal  matter  may  also  be  included.  This 
material  may  be  very  pure,  forming  on  decay  a  sapropel  (Fig.  291). 

Channels  and  Narrow  Lagoons  between  the  Land  and  Fringing  Coral  Reefs. 
— The  material  accumulating  in  these  is  both  vegetable  and  animal.  The  plants 
are  partly  algae,  but  the  remains  of  other  plants  are  also  included.  The  animal 
tissues  are  those  of  worms,  mollusks,  and  many  other  types.  There  is  always  a 
large  amount  of  mechanical  sediment  in  the  form  of  mud,  and  the  product  is 
black  bituminous  mud- rock  or  shale  (sapropelite),  which  rests  upon  the  pre- 
viously deposited  limestones  derived  from  the  coral  reefs,  etc. 

Mud-flats  Formed  in  the  Process  of  Filling  Lagoons.  — As  we  have  seen, 
there  is  a  stage  in  the  formation  of  marine  marshes  behind  barrier  beaches 
when  the  eel-grass  stage  is  succeeded  by  a  mud-flat  stage,  rich  in  decaying 


348  The  Organic  or  Biogenic  Rocks 

organic  matter,  which,  with  the  exception  of  the  eel-grass,  is  largely  of  animal 
origin.  Such  mud-flats  will  also  form  a  black  bituminous  mud-rock,  which 
underlies  the  coal  bed  formed  by  the  salt  peat  if  the  process  of  transformation 
is  complete. 

Mud-flats  of  Estuaries  and  of  River  Deltas.  —  In  the  estuaries  of  streams, 
i.e.  the  broad  stream  mouths  where  the  river  water  mingles  with  the  encroaching 
tide,  much  fine  mud  is  precipitated,  and  many  sea  animals,  killed  by  the  fresh 
water,  or  river  animals,  killed  by  the  sea-water,  will  be  embedded  in  this  mud 
and  the  decay  of  their  tissues  will  form  sapropelitic  material.  Such  muds  are 
formed  on  the  floor  of  the  Hudson  River  at  New  York  and  in  many  other 
estuaries.  Sometimes  vast  numbers  of  river  fish  are  killed  by  the  advancing 
sea-water,  and  these  form  extensive  fish  beds,  the  muds  being  saturated  by  the 
decaying  organic  matter  of  these  fish.  Again,  vast  masses  of  floating  sea 
animals,  such  as  Foraminifera,  pteropods,  etc.,  may  be  carried  into  the  estuary 
and  killed  by  the  fresh  water,  and  their  remains  sink  to  the  bottom,  where  the 
decaying  organic  matter  will  saturate  the  bottom  sediments.  On  the  mud  deltas 
of  great  rivers,  such  as  the  Mississippi,  the  Nile,  etc.,  much  organic  matter,  both 
vegetal  and  animal,  will  be  buried,  part  of  this  coming  from  the  river  and  part 
being  cast  upon  the  delta  from  the  sea  during  storms  and  high  water.  Chief 
among  this  material  will  be  the  seaweeds  and  the  animals  which  live  attached 
to  them.  In  this  manner  the  mud  becomes  saturated  with  decaying  organic 
matter,  which  will  be  a  mixture  of  coal-producing  higher  plants  and  organic 
slime-producing  algae  and  animal  tissues.  The  mud  ot  the  Nile  delta  contains 
only  from  5.5  to  nearly  8  per  cent  of  organic  matter,  but  that  of  the  Vistula 
carries  as  much  as  23.3  per  cent.  This  river  forms  a  deposit  of  black  mud,  locally 
called  pitch,  in  the  Bay  of  Danzig.  On  the  Mississippi  delta  many  mud 
lumps  or  mud  craters  are  formed,  from  which  large  volumes  of  gas,  the  product 
of  the  decay  of  the  organic  matter,  escape  (see  Fig.  126,  p.  182). 

Enclosed  Stagnant  Seas.  —  The  Black  Sea  is  an  excellent  example  of  a 
water  body  which,  while  still  maintaining  connection  with  the  sea  (Mediter- 
ranean), is  so  nearly  isolated  that  its  waters  are  practically  stagnant,  especially 
in  the  lower  part.  It  has  been  estimated  that  it  takes  1700  years  to  renew 
completely  the  lower  waters  of  this  deep  basin  through  the  shallow  inlets  and 
intermediate  salt  seas  that  connect  it  with  the  Mediterranean.  The  upper 
layers  (125  fathoms)  are  kept  fresher  by  the  inflowing  drainage  from  the  land, 
and  here  many  marine  organisms  live  and  die.  The  young  of  these  float  in 
the  surface  waters,  and  there  are  besides  many  other  surface-living  animals 
(plankton)  and  all  of  these  sooner  or  later  sink  to  the  bottom.  There  is  thus 
a  perpetual  rain  of  organisms  descending  through  the  waters  of  the  Black  Sea, 
and  these  remains  accumulate  upon  the  bottom  where,  because  of  the  stagnant 
water  and  lack  of  oxygen,  there  are  no  bottom  feeders,  only  bacteria.  These 
decompose  the  dead  animal  matter,  and  the  bottom  mud  thus  becomes  richly 
saturated  with  these  products  of  decay,  in  other  words,  it  becomes  a  rich 
sapropelite  in  which,  moreover,  much  sulphur,  usually  in  combination  with 
iron,  occurs. 

Highly  Saline  Lagoons.  —  As  we  have  seen,  the  waters  of  lagoons  sepa- 
rated by  a  bar  from  the  supplying  salt  water  body,  in  regions  of  much 
evaporation,  become  intensely  saline.  The  currents  flowing  into  such  lagoons 


Alteration  Products  from  Organic  Slime        349 

carry  large  numbers  of  animals  from  the  main  water  body,  and  these  are 
quickly  killed  by  the  brine  of  the  lagoon.  In  the  case  of  the  Kara  Bugas  Gulf, 
described  in  Chapter  XI,  untold  numbers  of  fish  and  floating  Foraminifera,  etc., 
are  carried  into  the  brine  from  ftie  Caspian  Sea.  As  there  are  no  scavengers, 
i.e.  animals  devouring  dead  organic  matter,  in  these  brines  as  is  the  case  in 
normal  sea^  water,  the  organic  matter  accumulates  and  is  embedded  with  the 
other  deposits  of  the  lagoon.  This  is  a  prolific  source  of  organic-decay  slime 
and  rich  deposits  of  sapropelite  are  formed. 


ALTERATION  PRODUCTS  FROM  ORGANIC  SLIME  PRODUCED  BY 
NON-VASCULAR  PLANTS  AND  BY  ANIMAL  TISSUES 

According  to  the  degree  of  admixture  of  mud  and  other  foreign 
material  with  the  decaying  organic  matter  from  non- vascular  plants 
(algae)  and  from  animal  tissues,  we  have  a  series  of  deposits  which 
ranges  from  nearly  or  quite  pure  organic  slime  (sapropel)  on  the 
one  hand,  to  mud  deposits,  impregnated  to  a  greater  or  less  degree 
with  this  slime,  on  the  other.  This  last  type  we  have  learned  to 
call  sapropelite,  and  it  is  by  far  the  most  common.  Pure  deposits 
of  the  slime,  however,  also  occur  and  are  known  from  their  alter- 
ation products,  of  which  cannel  coal  forms  the  most  important. 
The  sapropelites  are  generally  known  as  bituminous  muds  or  as 
bituminous  shales  when  consolidated.  The  organic  matter  con- 
tained in  them  is,  however,  often  concentrated  elsewhere  in  the 
form  of  petroleum  and  various  other  hydrocarbons  (asphalts) 
and  may  separate  out  as  natural  gas,  etc.  We  will  briefly  enu- 
merate the  chief  characteristics  of  each  of  the  important  types. 

Cannel  Coal.  —  This  is  generally  held  to  be  formed  from  the  fresh- water 
algae  which  accumulate  in  the  deeper  portions  of  ponds  and  swamps,  and  also 
often  to  a  large  extent  from  spores,  and  from  the  pollen  grains  of  higher 
plants,  blown  into  these  water  bodies.  Cannel  coal  has  a  compact  amorphous 
structure  and  a  dull  luster,  and  commonly  a  greasy  or  silky  appearance  on  the 
fresh  surface,  a  character  quite  distinct  from  that  of  coals  formed  from  the 
higher  plants.  This  is  often  well  brought  out  where  layers  of  cannel  coal 
alternate  with  layers  of  ordinary  bituminous  coal,  or  where  the  roots  of  higher 
plants  were  embedded  in  the  slime  which  formed  the  cannel  coal.  Such  layers 
of  bituminous  coal,  or  such  roots  of  higher  plants,  when  changed  to  coal,  will 
always  have  a  bright  luster  in  marked  contrast  with  the  dull  luster  of  the  cannel 
coal.  That  animal  tissues  also  add  to  the  slime  from  which  cannel  coal  is 
produced  is  shown  by  the  remains  of  their  hard  parts  (bones,  etc.)  in  the 
coal.  Thus  the  cannel  coal  of  Linton,  Ohio,  has  furnished  the  skeletons  of 
more  than  fifty  species  of  fishes  and  amphibians,  many  of  them  represented  by 
numerous  individuals.  Other  names  given  to  cannel  coal  are  boghead  and 


350  The  Organic  or  Biogenic  Rocks 

torbanite  (Scotland).  According  to  some  authorities,  the  waxy  and  resinous 
spores  of  higher  plants  are  more  important  in  the  formation  of  cannel  coal  and 
fresh-water  sapropelites  than  are  the  algae.  They  are  certainly  better  pre- 
served than  the  algae,  and  their  apparent  importance  may  be  more  largely  due 
to  this  fact. 

Jet.  —  This  is  a  deep  black,  brittle,  solid  bitumen,  segregated  in  more  or 
less  lens-shaped  masses  in  rocks  formed  of  muds  which  were  saturated  with 
organic  slime  (sapropelites).  It  appears  to  be  a  concentration  product  of  this 


FIG.  292.  —  Cliff  of  Jurassic  (Liassic)  sandstones  and  shales  at  Whitby  on 
the  North  Sea  coast  of  England.  In  these  strata  the  famous  Whitby  jet  is 
found. 

organic  matter  and  is  often  found  to  saturate  or  replace  pieces  of  fossil  wood 
embedded  in  the  muds  or  is  associated  with  fish  scales  and  the  remains  of  other 
organisms.  Jet  is  characterized  by  its  hardness  (which  is  greater  than  that  of 
asphalt),  its  conchoidal  fracture,  and  by  being  less  brittle  than  coal.  It  is 
susceptible  of  a  high  polish  and  is  much  used  for  ornamental  purposes.  Im- 
portant jet-producing  rocks  are  the  shales  of  Whitby,  England  (Fig.  292),  the 
oily  slates  of  Wiirttemberg,  and  the  similar  shales  of  ancient  Lycia  in  Asia  Minor, 
where  the  exposure  on  the  Gagas  River  forms  the  original  locality  from  which 
this  mineral  (often  called  gagatite  or  gagates)  was  obtained.  An  analysis  of 
jet  from  Wiirttemberg  gave  the  following  result. 

Carbon  (C),  71.0%  Nitrogen  (N),  trace 

Hydrogen  (H),  7.7%  Sulphur  (S),  trace 

Oxygen  (O),  23.3%  Ash,  0.9-2.9% 

Asphaltum  and  Special  Varieties  of  Solid  Hydrocarbons.  —  There  is  a 
considerable  number  of  solid  hydrocarbons  found  in  various  parts  of  the  world, 
and  these  are  probably,  for  the  most  part,  the  products  of  alteration  of  organic 
matter  of  the  non- vascular  plants  or  of  animal  tissues  and  therefore  members 


Alteration  Products  from  Organic  Slime        351 

of  the  sapropel  series.  A  volcanic  or  other  origin,  however,  has  also  been  sug- 
gested for  some  of  them.  They  are  generally  black  and  brittle,  and  distinguished 
from  coals  by  their  fusibility.  The  general  name  asphaltum  or  bitumen  is  given 
to  these,  and  their  distribution  is  world-wide.  To  the  pasty,  viscid  varieties  the 
general  name  maltha  is  applied.  When  sandstones  or  other  rocks  are  saturated 
with  asphalt,  the  mixture  is  called  asphalt  rock.  What  is  probably  the  most 
remarkable  occurrence  of  asphalt  is  found  on  the  island  of  Trinidad  off  the 
northeast  coast  of  South  America,  where,  in  what  is  regarded  as  the  crater  of  an 
old  mud  volcano  or  geyser,  occurs  the  famous  "  Pitch  Lake  "  of  Trinidad,  the 
material  of  which  is  an  emulsion  of  water,  gas,  bitumen,  and  some  other  organic 
substances  and  mineral  matter.  The  water  is  saline  and  contains  also  borates 
and  ammoniacal  salts;  the  gas  is  principally  hydrogen  sulphide  and  carbon 
dioxide.  The  bitumen  is  high  in  sulphur,  the  composition  when  purified  being, 

Carbon  (C),  82.33%  Sulphur  (S),  6.16% 

Hydrogen  (H),  10.69%  Nitrogen  (N),  0.81% 

In  other  hard  asphalts  the  sulphur  has  been  found  to  range  from  3.28  to  9.76 
per  cent,  and  in  soft  asphalts  or  malthas  from  0.6  to  2.29  per  cent. 

Some  of  the  more  important  varieties  of  solid  bitumen  are    (a)   Ozokerite, 
occurring  in  the  Tertiary  rocks  of  the  Caucasus,  the  Carpathians,  and  in  Utah, 


FIG.  293.  —  A  typical  view  of  an  oil  field,  showing  derricks  and  storage  tanks. 

and  forming  an  important  source  of  paraffin;  (b)  Albertite  and  (c)  Grahamite, 
similar  hydrocarbons  occurring  in  vein-like  fissures  in  the  country  rock  in  New 
Brunswick  and  West  Virginia,  respectively,  having  probably  been  injected  when 
in  a  fluid  state,  (d)  Uintaite  or  Gilsonite,  also  a  black,  brittle  and  lustrous 
mixture  of  hydrocarbons  found  in  the  Uinta  Mountains  of  Utah  and  in  many 
other  localities.. 

Petroleum  (Fig.  293). — This  takes  first  rank  among  the  products  of  decay  or 
distillation  of  the  organic  slime,  but  may  also  be  formed  by  the  distillation  of  the 
coaly  deposits  from  vascular  plants.  Nevertheless,  the  organic  matter  of  sapro- 
pelites  (the  sapropel)  i.e.  algae  and  spores  among  plants,  and  animal  tissues 


352  The  Organic  or  Biogenic  Rocks 

probably  forms  the  chief  source  of  petroleum.  Different  petroleum  deposits 
have  a  different  origin,  and  those  of  the  several  geological  horizons  will  be  referred 
to  again  in  their  proper  place  in  the  section  of  this  book  dealing  with  Historical 
Geology.  The  oldest  (Palaeozoic)  petroleum  deposits  of  North  America 
probably  were  derived  from  bituminous  shales  formed  either  in  mud-flats  or  in 
delta  deposits  (Trenton-Utica  oils),  in  lagoonal  deposits,  behind  coral  reefs 
(Onondaga-Marcellus  oils),  or  in  estuaries  (later  Devonian  and  younger  Palae- 
ozoic oils),  though  an  origin  in  enclosed  basins  of  the  Black  Sea  type  has  also 
been  suggested  for  some  of  these.  Although  the  source  of  the  oil  is  the  bitu- 
minous mud-rock,  the  accumulation  takes  place  in  more  porous  limestones  or 
sandstones  (oil  sands)  which  are  associated  with  them,  and  abounds  only  where 
special  structures  (anticlines,  domes,  etc.)  furnish  the  proper  conditions.  In 
the  following  diagrams  (Fig.  294)  the  relationship  of  the  bituminous  shales 
(the  oil  producers)  and  the  limestones  or  sandstones  (the  oil  storers)  is  shown. 


FIG.  294.  —  Diagram  showing  the  relationship  of  the  black  sapropelitic  shales, 
the  oil  producers  (fine  lines),  to  the  limestone  beds  on  the  left  and  the  sands  on 
the  right.  If  the  former  contain  porous  members,  or  areas  of  dolomitization 
(dotted  and  blocked)  these  will  form  oil  pools,  the  oil  passing  laterally  into  the 
reservoir  rock.  The  sandstones  on  the  right  will  also  receive  oil  from  the 
shales,  but  unless  there  is  a  capping-rock,  this  oil  will  become  dissipated. 

As  will  be  noted,  the  oil  will  have  to  pass  laterally  into  the  more  porous  rock 
which  replaces  the  bituminous  mud-rock,  from  the  organic  matter  of  which  the 
oil  is  formed. 

Many  if  not  most  of  the  petroleum  deposits  in  the  younger  (especially 
Tertiary)  rocks  of  America  and  Europe  appear  to  be  derived  from  the  alteration 
of  the  organic  material  of  algae,  especially  diatoms,  and  the  flesh  of  animals, 
ranging  from  protozoans  to  fish  or  even  higher  types.  In  nearly  all  of  the 
great  deposits,  these  organisms  appear  to  have  been  carried  into  enclosed 
bodies  of  water  such  as  the  Black  Sea  or  the  Kara  Bugas  Gulf,  where,  owing 
either  to  the  stagnant  character  of  the  water  or  to  its  high  salinity,  they  perished, 
their  organic  tissues  accumulating  in  the  sediments  because  of  the  absence  in 
these  waters  of  scavengers  or  animals  feeding  upon  such  matter.  Thus  the 
California  oils  are  believed  to  have  been  derived  from  the  soft  tissues  of  marine 
diatoms  which  accumulated  in  vast  quantities  under  special  conditions  which 
insured  the  non-consumption  or  incomplete  dissipation  of  the  decaying  organic 
matter.  Fresh-water  diatoms  accumulating  in  lakes  also  have  furnished  such 
materials.  The  oil  of  the  Caucasus  and  perhaps  of  some  of  the  Carpathian 
regions  may  be  largely  derived  from  the  organic  matter  of  fish  killed  in  large 
quantities  in  the  stagnant  or  highly  saline  waters  of  embayments  or  basins  of 
the  Black  Sea  type,  as  were  the  oil  and  asphalt  of  the  Alsace-Lorraine  region 
and  that  forming  to-day  in  the  vicinity  of  the  Kara  Bugas  Gulf,  where  vast 
numbers  of  fish  are  constantly  killed,  as  already  described. 


Alteration  Products  from  Organic  Slime        353 


Finally,  some  of  the  oil  of  the  Carpathian  region  may  have  originated 
from  the  Foraminifera  and  other  floating  (planktonic)  organisms  carried  into 
similar  lagoons,  in  some  of  which  salts  were  being  deposited. 

It  must  also  be  mentioned  that  petroleum  has  been  held  to  be,  in  some  cases 
at  least,  of  inorganic  origin,  formed  by  descending  waters  which  came  in  contact 

with  heated  metallic  carbides  in 

the  interior  of  the  earth's  crust, 
the  reaction  there  resulting  in 
the  formation  of  metallic  oxides 
and  the  liberation  of  hydro- 
carbon and  of  carbon  dioxide  as 
the  final  product.  This  theory, 
proposed  by  the  Russian  chemist, 
Mendelieff,  is,  however,  very 
generally  discarded  in  favor  of 
the  theory  of  organic  origin, 
which  is  fully  supported  by  the 
facts  known  in  the  cases  of  most 
large  petroleum  deposits. 

Natural  Gas.  —  This  is  es- 
sentially similar  in  origin  to 
petroleum,  representing  the 
volatile  hydrocarbons.  It  is 
commonly  associated  with  pe- 
troleum, but  may  also  occur 
where  this  is  not  found. 

Bituminous  Shales,  Oil 
Shales.  —  These  represent  the 
solidified  muds  of  lagoons,  stag- 
nant basins,  mud-flats,  etc., 
which  are  impregnated  with 
finely  disseminated  organic  mat- 
ter. Although  the  names  are  used  somewhat  loosely,  the  term  bituminous  shale 
is  best  applied  to  those  carrying  finely  divided,  partly  decomposed  remains  of 
vascular  plants,  while  the  oil  shales  are  saturated  with  the  product  of  decay  of 
non-vascular  plant  and  animal  tissues,  i.e.  the  true  sapropel,  and  these  form 
the  true  sapropelites.  Bituminous  shales  thus  pass  into  coal  deposits,  but 
petroleum  may  also  be  derived  from  them.  The  sapropelites,  however,  appear 
to  form  the  true  and  important  oil  shales. 


FIG.  295.  —  A  gushing  oil-well. 


CHAPTER  XIV 

ATMOSPHERIC   PRECIPITATES   AND   THEIR 
DERIVATIVES 

TYPES  or  ATMOSPHERIC  PRECIPITATES 

THE  atmospheric  precipitates  include  rain,  hail,  and  snow; 
their  derivatives  are  ice  and  the  special  form  of  the  latter  —  the 
glaciers.  There  are  other  atmospheric  precipitates,  such  as  nitro- 
gen compounds  formed  by  electrical  discharges,  etc.,  but  these  need 
not  be  considered  here,  though  they  may  at  times  become  of 
importance. 

Materials  in  the  Atmosphere 

As  has  been  stated  in  an  .earlier  chapter,  the  atmosphere  is  a 
mixture  of  oxygen  and  nitrogen  and  contains,  besides  minute 
quantities  of  other  substances,  a  fairly  definite  amount  of  carbon 
dioxide  (CO2)  and  a  variable  quantity  of  water  vapor  (H2O). 
The  carbon  dioxide  is  taken  from  the  atmosphere  by  the  higher 
plants,  which,  by  means  of  the  leaf-green  (chlorophyll)  in  their 
cells,  can,  under  the  influence  of  the  sunlight,  decompose  the  car- 
bon dioxide,  so  that  the  carbon  can  be  utilized  to  build  up  the 
tissues  of  the  plant  while  the  oxygen  is  given  off  again  through  the 
breathing  pores.  Carbon  dioxide  also  combines  with  certain  min- 
erals in  the  rocks  of  the  earth,  but  no  direct  precipitation  of  this 
substance  takes  place,  nor  are  any  notable  compounds  formed  in  the 
air  that  are  precipitated. 

If  we  omit  the  precipitates  formed  by  gaseous  emanations  from 
volcanoes,  the  only  important  substance  which  returns  to  the 
surface  of  the  earth  from  the  atmosphere  is  the  water  held  there  as 
invisible  vapor  and  the  various  forms  which  this  compound  assumes. 

The  Water  of  the  Atmosphere 

Sources  of  Water  Vapor.  —  The  great  source  whence  the  mois- 
ture of  the  air  is  derived  is  the  sea,  and  the  process  of  translation  of 
this  water  into  vapor  form  is  evaporation.  Evaporation,  however, 

354 


Types  of  Atmospheric  Precipitates  355 

takes  place  all  over  the  lands  as  well,  where  not  only  lakes  and 
rivers,  but  also  the  moisture  of  the  soil  is  drawn  upon  to  supply 
the  air  with  water  vapor.  Vegetation  takes  the  moisture  from  the 
ground,  and  this,  circulating  through  the  plants,  is  in  part  evapo- 
rated from  the  leaves,  sometimes  at  such  a  rate  that  the  plant 
droops. 

Absolute  Humidity  of  the  Air.  —  The  total  amount  of  water 
vapor  in  the  air  marks  its  absolute  humidity.  This,  however, 
signifies  little,  for  the  same  amount  of  vapor  in  the  air  at  high 
temperature  will  leave  that  air  very  dry,  while  at  low  temperature 
it  will  be  moist. 

Saturation  of  the  Air.  —  When  the  maximum  amount  of  water 
which  the  air  can  hold  at  any  given  temperature  is  reached,  the 
air  is  said  to  be  saturated.  A  cubic  foot  of  air  can  hold  half  a 
grain  of  water  at  o°  F.,  at  60°  it  can  hold  5  grains,  and  at  80°  it 
can  hold  1 1  grains.  The  air  of  a  room  40  by  40  by  15  feet  in  dimen- 
sions can  hold  nearly  20  pounds  of  water  when  the  temperature  is 
60°  F.,  or  nearly  enough  to  fill  a  common  water  pail.  At  80°  F. 
it  can  hold  more  than  twice  that  amount.  When  the  saturation 
point  of  the  air  is  reached,  the  slightest  fall  in  temperature  will 
cause  precipitation.  This  critical  point  is  called  the  dew  point. 

Relative  Humidity.  —  Ordinarily  the  air  holds  less  water  than  it 
is  capable  of  holding  at  that  temperature.  The  ratio  in  per  cent 
between  the  quantity  of  water  which  the  air  actually  contains 
and  the  maximum  amount  which  it  could  contain  at  that  tem- 
perature marks  its  relative  humidity.  Thus  in  the  case  of  the 
room  cited  above,  if  the  amount  of  water  which  the  air  can  contain 
is  20  pounds,  but  the  amount  which  it  actually  contains  is  only 
10  pounds,  the  relative  humidity  is  50  per  cent.  At  saturation 
(20  pounds  in  the  example  cited)  the  relative  humidity  is  100  per 
cent. 

The  average  relative  humidity  of  the  air  over  the  land  is  about 
60  per  cent,  while  over  the  oceans  it  is  about  85  per  cent.  It  is,  of 
course,  not  uniform,  especially  over  the  land,  where  great  variation 
exists.  When  the  relative  humidity  is  below  65  per  cent,  the  air 
is  said  to  be  dry,  no  matter  what  its  temperature.  Thus  it  appears 
that  regions  of  dry  air  are  far  more  extensive  than  those  of  moist, 
to  make  the  average  humidity  60  per  cent,  or  the  excessive  dry  ness 
of  some  regions  greatly  lowers  the  general  average,  in  spite  of 
the  fact  that  some  localities  have  a  very  high  humidity.  In  semi- 


356    Atmospheric  Precipitates  and  their  Derivatives 

arid  regions  the  relative  humidity  ranges  around  45  per  cent,  and 
in  desert  regions  between  25  and  30  per  cent.  The  lower  the 
relative  humidity,  the  greater  is  the  evaporation  from  the  surface 
of  the  earth. 

Precipitation  of  Atmospheric  Moisture 

When  the  air  is  saturated,  the  slightest  drop  in  temperature  will 
inaugurate  precipitation,  because  with  it  the  capacity  of  the  air 
for  holding  water  is  decreased,  and  the  excess  above  saturation 
must  be  eliminated.  If  the  temperature  of  the  air  is  above  freezing 
point,  this  precipitation  will  take  the  form  of  rain ;  if  below,  it  will 
be  snow  or  hail.  Clouds  merely  represent  the  preliminary  stage 
of  separation  of  the  water  vapor  from  the  atmosphere. 

The  rain  water,  in  so  far  as  it  is  not  re-evaporated,  either  runs 
off  as  surface  water  in  the  form  of  rivulets  which  unite  to  form  larger 
streams  (the  run-off),  or  it  sinks  into  the  ground  to  become  a  part 
of  the  subsurface  water  of  the  hydrosphere.  The  snow,  however, 
generally  accumulates  where  it  falls,  except  for  modification  by  the 
wind  (snow  drifts)  and  becomes  a  permanent  cover  of  unconsoli- 
dated  material  at  least  for  a  period  of  time,  if  the  temperature  of  the 
air  in  contact  with  it  is  below  the  freezing  point.  If  it  is  above 
that,  snow  melts  or  evaporates,  and  the  water  produced  by  melting 
becomes  a  part  of  the  hydrosphere,  running  off  as  surface  water  or 
sinking  in,  to  become  subsurface  water.  If  the  air  becomes  dry, 
i.e.  if  its  relative  humidity  falls  or  a  dry  wind  blows  across  the  snow 
field,  direct  evaporation  of  the  snow  takes  place,  that  is,  it  will 
pass  from  the  solid  directly  to  the  vapor  stage.  As  a  result  of  such 
evaporation  the  surface  of  the  snow  is  commonly  pitted  or  marked 
by  shallow  hollows  or  concavities. 

The  Snow-line 

The  lower  limit  of  the  permanent  snow  fields  of  the  earth  con- 
stitutes the  snow-line  which,  in  general,  corresponds  to  the  line 
which  connects  the  points  where  the  mean  summer  temperature 
of  the  air  is  o°  C.  ( +32°  F.).  There  are,  however,  many  modifica- 
tions of  this  relation,  due  chiefly  to  the  amount  of  precipitation 
during  the  winter  months,  the  amount  of  sunlight,  the  course  of 
drying  winds,  the  steepness  of  the  slope,  the  altitude,  and  the 
protection  in  ravines  and  shady  valleys.  The  height  of  the  snow- 
line  above  sea-level  varies  in  general  with  the  latitude.  In  the 


Glaciers  357 

Bolivian  Andes,  near  the  equator,  it  is  18,500  feet  on  the  western 
side  and  16,000  feet  on  the  eastern.  In  Lapland  (latitude  70°  N.) 
it  lies  3000  feet  or  more  above  the  sea,  and  in  Greenland  (lat.  6o°- 
70°  N.)  about  2200  feet.  In  higher  latitudes  it  approaches  sea-level. 
The  lower  limit  of  snow  fall,  and  the  snow-line,  or  lower  limit 
of  permanent  snow,  do  not  of  course,  correspond.  In  latitude 
40°  N.  snow  falls  at  sea-level,  but  the  permanent  snow-line  is  on  the 
average  3000  meters  higher.  Mountains  in  this  latitude,  below 
that  elevation,  have  permanent  snow  only  in  protected  ravines. 
In  the  tropics  the  difference  in  altitude  between  the  lower  limit  of 
snow  fall  and  the  snow-line  is  much  less. 

COMPACTING  AND  MODIFICATION  or  SNOW 

Granular  Snow.  —  When  precipitated,  snow  is  a  loose  aggregate 
of  needles  and  flakes  of  crystalline  form  and  structure.  This 
crystalline  character  is,  however,  soon  lost  by  partial  melting  and 
evaporation,  and  a  fine  granular  powder  results,  this  representing 
the  first  stage  in  alteration. 

Firn  or  Neve.  —  When  the  grains  of  snow  become  loosely  held 
together,  or  united  by  a  cement  of  ice,  the  neve  or  firn  is  produced. 
The  aggregate  thus  formed  is  full  of  air  bubbles,  and  it  represents 
the  second  stage  in  modification  (Fig.  304,  p.  367). 

Snow  Ice  or  Glacier  Ice.  —  In  this  substance  the  change  has 
progressed  so  far  that  the  mass  has  become  a  granular  crystalline 
ice  in  which  the  individual  crystalline  ice  grains  range  from  the  size 
of  a  pea  near  the  firn  border,  to  that  of  a  hen's  egg  in  the  lower 
portion.  The  crystals  are  intimately  united,  so  that  in  fresh  ice  they 
cannot  be  distinguished.  Ice  is  formed  slowly  from  snow  by  a 
process  of  compacting  which  eliminates  the  air  spaces  of  the  firn 
or  neve  and  brings  the  crystals  in  close  contact.  This  may  be 
produced  by  pressure,  by  percolating  waters  which  by  freezing 
replace  the  air  cavities,  or  by  the  growth  of  the  new  crystals  them- 
selves from  the  moisture  due  to  evaporation  of  the  smaller  crystals, 
or  that  resulting  from  partial  melting. 

GLACIERS 

Glaciers  are  produced  by  the  spreading  or  out-flow  from  the 
center  of  accumulation  of  the  ice  which  has  resulted  from  the 
compacting  of  the  snow.  According  to  the  mode  of  occurrence 


358    Atmospheric  Precipitates  and  their  Derivatives 

of  such  spreading  ice  masses,  we  may  recognize  (i)  true  glaciers,  i.e. 
valley  or  mountain  glaciers,  (2)  confluent  or  piedmont  glaciers, 
(3)  ice  caps,  and  (4)  glacial  sheets  or  continental  glaciers. 

True  Glaciers 

True  glaciers  are  confined  to  more  or  less  definite  channels, 
bounded  generally  by  rock  walls  and  comparable  in  many  respects 
to  streams  of  water  (Fig.  296).  Such  ice  streams  may  reach  a 


FIG.  296.  —  Ideal  glacier  landscape.  A,  firnorneve;  B,  mouth  of  glacier 
tunnel;  C,  glacial  stream;  a,  lateral  moraines;  b,  medial  moraine;  c,  glacial 
table ;  d,  terminal  moraine.  The  ridges  between  the  two  glaciers  are  serrated 
forming  aretes  or  grats.  (After  F.  Simony;  from  Ratzel,  Die  Erde.) 

length  of  fifty  miles  or  more,  as  in  the  great  Seward  glacier  of 
Alaska,  the  main  feeder  of  the  Malaspina  glacier  (see  map,  Fig.  320, 
p.  381)  in  which  the  width  at  the  narrowest  point  is  three  miles 
(Fig.  321,  p.  382).  Most  of  the  alpine  glaciers,  on  the-other  hand, 
are  not  over  a  mile  long,  though  the  largest,  the  Aletsch  Glacier, 
is  more  than  ten  miles  in  length.  The  thickness  of  the  alpine 
glaciers  rises  in  some  cases  to  800  and  in  exceptional  ones  to  1200 
feet. 

The  most  typical  glaciers  are  found  upon  the  mountain  sides 
and  in  well-defined  mountain  valleys,  the  best  known  examples  be- 
ing in  the  Alps.  They  are  commonly  called  valley  or  mountain 
glaciers,  the  latter  term  applying  especially  to  the  short  glaciers 


Glaciers  359 

which  lie  in  the  depressions  in  mountain  sides,  these  depressions 
being  often  due  to  the  erosive  work  of  the  glaciers  themselves. 
Typical  valley  glaciers,  also  called  dendritic  glaciers,  because  they 
usually  have  many  branches,  most  commonly  occupy  large  structural 
troughs  in  the  mountain  regions,  especially  in  high  latitudes,  though 
they  may  also  fill  old  erosion  valleys.  The  course  of  a  typical  valley 
glacier  may  be  straight,  but  is  more  often  a  winding  one.  It  may 


FIG.  297.  —  Abrupt  front  of  an  Alaskan  glacier,  with  a  deglaciated  valley  in 
front  partly  occupied  by  a  glacial  stream.  (Seward  division,  Gov't  R.  R. ;  by 
courtesy  of  Alaska  Engineering  Commission.)  Compare  Fig.  314,  p.  376, 
abrupt  glacier  front  on  coast. 

be  simple  throughout,  or  two  or  more  streams  may  unite  to  form  a 
main  trunk,  as  in  the  case  of  the  junction  of  several  rivers  into  a 
main  trunk  stream,  while,  moreover,  the  main  glacier  may  receive 
lateral  tributaries.  According  to  location,  altitude,  and  other 
causes,  the  lower  end  of  the  glacier  may  be  more  or  less  abrupt 
(Fig.  297)  or  it  may  spread  out  upon  the  flat  foreland  to  form  a  fan 
glacier,  which  by  the  union  with  similar  fans  of  neighboring  ice 
streams  forms  a  piedmont  glacier.  Examples  of  the  several  types 
will  be  described  in  some  detail. 

The  Great  Aletsch  Glacier  of  the  Alps  (Figs.  298,  299).  — In 
the  Bernese  Alps  of  central  Switzerland,  where  the  Jungfrau,  the 
Aletschhorn,  and  other  great  peaks  dominate  the  landscape,  we 
find  a  great  center  of  modern  alpine  glaciation.  From  the  southern 


360    Atmospheric  Precipitates  and  their  Derivatives 


FIG.  298.  —  Map  of  the  Aletsch  Glacier  and   the  surrounding  territory  in 
the  Bernese  Alps  of  Switzerland.    From  the  Swiss  Government  map. 


Glaciers 


361 


and  eastern  sides  of  these  mountain  ranges  descends  the  Great 
Aletsch  Glacier  toward  the  valley  of  the  Rhone,  without,  however, 
reaching  it.  This  glacier,  which  in  many  respects  is  the  finest  in 


the  Alps,  occupies  a  mountain  valley  for  about  ten  miles  of  its  length, 
and  is  bordered,  for  the  most  part,  by  high  ranges  and  peaks  on 
either  side,  the  largest  of  which  is  the  Aletschhorn  (4182  meters  or 


362     Atmospheric  Precipitates  and  their  Derivatives 


13,768  feet  high,  132  feet  higher  than  the  Jungfrau)  from  the  base 
of  which  tributary  glaciers  join  the  main  ice  stream  (Fig.  298). 


Valley  of  Viesch 


FIG.  300  a.  —  Diagrammatic  section,  showing  the  relative  position  of  the 
Marjelen  Lake,  the  Aletsch  Glacier  which  holds  it  and  the  valley  of  the  Viesch 
into  which  it  drains  when  full,  a,  b,  col  or  dividing  ridge  between  the  two 
valleys;  c,  vertical  cliff  of  ice  forming  the  dam.  (After  Lyell.) 

On  its  eastern  border  a  tributary  valley,  partly  free  from  ice,  is 
dammed  by  the  glacier  of  the  main  valley,  and  behind  this  ice 
dam  lies  the  famed  and  beautiful  Marjelen  Lake,  a  typical  example 


FIG.  300  b.  —  Section  of  glacier  lake  called  the  Marjelen  See.  a,  b,  c,  terrace 
of  detrital  matter  formed  on  the  margin  of  the  lake  when  full ;  d ,  surface  of  the 
lake  40  feet  below  its  usual  level ;  e,  mass  of  floating  ice  with  included  stones 
detached  from  the  dam;  /,  boundary  hill  composed  of  mica  schist.  (After 
Lyell.) 

of  a  lake  produced  by  ice-damming  of  a  valley  (Fig.  299).  This 
lake  is  periodically  drained,  partly  or  completely,  every  three 
to  five  years  by  the  opening  of  some  outlet  under  the  ice,  with  dis- 
astrous floods  in  the  valley  below.  Ordinarily,  however,  it  is  held 


Glaciers  363 

in  by  a  precipitous  wall  of  ice  of  the  Aletsch  glacier.  After  drainage 
it  fills  up  in  about  a  year  to  a  level  determined  not  by  the  height 
of  the  glacier  dam,  which  always  rises  much  above  the  lake,  but 
by  the  water-shed  or  col  which  separates  the  lake  from  the  valley 
of  the  Fiesch  (or  Viesch)  glacier  on  the  east  into  which  it  drains 
when  full  (Fig.  300  #).  Around  the  margin  of  this  lake,  terraces  or 
beaches  of  sand  and  gravel  are  built,  which  are  exposed  when  the 
lake  is  drained.  A  characteristic  section  of  such  a  beach  (Fig.  300  b) 
shows  a  surface  shelf,  sloping  from  5°  to  15°  toward  the  lake,  and 
about  1 6  paces  wide.  At  the  edge  of  this  slope,  which  marks  the 
level  of  the  lake  when  full,  there  is  a  sharp  change,  the  beds  de- 


FIG.  301  a.  —  The  Parallel  Roads  of  Glen  Roy.     (W.  Lamond  Howie.) 

scending  at  an  angle  of  29°.  This  is  the  underwater  slope  of  the 
deposit,  and  represents  the  angle  at  which  the  beds  (fore-sets) 
are  laid  down.  The  sands  are  bedded,  but  contain  no  organic 
remains,  since  the  temperature  of  the  water  is  always  near  freezing. 
Over  the  beach  and  bottom  are  scattered  many  large  blocks  of 
stone  left  by  icebergs  which  break  from  the  main  glacier  wall. 

In  one  of  the  glens  in  the  Scottish  Highlands,  known  as  Glen 
Roy,  there  are  several  old  beach  lines  at  successive  levels,  along  the 
sides  of  the  glen,  which  are  known  under  the  name  of  the  Parallel 
Roads  of  Glen  Roy  (Fig.  301  a,  6).  The  natives  regard  these  as 
roads  made  by  the  "gentry"  for  fishing  purposes  when  the  glen  was 
a  lake.  A  study  of  the  glen  has  shown  that  it  was  once  blocked 
by  a  glacier  at  the  lower  end,  as  is  the  Marjelen  Lake  to-day,  and 
that  it  was  filled  by  a  lake,  the  several  levels  being  due  to  the 


364    Atmospheric  Precipitates  and  their  Derivatives 


FIG.  301  b.  —  Photograph  of  one  of  the  terraces 
or  "parallel  roads"  in  Glen  Roy,  Scotland,  showing 
the  distinct  notch  which  it  forms  upon  the  side  of 

,  ,  j       ,         j  if  r  • 

the  glen,  and  the  dense  growth  of  peat-forming 
vegetation  ;  largely  heather.     (Photo  by  author.) 


progressive  opening  of  cols  permitting  drainage,  at  successively 
lower  levels,  into  other  valleys  after  the  manner  of  the  drainage  of 
the  Marjelen  Lake  into  the  Viesch  Valley,  the  col  of  which  deter- 

mines the  height  of 
the  lake  and  the 
beaches  (Fig.  302). 

From  the  foot  of 
the  Aletsch  Glacier 
issues  the  turbulent 
Massa  River,  which 
joins  the  Rhone  sev- 
eral miles  below,  and 
more  than  doubles 
the  volume  of  that 
stream.  It  has  been 
sai^  that  other  gla- 
ciers  send  forth  tor- 
r^fc  err..  fi,Q  ,v~ 

1CJ.1LS      llVJlil       LUC      ICC 

caverns  at  their  foot  ; 

this  one  alone  pours 
out  a  river.  The  Rhone  carries  down  to  Naters  the  drainage  of 
its  own  glaciers  supplemented  by  a  dozen  other  ice-fed  streams; 
yet  these  combined  waters  are  far  exceeded  in  volume  by  those 
brought  by  the  single  stream  from  the  Aletsch  Glacier.  It  has 
been  questioned,  indeed,  whether  the  united  torrents  of  any  four 
glaciers  in  the  Alps  could  equal  that  single  stream. 

A  fine  view  of  the  glacier  is  obtained  from  the  summit  of  the 
Eggishorn  (Fig.  299),  a  peak  just  south  of  Marjelen  Lake  and 
opposite  the  tributary  Aren  or  Middle  Aletsch  glacier,  which  heads 
in  an  amphitheater  in  which  lies  the  middle  Aletsch  neve,  at  the 
eastern  foot  of  the  Aletschhorn  (see  map,  Fig.  298).  Farther  down, 
on  the  same  side,  is  the  little  Trist  or  Upper  Aletsch  Glacier  or 
glacieret,  which  at  present  does  not  reach  the  main  ice  stream  but 
rests  in  a  valley,  the  floor  of  which  is  high  above  the  surface  of  the 
great  glacier.  This  little  glacier  is  therefore  called  a  hanging 
glacier  or  glacieret,  being  hung,  as  it  were,  upon  the  valley  side  of 
the  great  one.  In  former  times  this  did,  however,  extend  to  the 
main  glacier.  It  heads  in  the  upper  Aletsch  neve  south  of  the 
Aletschhorn,  and  another  one,  the  Beichfirn  or  neve,  is  also  tribu- 
tary to  it.  These  neves  lie  in  more  or  less  horseshoe-shaped  valleys 


Glaciers 


365 


366    Atmospheric  Precipitates  and  their  Derivatives 

or  cirques,  and  between  them  rise  the  pyramidal  peaks  of  triangular 
base  to  which  the  designation  horn  is  applied. 

The  great  Aletsch  Glacier  itself  heads  in  a  group  of  large  neves 
which  extend  north  of  the  Dreieckhorn  and  Aletschhorn  and 
toward  the  foot  of  the  Jungfrau.  On  the  east  lies  the  Wannehorn, 
the  western  face  of  which  is  marked  by  a  number  of  small 
cirques,  the  neves  of  which  are  tributary  to  the  Great  Aletsch 
Glacier. 


FIG.  303.  —  Typical  glacial  cirque  with  large  amount  of  moraine  deposited 
in  foreground.  -Massif  de  Pelvoux,  Alpes  d'Oisaus,  France.  (Courtesy  of 
Prof.  D.  W.  Johnson.) 

These  cirques,  when  well  developed  (Fig.  303),  are  semicircular 
or  amphitheater-shaped,  with  rough,  precipitous  walls,  and  com- 
paratively smooth  floors  filled  with  neve.  Their  precipitous  walls 
are  due  to  the  plucking  and  sapping  action  of  the  ice  which  freezes 
to  the  blocks  and  in  the  cracks  at  the  base  of  the  cliff,  and  under 
the  influence  of  partial  melting  and  refreezing  pushes  away  from 
the  wall,  carrying  the  plucked  or  quarried  blocks  with  it.  As  the 
ice  moves  away  from  the  walls  of  the  cirques  a  crevasse  comes 
into  existence  between  the  rock  wall  and  the  ice.  This  crevasse, 
called  the  bergschrund,  may  vary  in  width  from  two  or  three  to 
more  than  eighty  feet,  and  in  depth  to  150  feet  or  more.  At  the 


Glaciers 


367 


bottom,  much  disruption  of  rock  takes  place  by  the  freezing  of 
the  water  which  drips  into  it. 

In  mountains  which  were  formerly  covered  by  glaciers,  such 
cirques  are  characteristic  features,  and  their  presence,  readily 


FIG.  304.  —  Glacier  of  Three  Sister  peaks,  Oregon,  between  West  and  North 
Sister  (the  latter  shown  in  the  view).  —  Showing  junction  of  glacier  and  neve" 
and  marginal  crevasses.  (Photo  by  I.  C.  Russell,  Aug.  18,  1903,  U.  S.  G.  S., 
by  courtesy  of  Popular  Science  Monthly.} 

recognized  by  their  peculiar  form,  is  in  itself  a  proof  that  such 
mountains  formerly  held  glaciers,  even  though  these  have  entirely 
disappeared.  As  cirques  are  progressively  cut  backwards  into  the 
mountains,  their  walls  increase  in  height,  and  if  cirques  approach 
each  other  from  opposite  directions,  the  space  between  them  is 
narrowed,  and  eventually  only  a  sharp,  narrow,  serrated  ridge 


368    Atmospheric  Precipitates  and  their  Derivatives 


remains  between  them,  from  which  arise  the  sharp  horns  (Fig.  296). 

Later  still,  parts  of  the  ridge  will  be  lowered  to  form  a  connecting 

col  between  two  cirques  (see  further,  section  on  glacial  erosion  in 

Chapter  XXIII). 

The  neves  of  the  Aletsch  really  occupy  three  large  and  many 

small  cirques  or  embayments  in  the  mountain  complex.     On  the 

west  between  the  Dreieckhorn  and  the 
Gletscherhorn,  lies  the  great  Aletsch 
neve  which  also  feeds  the  Lang  Glacier 
descending  to  the  southwest.  On  the 
northwest  lies  the  Jungfrau  firn  or 
neve,  which  is  bounded  by  the  Glet- 
scherhorn, Jungfrau,  and  the  Monch  on 
the  west  and  north,  and  the  Trugberg 
ridge  on  the  east.  The  Ewigschnee 
Feld  to  the  east  of  the  Trugberg  ridge 
completes  the  trinity,  while  a  fourth 
smaller  one  north  of  the  Faulberg  de- 
scends from  the  Griinhorn  Liicke  on 
the  east.  Crevasses  are  not  uncom- 
mon on  these  neves,  but  the  berg- 
schrund  is  not  everywhere  developed. 
In  some  cases  the  neves  connect  across 
the  cols  or  passes  with  others  which 
feed  glaciers  descending  in  other  direc- 
tions. 


FIG.  305.  —  Section  of  an 
old  glacial  pot-hole  filled  with 
debris,  Christiania.  (After 
Brogger.) 


The  surface  of  the  Aletsch  glacier,  which  is  about  two  miles 
wide,  is  often  almost  free  from  stones  and  rock  debris  except  along 
its  sides,  where  the  material  falling  from  the  bounding  cliffs  forms 
lateral  moraines.  This  glacier,  unlike  most  other  large  glaciers  of 
the  Alps,  has  as  a  rule  no  medial  moraine  or  only  a  feeble  one. 
Along  the  center,  the  ice  is  mostly  solid,  though  narrow  fissures  or 
crevasses  frequently  open  in  it.  Such  crevasses  are  much  more 
pronounced,  however,  on  its  eastern  margin,  which  has  a  convex 
form  (see  also  Fig.  304).  The  melting  of  the  ice  upon  the  surface 
produces  pools  and  streams,  which  occasionally  combine  into 
small  rivers.  These  tumble  into  the  crevasses  and  produce  vertical 
chimneys  or  moulins,  which  carry  the  waters  to  the  bottom  of  the 
glacier  where,  by  their  whirling  motion,  they  often  cut  "  pot- 
holes "  in  the  bed  of  the  ice  stream.  In  regions  from  which  glaciers 


Glaciers 


369 


have  departed,  such   pot-holes   are    sometimes   seen   filled   with 
debris  (Fig.  305). 

The  Mer  de  Glace  (Figs.  306,  307).  —  From  the  northern  slope 
of  the  Mont  Blanc  glacial  field  on  the  Franco-Swiss  border  descends 
the  glacier  Mer  de  Glace 
toward  the  Chamonix  Val- 
ley. At  the  head  of  the 
glacier  is  a  complex  of 
cirques,  which  radiate  out- 
ward from  the  stem  glacier 
to  which  they  are  tribu- 
tary, as  the  veins  of  a 
maple  leaf  radiate  from  the 
petiole.  From  these 
cirques,  short  glaciers  or 
glacierets  unite  to  form  the 
trunk  glacier,  which  flows 
toward  the  Chamonix 
Valley.  This  is  the  more 
common  type  of  glacier 
found  in  the  Alps,  and 
though  the  branches  are 
given  different  names,  they 
are,  in  a  manner,  only 
tributary  heads  of  one  prin- 
cipal stream.  These  smaller 
glaciers  commonly  have  a 
much  steeper  slope  than 
the  trunk  glacier,  and  many 
of  them  suggest  ice  cas- 
cades, being  indeed  so 
named,  as  for  example  the 
Cascade  du  Talefre,  which 
joins  the  Mer  de  Glace 


FIG.  306.  — •  Map  of  the  Mer  de  Glace, 
French  Alps,  showing  four  medial  moraines 
and  the  terminal  glacial  stream,  the  Ar- 
veyon.  (After  Le  Conte.)  The  feeding 
glaciers  are  :  on  the  right,  Glacier  du  Geant, 
with  ice  cascades  and  seracs,  and  its  tribu- 
tary, the  Glacier  des  Periades;  in  the 
center,  the  Glacier  de  Leschaux ;  and  on  the 
left,  the  Glacier  de  Talefre  —  also  showing 
ice  cascades  and  seracs. 


from  the  east,  and  the 
Cascade  of  the  Glacier  du  Geant  on  the  west  (see  map,  Fig.  306). 
Such  steep  glaciers,  occupying  the  depressions  in  the  mountain 
sides,  are  more  properly  termed  mountain  glaciers,  and  they  are 
the  most  common  among  the  2000  or  more  glaciers  of  the  Alps, 
most  of  which  are,  however,  less  than  a  mile  in  length. 

v 


370    Atmospheric  Precipitates  and  their  Derivatives 


Each  of  these  tributary  glaciers  has  its  lateral  moraine  of  rock 
debris,  partly  derived  from  the  lateral  wall,  but  also  in  part  brought 
up  from  below  (see  page  493)  by  movements  within  the  ice.  As 
two  streams  unite,  the  lateral  moraines  on  adjoining  sides  combine 
to  form  a  medial  moraine,  a  more  or  less  continuous  line  of  rock- 
fragments,  sand,  etc.,  which  stretches  along  the  central  portion 
of  the  confluent  glacier  thus  formed.  In  the  Mer  de  Glace  there 
are  several  such  medial  moraines  forming  parallel  lines  upon  the 
trunk  glacier,  which,  moreover,  has  its  own  lateral  moraines. 


r  -^ 


FIG.  307.  —  The  Mer  de  Glace  near  Chamonix,  French  Alps.  Note  the 
snubbed  lower  ends  of  the  mountain  ridges  indicating  the  beginning  of  the  U-- 
shaped glacial  valley  form.  (Photo  D.  W.  Johnson.). 

At  the  foot  of  the  glacier  where  the  ice  undergoes  melting,  this 
material  comes  to  rest  to  form  the  terminal  moraine,  which  is  also 
built  in  part  of  material  carried  along  within  or  on  the  bottom  of 
the  ice  (englacial  and  subglacial  detritus).  From  the  melting  of 
the  ice,  glacial  streams  arise,  these  heading  often  far  back  in 
tunnels  under  the  ice  as  subglacial  streams. 

Other  Glaciers  of  Similar  Character.  —  Though  the  glaciers  of 
the  Alps  are  the  most  familiar,  and  in  many  respects  best-studied 
there  are  many  others  in  different  parts  of  the  world  which  show 
the  characters  so  far  described  on  a  much  larger  scale.     In  the 
Karakoram  Himalayas,  several  long  and  narrow  valley  glaciers  are 


Glaciers 


found,  which,  moreover,  have  numerous  lateral  tributaries  similar 
to  streams  of  water.  These  form  the  so-called  dendritic  or  true 
valley  type,  of  which  the 
Great  Aletsch  is  a  small 
example  with  few  tribu- 
taries. Of  these  larger  ex- 
amples, the  Hispar  Glacier 
(Fig.  308)  has  a  length  of 
over  36  miles,  and  is  char- 
acterized by  numerous  trib- 
utaries on  both  sides,  which  FlG  30g  _  Hispar  Glacier>  a  typical 
join  it  approximately  at  valley  glacier,  with  numerous  tributaries, 
right  angles.  The  main  Karakoram,  Himalayas.  (After  Martin 

Conway.)      Ice     and     streams    in    black 

glacier  here  is  very  straight.      dramage  territory  in  dotted  line. 
The  Baltoro  Glacier  of  the 

same  region  and  of  similar  length  occupies  a  curving  valley,  and 
its  tributaries  also  join  mostly   at    right   angles.     They   supply 


Miles: 


FIG.  309.  —  Three  of  the  largest  ice  tongues  of  the  Swiss  Alps  superposed  on 
the  same  scale  over  Hubbard  Glacier,  Alaska.     (Canadian  Geol.  Survey.) 


372    Atmospheric  Precipitates  and  their  Derivatives 


Glaciers 


373 


morainal  material  from  different  sources,  and  as  a  result  this  glacier 
has  15  moraines  of  different  colors.  The  Tasman  Glacier  of  New 
Zealand  is  another  example.  The  Hubbard  Glacier  ?f  Alaska 
(location:  map,  Fig.  320,  p.  381)  exceeds  the  combined  area  of 
the  Aletsch,  Rhone,  and  Mer  de  Glace,  as  shown  in  the  diagram 
on  page  37 1  (Fig.  309). 

Characters  of  the  Glacial  Valley.  —  When  such  glaciers  shrink 
with  a  change  in  climate,  or  disappear  entirely,  their  beds  become 
exposed  and  show  characteristic  features.  The  form  of  the  valley 


FIG.  311.  —  Rounded  rock  surfaces  or  Roches  Moutonnecs,  due  to  erosion  by 
a  former  glacier,  Colorado.     (After  Hayden.) 

often  resembles  in  section  the  letter  U,  with  the  sides  approximately 
perpendicular  and  the  bottom  rounded  (Fig.  310).  This  is  due 
to  the  erosive  action  of  the  ice  and  is  the  characteristic  form  of  a 
young  glaciated  valley  or  one  deepened  by  glacial  work.  Many 
ancient  valleys  in  regions  now  entirely  free  from  ice  have  this 
form,  and  point  to  former  glacial  occupancy  and  erosion  or  deepen- 
ing by  ice  (see  also  Chapter  XXIII). 

The  eroded  bottom  of  such  a  valley  often  shows  hummocky 
surfaces,  sloping  and  smooth  on  the  side  from  which  the  glacier 
moved  (stoss-side)  and  with  striated  surfaces,  but  rough  and 


374    Atmospheric  Precipitates  and  their  Derivatives 


cliff ed  on  the  side  away  from  the  movement  (lee  side).  Such 
rock  surfaces  when  seen  from  above  have  a  fanciful  resemblance 
to  a  flock  of  crouching  sheep,  on  which  account  the  French  have 
called  them  roches  moutonnees,  a  name  which  has  been  generally 
adopted  for  such  erosion  surfaces  (Fig.  311).  They  are  not  con- 
fined to  glaciated  valley  floors,  but  occur  in  regions  of  continental 
glaciation  as  well. 

Where  tributary  valleys  join  the  main  valley  they  are  generally 
found  to  do  so  at  a  point  much  above  the  floor  of  the  latter. 
The  lateral  valleys  are  indeed  hanging  valleys,  there  being  an 

abrupt  descent  from 
the  valley  bottoms  at 
their  mouths  to  the 
bottom  of  the  main 
trough.  This  is  due  to 
the  fact  that  the  main 
ice  stream  deepens  its 
valley  more  readily 
than  the  smaller  lateral 
glaciers  deepen  theirs, 
and  although  the  sur- 
faces of  lateral  glaciers 
ussually  accord  with 
that  of  the  main  stream 
to  which  they  are  tribu- 
tary, .their  bottoms  do 


FIG.  312.  —  Diagram  to  illustrate  the  relation- 
ship of  main  and  tributary  glaciers.  The  sur- 
faces of  the  two  glaciers  are  in  accord,  but  the 
valley  of  the  main  glacier  is  deepened  much 
below  that  of  the  tributary  glacier.  On  the 
melting  of  the  glaciers  the  valley  of  the  tribu- 
tary glacier  will  be  a  hanging  valley,  its  mouth 
joining  the  main  valley  at  some  height  above 
its  floor.  A  river  flood-plain  deposit  is  formed  in 
the  main  valley  after  abandonment  by  the  ice, 
while  an  alluvial  fan  forms  at  the  mouth  of  the 
tributary  hanging  valley.  (After  Davis.) 

not     correspond    (Fig. 

312).  Along  the  borders  of  the  great  fjords  in  the  "  inside  passage  " 
to  Alaska,  there  are  very  many  fine  examples  of  such  hanging 
valleys.  These  fjords  themselves  represent  the  old  valleys  of  the 
main  or  trunk  glaciers,  now  abandoned  by  the  ice  but  filled  instead 
by  water  from  the  sea.  (See  further,  Chapter  XXIII.) 

Movement  of  Mountain  and  Valley  Glaciers.  —  Experiments 
have  been  made  on  the  Aar  Glacier,  in  the  north  of  the  Bernese 
Alps,  where  Agassiz  conducted  his  pioneer  studies,  on  the  glaciers 
at  the  head  of  the  Rhone  valley  and  on  others,  to  determine 
the  character  and  rate  of  movement  of  valley  and  mountain 
glaciers.  A  line  of  stakes  driven  into  the  ice  across  the  glacier 
was  accurately  located  by  instruments  with  reference  to  the  rock 
walls.  Measurements  of  their  advance  were  made  at  intervals  to 


Glaciers 


375 


determine  the  rate  and  mode  of  progress.     A  significant  fact 
discovered  was  that  the  center  of  the  ice  stream  moved  faster 


FIG.  313.  —  Map  showing  the  movement  of  the  ice  of  the  Rhone  Glacier  in 
Switzerland,  between  the  years  1874  anc*  1882  at  B,  C,  and  D ;  the  retreat  of  the 
ice  front  by  melting  during  the  same  period,  and  the  several  terminal  moraines. 
(After  Heim;  from  Chamberlin  and  Salisbury,  Geology.  By  permission  of 
Henry  Holt  &  Co.)  (See  also  Figs.  319  a  and  b,  p.  380.) 

than  the  sides,  for  the  alignment  of  the  stakes,  originally  straight, 
became  more  and  more  curved  downstream  at  the  center.  This 
is  well  shown  in  the  preceding  map  of  the  Rhone  Glacier  from 


376    Atmospheric  Precipitates  and  their  Derivatives 

which  the  Rhone  River  arises  (Fig.  313)',  the  movements  indicating 
the  advances  between  the  years  1874  and  1882.  The  fluctuation 
in  the  position  of  the  front  of  the  glacier  is  also  shown,  and  the 
locations  of  the  terminal  moraines  at  earlier  periods  are  indicated. 
In  general,  the  rate  of  movement  of  Swiss  glaciers  ranges  from 
one  or  two  inches  to  four  feet  or  more  a  day,  but  glaciers  in  other 
parts  of  the  world  have  shown  a  greater  rate  of  advance.  Thus 
the  Muir  Glacier  of  Alaska  (Fig.  314)  has  moved  at  the  rate  of 
seven  feet  or  more  a  day,  while  some  Greenland  glaciers  have 


FIG.  314.  —  Front  of  Muir  Glacier,  Glacier  Bay,  Alaska,  from  the  east.  In 
the  distance  is  Morse  Glacier.  (U.  S.  G.  S. ;  courtesy  of  Prof.  D.  W.  Johnson.) 
(See  also  Fig.  318,  p.  379.) 

moved  in  summer  time  as  much  as  50  or  60  feet  a  day.  These 
glaciers  are,  however,  of  a  different  type,  being  tongues  from  an  ice 
mass  of  great  extent. 

The  top  of  the  glacier  moves,  on  the  whole,  faster  than  the 
bottom,  as  was  shown  by  a  line  of  stakes  driven  into  the  side  wall 
of  a  glacier  in  a  favorable  spot.  Here,  after  a  while,  the  alignment 
of  the  stakes  changed  from  vertical  to  forward  sloping  at  the  top. 

Surface  Features  Due  to  Ablation.  —  The  surface  of  a  glacier  is 
affected  by  the  heat  of  the  sun  and  by  drying  winds,  with  the 
result  that  irregularities  are  produced  by  melting  and  evaporation. 


Glaciers 


377 


FIG.  315.  — '' Glacier  -Tables,"  ice-pillars  pro- 
tected by  slabs  of  rock.  Parker  Creek  Glacier, 
California.  (After  Russell.) 


When  the  ice  is  much  fissured  by  crevasses,  as  along  the  outer 
margins  where  stresses  occur,  or  where  the  ice  passes  over  an 
abrupt  change  in 
the  slope  of  the  bot- 
tom, the  melting 
along  these  cre- 
vasses may  pro- 
duce a  rough  ridge 
or  pinnacle  topog- 
raphy. Where  the 
surface  is  protected 
by  rocks  and  other 
debris  of  the  mo- 
raines, which, 
though  absorbing 

heat  on  the  surface,  do  not  readily  conduct  it  downwards,  the  sur- 
rounding exposed  ice  will  melt  more  rapidly,  leaving  the  protected 
portions  in  relief.  A  variety  of  special  features  may  thus  be 
formed,  among  which  ice  tables  consisting  of  large  blocks  of  stone 
supported  by  ice  pillars  are  perhaps  the  most  striking  (Fig.  315). 

Small  stones  and  dust  particles,  on  the 
other  hand,  by  absorbing  the  heat, 
will  cause  a  more  rapid  melting  and 
will  sink  into  the  ice,  forming  depres- 
sions (dust  wells).  Thin,  bouldery  mo- 
raines are  sometimes  sunk  below  the 
surface  in  this  manner,  whereas  a  thick 
moraine  comes  to  rest  upon  a  ridge  of 
ice,  and  the  spreading  of  the  debris 
down  the  slope  of  the  ridges  may  lead 
to  a  partial  covering  of  the  ice  sur- 
face, especially  in  the  lower  part  of 
the  glacier. 

The  Expanded  Foot  of  Glaciers.  — In 
Arctic  regions  the  valley  glaciers  con- 
tinue to  the  flatter  lowland  or  to  the 
sea-margin,  where  they  expand  in  a 
broad,  flat  lobe,  a  sort  of  fan-like  ice 


"»--"•     ~    0 


FIG.  316.  —  Map  of  Baird 
Glacier,  a  typical  expanded- 
foot  glacier,  and  of  Miles 
Glacier,  with  partly  ex- 
panded foot  and  marginal 
lake  formed  by  expansion  of 
Copper  River,  Alaska.  (After 
Tarr  and  Martin.) 


delta  or  ice  apron,  or  better,  an  ice  lake  without  retaining  margins. 
The  Foster  Glacier  of  Alaska  expands  into  such  a  foot  on  the  coast, 


378    Atmospheric  Precipitates  and  their  Derivatives 

and  the  Baird  Glacier  of  Alaska  expands  in  a  similar  manner  in  the 
valley  of  the  Copper  River,  which  it  has  pushed  to  the  opposite 
side  (Fig.  316).  This  is  indeed  the  best  known  example  of  such 
an  expanded  foot  of  a  glacier.  The  Miles  Glacier,  from  the 
opposite  side  of  the  same  valley,  expands  in  a  less  regular  manner 
and,  moreover,  forms  a  lake  of  the  river  along  its  front.  Other 
examples  in  Alaska  are  the  Davidson  Glacier  on  the  Lynn  Canal 
and  the  Mendenhall  Glacier.  When  several  adjoining  glaciers 
from  the  same  upland  coalesce  in  their  expanded  portions,  the 


FIG.  317.  —  Spencer  Glacier,  Alaska  (Oct.  1918).  This  is  melting  back  and 
shows  a  sloping  and  crevassed  front,  and  a  well-developed  terminal  moraine, 
especially  on  the  left.  (Seward  Div.  Gov't  Railroad;  by  courtesy  Alaska 
Engineering  Commission.) 

piedmont  type  is  produced.  Conversely,  a  shrinking  piedmont 
glacier  will  become  resolved  into  the  several  component  expanded- 
foot  portions  of  the  corresponding  glaciers. 

Retreat  of  Glacier  Front.  —  When  the  rate  of  melting  is  in 
excess  of  that  of  advance,  the  glacier  front  will  move  backward 
or  "  retreat."  The  fronts  of  many  Alaskan  glaciers  have  been 
retreating  in  recent  times,  in  spite  of  the  fact  that  the  glacier 
as  a  whole  advances.  The  front  of'  Spencer  Glacier  (Fig.  317)  is 
melting  away,  leaving  a  part  of  the  valley  uncovered.  The  front 
of  the  Muir  Glacier  (Fig.  314,  p.  376),  which  faces  the  sea,  has 


Glaciers 


379 


Co 

M 

00 

I 

I 

HO 


p    C 

It 


o 


3  - 

5  5* 


c 


5 

Si 

5   3 


380     Atmospheric  Precipitates  and  their  Derivatives 


FIG.  319  a.  —  Front  of  the  Rhone  Glacier  in  1875.     (After  Walther.)     (See 
also  Fig.  313,  p.  375.) 


FIG.  319  6.  —  Front  of  the  Rhone  Glacier  in  1900.     (After  Walther.) 


382     Atmospheric  Precipitates  and  their  Derivatives 

retreated  steadily  between  1899  and  1911,  the  retreat  between 
1907  and  1911  being  2000  feet.  The  positions  of  the  front  in  1899, 
1903,  and  1907  are  shown  in  the  diagrams  (Fig.  318,  p.  379).  In 
the  next  two  figures  the  change  in  the  front  of  the  Rhone 
Glacier  is  shown  between  the  years  1875  (Fig.  319 a)  and  1900 
(Fig.  319^).  In  speaking  of  the  retreat  of  the  glacier  we  must 
remember  that  it  is  the  front  of  the  ice  which  alone  changes  its 
position,  not  because  the  glacier  as  a  whole  moves  backward, 


FIG.  321. — Surface  of  Seward  Glacier,  Alaska.  The  summit  of  Mt.  St. 
Elias  is  seen  in  the  distance  beyond  the  hills  bordering  the  glacier.  Drawn 
from  a  photograph.  (After  Russell,  Glaciers  of  North  America.  By  permission 
of  Ginn  &  Co.) 

but  because  it  melts  away  at  the  front.  The  water  resulting  from 
the  melting  forms  the  glacial  stream,  while  the  rock  debris,  which 
was  carried  in  and  upon  the  ice,  remains  behind  and  forms  the 
terminal  moraine.  (See  further,  Chapter  XVI.) 


The  Piedmont  Type  of  Glacier 

The  Malaspina  Glacier.  —  At  the  foot  of  Mount  St.  Elias  and 
west  of  Yakutat  Bay  in  Alaska  lies  the  Great  Malaspina  Glacier 
or  ice  fan  which,  with  its  moraines  and  secondary  deposits,  here 
forms  the  border  of  the  North  Pacific  Ocean  (Fig.  320).  This  ice- 
mass  is  formed  by  the  confluence  of  the  basal  expanded  portions 
of  a  number  of  valley  glaciers  which  descend  from  the  mountainous 


Glaciers 


383 


regions,  the  largest  of  these  being  the  Seward  Glacier  (Fig.  321), 
while  others  are  the  Agassiz  and  the  Tyndall  glaciers. 

The  surface  of  the  Malaspina  Glacier  has  an  exceedingly  gentle 
slope,  while  the  mass  as  a  whole  is  relatively  stagnant.  Its  area 
is  about  1500  square  miles  (about  the  size  of  Rhode  Island)  and 
its  marginal  thickness  perhaps  1000  feet.  Along  parts  of  the  foot 
of  the  Malaspina  Glacier  an  extensive  terminal  moraine  is  de- 


FIG.  322. — Moraine-covered  border  of  Malaspina  Glacier  from  Blossom 
Island.  Drawn  from  a  photograph.  (After  Russell,  Glaciers  of  North  America. 
By  permission  of  Ginn  &  Co.) 

veloped,  while  other  parts  deploy  into  the  sea  and  from  them 
fragments  are  broken  off  to  form  icebergs..  In  some  portions 
where  the  partial  melting  of  the  ice  surface  has  permitted  the 
accumulation  upon  the  ice  of  a  mantle  or  covering  of  debris  (Fig. 
322)  a  luxuriant  vegetation  has  sprung  up,  and  in  places  this 
glacier  actually  supports  a  forest  growth  (see  map,  Fig.  320)  with 
trees,  some  of  which  have  reached  a  trunk  diameter  of  three  feet. 
In  many  places  stagnant  pools  of  water  occur  in  which  sediments 
are  deposited,  while  over  the  central  portion,  where  debris  is 
absent,  there  are  many  crevasses  into  which  fall  the  streams  from 
the  melting  ice.  Glacial  streams  issue  from  tunnels  beneath  the 
ice  in  several  places  (Fig.  323). 


384     Atmospheric  Precipitates  and  their  Derivatives 

Other  Piedmont  Glaciers.  —  Other  examples  of  piedmont 
glaciers  are  Bering  Glacier,  west  of  the  Malaspina,  and  of  about 
the  same  size,  and  the  smaller  Alsek  Glacier  to  the  east.  In  Chile, 


FIG.  323.  —  Entrance  to  ice  tunnel  in  Malaspina  Glacier.  Drawn  from 
a  photograph.  (After  Russell,  Glaciers  of  North  America.  By  permission  of 
Ginn  &  Co.) 

south  of  S.  lat.  42°,  lie  the  San  Rafael  piedmont  glacier  and  some 
others.  Piedmont  glaciers  also  existed  in  the  Alps  and  in  the 
Rocky  Mountains  dufing  the  Pleistocene  period,  as  is  shown  by 
the  abandoned  moraines  and  other  features.  - 


ICE-CAPS 

The  Vatna  Jokull.  —  In  the  southeastern  part  of  Iceland  lies  the 
plateau  of  Vatna  Jokull,  the  largest  of  the  many  ice-covered  pla- 
teaus or  jokulls  found  in  that  northern  island  (Fig.  324).  It  has  an 
area  of  8500  square  kilometers,  being  covered  throughout  by  an 
ice-cap  which  forms  an  arched  dome  with  slopes  descending  from 
1900  meters  in  the  center  to  800  meters  near  the  margin,  where  it 
sends  out  ice  tongues  to  the  lower  levels  (20  to  100  meters).  As 


Continental  Glaciers 


385 


there  are  few  or  no  projecting  rocky  peaks  rising  through  it  (except 
near  the  margins)  this  ice-cap  is  largely  free  from  debris  and  very 
white.  Iceland  has  several  such  ice-caps  and  they  form  a  transition 
to  the  more  extensive  inland  ice  mass,  such  as  that  of  Greenland. 
Other  Examples.  —  Similar  caps  or  plateau  glaciers  are  found 
in  Scandinavia,  but  these  are  smaller  and  often  elongated ;  they 
also  send  out  tongues  in  all  directions.  The  ice  mantle  of  Red 


FIG.  324.  —  The  Ice-Cap  of  Vatna  Jokull  in  Iceland.     Ice  in  black. 
Th.  Thoroddsen,  Petermann  Mitt.  1906.) 


(After 


Cliff  Peninsula,  north  of  Inglefield  Gulf  in  Greenland,  is  another 
example,  and  still  others  are  found  in  the  Kerguelen  Islands,  and 
on  the  summit  of  Kilimandjaro  in  Africa.  All  such  ice-caps  imply 
conditions  of  exceptional  precipitation  and,  on  the  whole,  low 
temperatures. 

CONTINENTAL   GLACIERS 

The  Greenland  Ice  Cover  (Fig.  325).  —  Related  to  the  preceding 
type,  but  of  vastly  greater  extent  and  thickness,  is  the  ice  mantle 
which  covers  Greenland.  This  is  believed  to  have  the  character 
of  a  very  flat  dome,  the  highest  portion  of  which  lies  somewhat 
to  the  east  of  the  median  line  of  the  continent,  and  the  thickness 
of  which  approximates  perhaps  3000  feet.  It  forms  a  continuous 
mantle,  except  for  a  narrow  marginal  portion  in  which  the  under- 
lying rocks  are  exposed,  this  portion  varying  in  width  from  five  to 
twenty  miles,  though  in  places  the  ice  reaches  the  coast.  In  two 


386     Atmospheric  Precipitates  and  their  Derivatives 


localities,  However,  the  uncovered  margin  reaches  a  width  of  from 
60  to  100  miles. 

Only  the  northern  and  southern  portions  of  this  ice  field  have 
been  crossed,  the  great  central  area  being  entirely  unknown,  but 
there  is  little  reason  to  think  that  it  is  other  than  a  vast  field  of 

snow  and  ice.  In  this  central  por- 
tion the  surface  is  probably  nearly 
flat,  rising  perhaps  10,000  feet  above 
sea-level.  Around  the  margins,  how- 
ever, for  a  width  ranging  from  75  to 
100  miles,  the  outward  slope  is  rather 
abrupt,  often  being  so  steep  as  to  be 
difficult  of  ascent.  This  slope  is  broken 
into  a  series  of  broad  terraces  or  steps, 
though  the  rock  margins  of  the  con- 
tinent, where  uncovered,  are  generally 
mountainous,  reaching  heights  be- 
tween 5000  and  8000  feet  on  the  east 
coast,  but  not  over  6000  feet  on  the 
west.  Within  the  margin  of  the  ice, 
high  rock  peaks  occasionally  project 
through  it,  forming  nunataks.  From 
the  margin  of  the  ice  mass  many 
tongues  and  lobes  project  outward 
into  the  valleys  of  the  coastal  region. 
The  edge  of  the  ice  mass,  where 
not  extended  in  glaciers,  is  often  very 

precipitous  or  even  overhung  by  an  ice  cornice.  This  is  most 
marked  in  northern  Greenland,  but  becomes  the  exception  farther 
south.  Crevasses  are  common  in  some  parts  of  the  surface  of  the 
marginal  portion,  and  many  dimples  and  "  basins  of  exudation  " 
occur  in  some  sections  above  the  margin. 

Where  the  ice  moves  outward  between  adjoining  nunataks,  a 
series  of  crescent-shaped  moraines  of  debris  is  often  formed,  these 
moraines  extending  from  one  nunatak  to  another  and  having 
their  convexity  pointing  outward.  These  are  held  to  be  formed 
by  material  carried  up  from  the  lower  or  basal  portions  of  the  ice, 
the  upward  movements  being  produced  by  the  formation  of 
curved  shearing  planes  due  to  obstructions,  and  the  outward  con- 
vexity to  the  accelerated  motion  of  the  central  portion  of  the  ice 


FIG.  325.  —  Map  of  the 
Greenland  Ice-Cap.  (After 
Stieler;  from  Chamberlin  and 
Salisbury,  Geology.  By  per- 
mission of  Henry  Holt  &  Co.) 


Icebergs  387 

between  the  nunataks  and  marginal  retardation  by  them.  The 
nunataks  themselves  furnish  only  minor  amounts  of  debris  to  the 
surface  of  the  ice,  though  in  some  cases  morainal  ridges  extend 
from  them  marginward. 

The  chief  portion  of  the  ice  which  encloses  rock  debris  lies  within 
the  basal  100  feet  of  the  masses  exposed  in  the  sections.  Here 
much  englacial  material  is  found,  and  this  appears  to  be  largely 
derived  from  the  bottom  of  the  ice  sheet  and  carried  upward  by 
obliquely  outward-rising  shearing  planes.  In  many  portions  along 
the  margins  heavy  terminal  moraines  accumulate,  being  chiefly 
derived  from  the  subglacial  and  englacial  material  brought  there 
by  the  outward  moving  ice. 

Comparatively  little  water  issues  from  the  margins  of  the  ice- 
sheet  of  Greenland,  though  small  streamlets  appear  beneath  the 
ice  border,  bringing  sand  and  gravel  which  they  distribute  among 
the  coarser  morainal  material. 

The  Antarctic  Ice-Sheet.  —  The  Antarctic  continent,  which  is 
larger  than  Europe,  is  covered  by  a  great  ice-sheet  similar  to  that 
which  covers  Greenland,  and  which  like  the  latter  is  dome-shaped, 
with  a  height  of  10,500  feet  above  the  sea  at  the  pole  (Amundsen). 
Many  great  mountain  masses  or  nunataks  project  through  the  ice, 
rising  to  heights  of  1 5 ,000  feet.  Along  the  margin  the  ice-sheet  sends 
out  valley  glaciers  which  reach  the  sea,  while  part  of  the  great  sheet 
itself  abuts  upon  the  ocean  as  in  the  case  of  the  floating  ice  shelf 
or  Great  Ice  Barrier  of  Victoria  Land,  with  ice-cliffs  many  miles 
in  length  and  in  places  rising  to  heights  of  280  feet,  though  else- 
where low  enough  to  permit  landing  from  a  ship  alongside  of  it. 

ICEBERGS 

Where  glaciers  enter  the  sea  or  other  water  bodies  they  advance 
on  the  bottom  until  a  depth  equal  to  their  thickness  is  reached, 
when  portions  are  detached  from  their  front  and  float  away  as  ice- 
bergs (Fig.  326).  Such  icebergs  may  become  tilted  or  even  over- 


FIG.  326.  —  Glacier  descending  into  the  sea,  where  its  front  is  buoyed  up  by  the 
water  and  becomes  broken  up  into  icebergs.     (After  A.  Helland.) 


388     Atmospheric  Precipitates  and  their  Derivatives 


FIG.  327  a.  —  A  large  iceberg. 

turned,  and  they  soon  lose  their  load  of  debris.  In  some  cases, 
however,  this  may  be  carried  far  to  sea  before  the  melting  of  the 
iceberg  permits  its  deposition  on  the  bottom.  In  certain  cases 

where  many  successive 
icebergs  melt  near  the 
same  point,  submarine 


banks  composed  of  ice- 
rafted  material  may  be 
built  up.  The  Grand 
Banks  off  Newfoundland 
have  been  regarded  as  in 
part,  at  least,  formed  by 
such  debris. 

Only  a  small  portion 
of  an  iceberg  (in  pure  ice 
only  one  ninth)  appears 
above  water  (Figs. 
3270,  b),  and  while  ice- 
bergs from  Greenland 
seldom  exceed  100  feet 
in  exposed  height,  some 
in  the  Antarctic  region 


FIG.  327  b.  —  Floating  iceberg,  showing  the 
proportion  of  visible  to  submerged  ice.  (From 
Kayser's  Lehrbuch.) 


have  been  found  to  rise  500  feet  or  more,  with  a  length  of  several 
miles.     This  would  make  these  bergs  blocks  of  enormous  size. 


Causes  of  Ice  Movement  389 

CAUSES  OF  ICE  MOVEMENT 

The  subject  of  the  causes  of  glacial  movements  belongs  to  physics 
rather  than  to  geology,  but  may  be  briefly  considered.  By  many 
(following  Forbes)  ice  has  been  considered  a  viscous  substance 
which  in  large  masses  will  flow  under  the  influence  of  its  own 
weight  like  pitch  or  asphalt.  Freezing  of  descending  waters  and 
the  consequent  expansion  was  regarded  by  Agassiz  and  Charpentier 
as  the  chief  cause  of  glacier  motion.  Partial  melting  and  refreezing 
within  the  mass,  the  momentary  liquefaction  of  minute  portions  of 
the  mass  while  the  ice  as  a  whole  remains  solid,  —  may  produce 
a  condition  of  flowage  of  the  ice.  This  melting  may  be  the  result 
of  pressure  rather  than  applied  heat.  Expansion  and  contraction 
have  also  been  appealed  to  and  so  have  repeated  fracturing  and 
refreezing  (regelation) .  Finally,  the  growth  of  the  ice  crystals 
themselves  has  been  invoked,  and  the  influence  of  gravity  no  doubt 
is  effective  in  glaciers  upon  a  sloping  surface.  Altogether  the 
subject  is  too  complicated  for  elementary  treatment,  and  we  may 
merely  remark  that  different  combinations  of  the  above  men- 
tioned causes,  and  perhaps  others,  are  effective  in  producing  the 
movement  of  different  glaciers  and  perhaps  of  the  same  glacier  at 
different  times. 


CHAPTER  XV 

DESTRUCTION   OF  ROCKS  AND   THE  FORMATION 
OF   CLASTIC   MATERIAL 

ALL  rocks  are  subject  to  destruction,  and  the  product  of  such 
destruction  may  be  visible  fragmental  or  clastic  material,  or  it 
may  be  changed  by  great  heat  into  molten  material  (magma),  or 
finally  it  may  be  invisible  as  the  result  of  solution  in  water,  or  of 
vaporization.  Molten,  dissolved,  or  vaporized  rock  material  will 
be  redeposited  under  favorable  conditions  as  igneous  rock,  as 
aqueous  precipitates,  or  as  gaseous  sublimates  (sulphur,  etc.), 
respectively,  or,  by  the  intervention  of  animals  and  plants,  as 
organic  deposits.  Fragmental  material,  on  the  other  hand,  will 
produce,  when  reconsolidated,  a  new  type  of  rock,  the  clastic  rock, 
and  to  this  attention  is  now  invited. 


AGENTS  ACTIVE  IN  THE  FORMATION  OF  FRAGMENTAL  OR 
CLASTIC  MATERIAL 

In  general,  we  may  note  that  there  are  two  methods  of  breaking 
rocks  into  fragments,  the  chemical  and  the  mechanical.  In  the 
first  case,  the  fragmentation  is  produced  by  alteration  of  the  ma- 
terial of  the  rock,  either  by  abstraction  of  some  of  the  component 
material,  in  solution,  etc.,  or  by  the  addition  of  material  such  as 
oxygen,  water,  carbon  dioxide,  etc.,  or  by  both.  The  chemical 
method  of  rock-breaking  is  called  decomposition,  and  the  products 
of  decomposition  differ  from  the  material  of  the  original  rock.  In 
the  second  method  of  rock-breaking,  the  mechanical,  the  rocks 
are  merely  broken  into  fragments,  or  are  separated  into  their  com- 
ponent minerals.  This  is  called  disintegration.  When  both  modes 
of  change  go  on  together,  under  the  influence  of  the  atmosphere  and 
its  contained  water  vapor  and  gases,  with  accompanying  tempera- 
ture changes,  etc.,  the  process  is  called  weathering  of  rocks. 

390 


Processes  of  Erosion  391 

The  agents  active  in  the  production  of  fragmental  material  from 
the  rocks  of  the  earth's  crust,  are  the  following : 

1.  The  Atmosphere.  — This  operates  by  the  action  of  its  con- 
stituents, its  contained  gases  and  vapors,  by  its  transmission  of  the 
heat  of  the  sun,  and  the  escape,  by  radiation,  of  the  heat  waves 
from  the  earth,  and  by  the  mechanical  action  of  wind. 

2.  The  Hydrosphere.  —  This  operates  by  solution  of  rock  and 
alteration  of  its  constituents,  and  by  mechanical  activities  of  its 
movement,  as  in  the  case  of  waves,  river-currents,  rains,  etc. 

3.  The   Pyrosphere.  —  This    agent    operates    chiefly    through 
igneous  explosions,  which  shatter  the  rock. 

4.  Movements  of  the  Lithosphere.  —  The  grinding  and  frac- 
turing of  rocks  in  the  movement  of  one  rock  mass  over  or  against 
another,  as  in  faulting,  represent  the  chief  work  of  this  agent. 
Here  also  are  properly  placed  the  movements  of  large  ice  masses, 
such  as  glaciers,  over  other  rock  surfaces,  and  their  destructive 
work. 

5.  The  Biosphere.  —  This  operates  by  various  rock-destroying 
activities  of  plants  and  animals,  including  man. 

PROCESSES  OF  EROSION 

Breaking  up  or  fragmentation  (clastation)  of  rock  is  one  form 
of  erosion,  and  the  product  of  such  work  in  general  remains  near 
the  scene  of  operation  unless  other  agencies  are  active.  Erosion 
is,  however,  accomplished  in  another  manner,  namely  by  the  break- 
ing off  and  removal  of  material  by  the  agent  of  erosion.  This  is 
called  ablation,  and  also  becomes  the  first  step  in  the  transporta- 
tion of  material.  Ablation  may  be  effected  mechanically  by  the 
denuding  or  stripping  of  the  surface  of  material  loosened  by  pro- 
cesses of  clastation  or  rock-breaking  in  place.  Such  removal  is 
called  denudation.  A  second  process  is  that  of  corrasion,  where  ma- 
terial is  ground  or  filed  away  by  means  of  "  tools  "  which  are  carried 
by  the  agent.  Thus  wind  carrying  sand  grains  or  dust  will  corrade 
any  surface  over  which  it  blows,  after  the  manner  of  the  artificial 
sand  blast  used  in  renovating  old  stone  structures,  such  as  the  stone 
fronts  of  buildings,  etc.  The  wear  which  these  sand  grains  them- 
selves experience  is  call  abrasion.  A  third  method  is  analogous  to 
quarrying  by  man,  in  that,  by  a  process  of  undermining  and  loosen- 
ing, large  masses  of  rock  are  removed  from  their  original  location. 
Finally,  chemical  ablation  takes  place,  when  the  rock  mass  under- 


392 


The  Formation  of  Clastic  Material 


goes  solution,  melting,  or  other  chemical  change  which  at  the  same 
time  removes  the  material.  This  is  expressed  by  the  general  term 
corrosion.  It  is  sometimes  spoken  of  as  chemical  denudation.  We 
may  tabulate  these  processes  as  follows : 


Erosion 


I.  Fragmentation  or  dastation.  — 
Breaking  or  shattering  material  in 
situ  not  necessarily  accompanied 
by  removal. 

II.  Ablation  or  the 
separation  and  simul- 
taneous removal  of 
rock  material  by  the 
same  agents. 


(a)  Mechanical 


(6)  Chemical 


(a)  Physical,  or  dis- 
integration. 

(b)  Chemical,  or  de- 
composition. 

'  i.  Denudation. 

2.  Corrasion     and 

abrasion. 

3.  Quarrying. 
Corrosion. 


DESTRUCTIVE  WORK  OF  THE  ATMOSPHERE 

Weathering 

By  weathering  we  understand  all  the  rock-modifying  influences 
of  the  atmosphere,  other  than  the  mechanical  activities  of  the 
wind.  These  weathering  processes  are  both  chemical  and  mechan- 
ical and  belong  primarily  to  the  division  of  rock-breaking  or  clasta- 
tion,  though  some  corrosion  may  also  occur. 

Daily  and  Seasonal  Changes  of  Temperature ;  Insolation.  —  All 
rocks  are  affected  to  a  greater  or  less  degree  by  the  daily  and  seasonal 
changes  in  temperature,  and  although  the  source  of  the  heat  which 
affects  the  rocks  is  the  sun,  the  presence  of  the  atmosphere  modifies 
this,  so  that  the  effects  or  changes  produced  are  properly  classed 
with  the  activities  of  the  atmosphere.  In  common  with  nearly 
all  substances,  rocks  expand  under  the  influence  of  heat  and  con- 
tract under  that  of  cold.  Illustrations  of  the  expansive  effect  of 
the  sun's  heat  on  substances  are  seen  in  the  lengthening  on  hot 
summer  days  of  the  rails  on  a  car  track,  which  are  purposely  placed 
short  of  contact  to  allow  for  such  expansion,  which  would  otherwise 
cause  a  buckling  of  the  rails.  On  very  cold  days  the  ends  of  the 
rails  are  separated  by  an  appreciable  gap.  The  buckling  of  con- 
crete sidewalks  from  prolonged  exposure  to  the  sun  is  another  ex- 
ample, and  the  repeated  removals  of  such  bucklings  and  filling  of 
the  gaps  with  new  cement  is  necessary,  and  noticeable  in  most 
older  cement  walks.  The  chief  effects  produced  on  rock  masses 
by  these  changes  in  temperature  are  exfoliation  and  granular  dis- 
integration. 


Destructive  Work  of  the  Atmosphere  393 


Exfoliation.  —  Where  changes  of  temperature  between  day  and 
night  are  great,  as  in  desert  regions,  the  effect  upon  the  rocks  is  very 
marked.  During  the  day  the  exposed  rock  surfaces  are  intensely 
heated,  especially  in  the  case  of  the 
dark-colored  igneous  rocks,  which  ab- 
sorb more  heat  than  those  of  lighter 
color.  The  daily  range  in  tempera- 
ture on  the  surfaces  of  some  rocks  has 
been  estimated  as  high  as  80°  C.  (144° 
F.).  As  rock,  on  the  whole,  is  a  poor 
conductor  of  heat,  the  surface  is  chiefly 
affected,  though  the  heat  passes  gradu- 
ally inward.  Hence  the  surface  layers 
are  subjected  to  an  expansive  force, 
while  the  deeper  layers  are  not  so 

affected.     On  cooling  by  radiation  in     basic  igneous  rock.     Note  the 

successive    shells    which    are 
the  clear  atmosphere,  the  temperature    peeling  off     (From  specimen 

of  the  outer  layers  will    sink  rapidly     in  Columbia  University. 
and   may   easily  pass   below   that  of 
the  inner  part  of  the  rocks.     As  a  re- 
sult, the  surface  portion  for  some  distance  inward  is  subjected  to 
a  series  of  strains  which  will  result  in  the  flaking  or  peeling  off  of 
such  outer  layers.     As  the  angles  of  rocks  are  most  exposed,  being 
subjected  to  heating  from  all  sides,  they  will  fall  off  first,  and  the 


FIG.     328  a.  —  Concentric 
exfoliation    in  a    fine-grained 


B. 

Reduced. 


FIG.  328  b.  —  Concentric  exfoliation  or  "  spalling  "  of  biotite  granite.  Ridge 
south  of  Morrison  Creek,  Yosemite  Quadrangle,  Cal.  (Photo  by  Turner,  from 
U.  S.  G,  S. ;  courtesy  of  Popular  Science  Monthly.) 


394 


The  Formation  of  Clastic  Material 


result  will  be  the  production  of  a  curved  or  rounded  outline,  and 
the  subsequent  layers  peeled  off  will  also  be  curved.  Thus  con- 
centric exfoliation  or  the  successive  peeling  off  of  layers  of  rock 
results,  a  phenomenon  very  marked  in  most  dark  and  fine-grained 
rocks  in  such  regions  (Figs.  328  a-c).  When  the  changes  in  tem- 
perature and  the  consequent  expansion  and  contraction  are  very 
rapid,  large  masses  of  rock  may  be  thrown  off  with  some  violence. 
Over  large,  nearly  flat  surfaces,  the  effect  of  insolation  is  such  as 
to  produce,  a  series  of  planes  of  separation  parallel  to  the  surface 


FIG.  328  c.  —  Concentric  exfoliation  in  a  dike  of  basic  igneous  rock.     (From 
Ratzel,  Die  Erde.) 

and  at  progressively  greater  distances  apart  in  depth.  Such  ex- 
pansion joints  or  planes  are  seen  in  the  faces  of  most  granite 
quarries,  and  they  greatly  facilitate  the  process  of  quarrying,  but 
limit  the  size  of  the  blocks  obtainable  near  the  surface  (Fig.  329). 
Granular  Disintegration.  —  Besides  affecting  the  rock  mass  as  a 
whole,  changes  in  temperature  have  a  further  detailed  effect  upon 
the  minerals  or  particles  of  which  the  rock  is  composed,  especially 
if,  as  is  usually  the  case,  these  are  not  all  of  one  kind.  A  granite, 
for  example,  exposed  to  such  influences,  will  have  its  several 
minerals  affected  in  different  degrees.  The  dark  minerals  (horn- 


Destructive  Work  of  the  Atmosphere          395 


blende,  black  mica)  will  absorb  heat  more  readily  and  also  give 
it  up  more  quickly  than  the  light  minerals.     Moreover,  the  ability 


FIG.  329.  —  Horizontal  jointing  in  a  granite  ledge,  due  to  expansion  and 
contraction  of  the  surface  layers  under  insolation.  (Courtesy  of  Prof.  J.  B. 
Woodworth.) 

to  react  to  heat  and  cold  is  not  the  same  in  the  feldspar  as  in  the 
quartz,  and  both  differ  in  this  respect  from  the  dark  minerals.  In 
other  words,  each  min- 
eral has  its  own  coefficient 
of  expansion  and  con- 
traction. As  a  result, 
internal  stresses  are  set 
up  and  the  minerals  will 
tend  to  separate  one  from 
the  other  and  finally  fall 
apart,  producing  a  sand 
of  loose  minerals.  This 
granular  disintegration, 
as  it  is  called,  —  the  sep- 
aration of  the  rock  into 


its  component  minerals, 
—  is  a  characteristic 
feature  observable  in  all 
granite  and  similar  rock 


FIG.  330  a.  —  Slopes  of  crystalline  sand  on 
the  sides  of  Pikes  Peak,  Colorado.  The  sand 
is  the  product  of  granular  disintegration  of  the 
granitic  rock,  and  consists  of  quartz,  feld- 
spar, and  ferro-magnesian  mineral  fragments. 
(Photo  by  the  author.) 


The  Formation  of  Clastic  Material 


masses.     The  slopes  of  Pikes  Peak,  which  is  a  mountain  largely 
composed  of  coarse  granite,  are  covered  with  long  screes  of  such 

disintegration  products, 
which  have  the  appear- 
ance of  a  coarse  sand 
with  a  smooth  surface 
which  slopes  at  the  angle 
at  which  such  material 
will  come  to  rest,  that 
is,  the  angle  of  repose 
(Fig.  330  a).  Similar 
products  of  granular  dis- 
integration may  be  seen 
around  large  granite 
boulders  on  exposed  hill- 
sides, even  in  fairly  moist 
regions  (Fig.  3306). 
Such  boulders  are  them- 
selves often  the  product 


FIG.  330  b.  —  Disintegrating  granite  boulder, 
Dogtown  Common,  Cape  Ann,  Mass.  Half 
of  the  boulder  has  crumbled  into  crystalline 
sand.  (Photo  by  the  author.) 


of  granular  disintegra- 
tion (Fig.  330  c).  Granite  surfaces  often  show  areas  of  bare  rock 
separated  by  fissures,  into  which  the  product  of  disintegration  has 


FIG.  330  c.  —  Granite  boulders,  the  result  of  decomposition  and  disintegra- 
tion of  granite  in  situ;  resting  on  granite  ledge.  Graniteville,  Mo.  (Gardner  col- 
lection of  photographs,  No.  7908.  Courtesy  Geol.  Dept.,  Harvard  University.) 


Destructive  Work  of  the  Atmosphere          397 


been  washed  by  rains,  and  here,  in  the  moister  climates,  where 
this  material  is  further  decomposed,  lines  of 'vegetation  will  spring 
up.  In  moist  climates, 
dark  igneous  rocks  are 
often  reduced  to  a  mass 
of  residual  boulders  by 
combined  disintegration 
and  decomposition  (Figs. 

331 «,  *)• 

Finally,  it  should  be 
noted  that  the  minerals 
themselves  are  affected  FIG.  331  a.— Boulders  of  disintegration  and 
by  these  temperature  decomposition.  Part  of  a  Camptonite  dike 
Changes,  because  min-  weathered  until  only  scattered  residual  blocks 

of  the  original  rock  remain, 
erals  with  several  distinct 

crystal  axes  react  differently  on  the  several  faces.    Thus  a  crystal 
of  feldspar  is  itself  subjected  to  internal  stresses  which  open 


FIG.  331  b.  —  Boulders  of  disintegration.  Residual  masses  left  by  weathering 
of  a  diabase  dike,  Medford,  Mass.  The  dike  had  been  left  in  relief  by  quarry- 
ing operations,  and  weathered  into  a  mass  of  spheroidal  boulders.  As  the  dike 
finally  crumbled  completely,  the  boulders  were  thrown  together  into  the  heap 
here  shown.  (Photo  H.  W.  Dyer ;  courtesy  of  Prof.  Elizabeth  Fisher,  Wellesley 
College.) 


398 


The  Formation  of  Clastic  Material 


minute  fractures  along  the  cleavage  lines.  Into  these  fine  fractures 
air  and  moisture  will  penetrate  and  effect  the  decomposition  of 
the  mineral  along  the  sides  of  the  fracture. 

The  Talus  Slope  (Fig.  344).  —  By  the  disruption  and  disintegra- 
tion of  the  rock  masses,  loose  material  of  all  sizes  is  formed,  and  this 
will  accumulate  at  the  foot  of  every  cliff,  forming  a  talus,  the  sur- 
face slope  of  which  varies  with  the  coarseness  of  the  material.  In 
dry  climates,  the  talus  is  chiefly  formed  by  the  processes  above 
described,  but  in  cold  climates  where  the  atmosphere  is  moist,  frost 
action  may  become  an  important  factor  in  its  production.  By  the 
combined  action  of  these  agencies  it  is  brought  about  that  most  ex- 
posed mountain  peaks  are  heaps  of  coarse,  loose  material,  generally 
burying  the  ledges,  while  long  slopes  of  finer  material  are  formed 
in  every  favorable  locality. 

Land  Slides. — When  a  talus  slope  becomes  saturated  with  rain- 
water during  a  moist  season,  its  potential  angle  of  slope  is  lowered, 
because  the  entire  mass  becomes  more  mobile  than  when  dry.  For 

a  time,  the  angle  of  slope 
may  be  mainiained,  but 
at  a  critical  moment 
the  cohesive  force  may 
be  overcome  and  the 
entire  mass,  or  a  large 
part  of  it,  will  slide 
down  the  mountain's 
slope,  forming  an  earth 
and  rock  avalanche  or 
rock  slide  (Fig.  332), 
which  may  produce  dis- 
astrous effects  in  the 
valley  bottoms  below, 
destroying  farmlands 
and  buildings,  and  dam- 
ming narrow  valleys  so  that  the  upper  portions  may  be  converted 
into  closed  basins  in  which  lakes  will  form.  By  the  final  overflow 
of  the  lake-water,  and  the  accompanying  destruction  of  the  earth 
dam,  disastrous  floods  may  sweep  the  valley  below.  In  one  of  the 
upper  branches  of  the  Ganges  River  in  the  Himalayas,  a  slide, 
bringing  down  800,000,000  tons  of  rock  debris  in  three  days,  built 
a  dam  across  the  narrow  valley  nearly  a  thousand  feet  deep.  Be- 


FIG.  332. — Land-slide,   Ausable  Lake,  N.  Y. 
(A.  D.  Savage,  Photo.) 


Destructive  Work  of  the  Atmosphere          399 


hind  this  a  lake  accumulated, 
reaching  a  length  of  four  miles 
in  a  year.  Then  the  overflow 
partly  destroyed  the  dam,  al- 
lowing 400,000,000  cubic  yards  of 
water  to  discharge  in  about  four 
hours  and  flooding  the  valley  be- 
low to  a  depth  of  100  to  170 
feet,  destroying  every  vestige  of 
habitation  for  a  distance  of  150 
miles  down  the  valley.  Even  on 
gentle  slopes  the  saturation  of 
the  talus  by  water  will  produce 
disastrous  slides. 

Frost  Work.  —  Water  on  freez- 
ing expands  one  tenth  of  its 
volume  and  so  becomes  a  power- 
ful agent  in  disrupting  rock. 
Where  the  moisture  of  the  air  is 
condensed  on  the  cold  surfaces  of 
rocks,  long  ice  crystals  may  form, 
and  if  these  develop  in  crevices, 
their  growing  force  pushes  the 

walls  apart  and  eventually  loosens  the  smaller  fragments,  which 
then  fall  to  the  bottom  of  the  cliff  to  aid  in  building  the  talus.  By 
the  combined  action  of  frost  and  insolation  remarkable  erosion 

forms  may  be  pro- 
duced, an  example 
of  which  is  seen  in 
the  "  old  man  of  the 
mountain "  in  the 
White  Mountains 

(Fig-  333)- 

Boulders  on  a  rock 
FIG.  334.  —  Diagram  showing  three   stages  in     or  other  hard  surface 
the  downward  progress  of  a  boulder  on  a  sloping  .          ... 

hillside  under  the  action  of  frost.  The  original  are  otten  ll±ted  to  a 
position  is  shown  by  heavy  dotted  lines.  Frost  slight  degree  by  the 
crystals  forming  under  the  boulder  raise  it  at  right  formation  of  innu- 
angles  to  the  surface  (fine  dotted  lines) ;  when  ,  .  . 

these  supporting  crystals  melt  the  boulder  settles  merable  lce  crystals 
back  at  right  angles  to  the  horizon  (solid  outline),  beneath  them.  If  the 


FIG.  333.  —  The  Old  Man  of  the 
Mountain,  Franconia  Notch,  White 
Mountains,  N.  H.  Illustrating  pe- 
culiar result  of  rock  weathering. 


400  The  Formation  of  Clastic  Material 

surface  on  which  the  boulder  rests  has  a  slope,  lifting  by  the  crys- 
tals will  be  at  right  angles  to  this  slope.  On  the  melting  of  the 
ice,  the  boulder  will  settle  back  vertically,  and  thus  make  a  slight 
advance  down  hill.  In  this  manner  a  large  boulder  may  travel  for 
considerable  distances  down  hill  in  the  course  of  time  and  may 
eventually  reach  the  edge  of  a  precipice  over  which  it  will  finally 
fall  (Fig.  334). 

Frost  acts  in  a  similar  manner  on  pebbles  and  on  the  finer  ma- 
terial which  forms  the  soil.     The  effect^  of  the  frost  upon  the  soil 


FIG.  335.  —  View  of  the  Slumgullion  rock-flow  from  its  source  to  Lake  San 
Cristobal,  which  was  formed  when  the  great  slide  dammed  up  the  river  valley. 
(Photo  by  W.  Cross,  U.  S.  G.  S.  Courtesy  of  D.  W.  Johnson.) 

is  one  of  expansion,  by  filling  it  with  ice-crystals,  which  lift  and 
push  apart  the  particles.  This  may  readily  be  seen  in  dirt  paths 
on  a  frosty  morning,  in  the  spring  or  autumn.  When  the  ice 
crystals  melt,  they  leave  the  soil  full  of  small  cavities  which  give 
the  mass  a  loose  spongy  character.  Under  the  beat  of  the  rain, 
the  running  and  soaking  in  of  water,  the  movement  of  the 
groundwater,  the  trampling  of  animals,  etc.,  this  uplifted,  loosened 
soil  is  again  compacted,  and  in  this  process  slowly  moves  down  hill. 


Destructive  Work  of  the  Atmosphere          401 

By  the  alternate  freezing  and  thawing  of  the  moisture  in  a  talus 
slope,  this  may  undergo  a  slow  process  of  creeping  down-hill,  form- 
ing a  "rock  glacier"  (Figs.  335,  336).  Such  creeping  talus  masses, 
often  of  considerable  length,  have  been  found  in  many  mountain 
regions  where  intense  cold  prevails  for  part  of  the  year.  Their 
movement  is  often  indicated  by  the  complete  absence  of  vegetation, 
even  over  the  lower,  gentler  slopes,  plants  being  unable  to  main- 
tain a  foothold  on  account  of  the  motion. 


FIG.  336.  —  View  northeast  across  Cleveland  gulch  at  landslide  mass  and 
conglomerate  rock-glacier.  Silverton  Quadrangle,  Colo.  (Willis,  Photo ;  from 
U.  S.  G.  S.) 

Soil  on  hillsides  also  creeps  under  the  influence  of  freezing  mois- 
ture, as  well  as  gravitative  control,  and  this  creep  is  well  seen  where 
the  underlying  rock  is  exposed  in  cuts  or  otherwise,  and  where  it  is 
composed  of  vertical  or  steeply  inclined  beds,  especially  shales 
or  schists.  The  upper  ends  of  these  beds  below  the  soil  are  then 
frequently  found  bent  forward  as  the  result  of  the  dragging  effect 
of  the  creeping  soil  (Fig.  337). 

Chemical  Work  of  the  Atmosphere.  —  The  atmosphere  consists 
of  a  mixture  of  oxygen  and  nitrogen,  the  former  being  somewhat 
less  than  21  per  cent  of  the  volume,  and  the  latter  something  over 
78  per  cent.  Besides  this,  there  is  a  small  quantity  of  the  element 
argon  and  other  rare  gases  (less  than  one  per  cent  by  volume)  and 


402 


The  Formation  of  Clastic  Material 


a  fairly  constant  admixture  of  carbon  dioxide  (CO^  about  0.03  per 
cent  by  volume)  and  water  vapor  (HkO)  in  very  variable  quantity. 
Minute  quantities  of  other  compounds,  such  as  ozone,  nitric  acid 
and  ammonia,  are  also  found,  together  with  free  hydrogen,  sulphur 
compounds  and  other  substances.  The  agents  active  in  producing 
chemical  changes  in  the  rock  are,  however,  few,  comprising  chiefly 
oxygen,  carbon  dioxide  and  water  vapor.  Accordingly  the  chem- 
ical- activities  of  the  atmosphere  may  be  grouped  under  (a) 


FIG.  337.  —  Nearly  vertical  beds  of  shale  bent  over  and  broken  at  the  top, 
by  superincumbent  weight,  or  creep  of  the  overlying  soil.  Columbia,  Pa. 
(Walcott,  Photo  U.  S.  G.  S.) 

oxidation,  (b)  carbonation  and  (c)  hydration  and  dehydration. 
Special  combinations  of  these  produce  other  changes  such  as  kao- 
linization  and  laterization. 

Oxidation.  —  The  oxygen  of  the  air  accomplishes  its  chief  work  by  uniting 
with  the  iron  and  sulphur  of  the  rock-forming  minerals  and  with  the  organic 
matter  present.  This  is  called  oxidation.  The  iron  of  the  ferro-magnesian 
minerals  and  of  other  iron  compounds  in  the  rocks  is  transformed  into  the  oxide 
(Fe2O3),  or  in  the  presence  of  moisture  into  the  hydrous  oxide  of  iron.  The 
former  has  a  red  color  (the  mineral  hematite,  etc.)  and  the  latter  a  yellow  or 
ocher  color  (limonite,  etc.).  In  their  formation  there  is  generally  an  in- 
crease in  volume  of  the  material,  this  being  especially  great  when  limonite  is 
formed  by  simultaneous  oxidation  and  hydration.  When  magnetite  (Fe3O4)  is 


Destructive  Work  of  the  Atmosphere          403 

altered  to  limonite  by  these  processes,  the  increase  in  volume  is  64  per  cent. 
On  the  other  hand,  if  iron  carbonates  are  oxidized,  the  loss  of  carbon  dioxide 
is  not  wholly  compensated  for  by  the  addition  of  oxygen  and  water,  and  the 
volume  decreases.  By  the  oxidation  of  the  sulphur  of  the  rock  minerals,  such 
as  pyrite,  both  sulphurous  and  sulphuric  acids  may  be  formed,  which  unite 
with  the  iron  or  other  substances  to  form  new  compounds  (sulphites  and  sul- 
phates). By  the  oxidation  of  the  organic  matter,  carbon  dioxide  (CO2)  and 
water  are  produced. 

Carbonation.  —  This  most  commonly  affects  the  silicate  minerals,  the  carbon 
dioxide  combining  with  the  basic  elements  to  form  carbonates,  while  silica  is 
set  free.  Besides  the  carbon  dioxide  of  the  atmosphere,  there  is  generally  an 
abundant  supply  of  this  gas  furnished  by  the  oxidation  of  the  organic  matter, 
and  this  also  becomes  active  in  attacking  the  rock,  altering  some  of  the  min- 
erals. Water  takes  up  carbon  dioxide,  from  the  air,  and  from  decaying  organic 
matter  in  the  soil  and  in  stagnant  ponds ;  indeed  more  CO2  is  obtained  from 
organic  matter  than  from  the  air.  Water  thus  charged  with  CO2  is  a  power- 
ful solvent,  attacking  and  dissolving  many  minerals.  We  shall  return  to  this 
subject  in  the  study  of  groundwater. 

Hydration.  —  This  is  the  union  of  water  with  the  mineral  substances  of  the 
rocks  and  is  the  most  important  single  process  of  change,  but  hydration  by  at- 
mospheric moisture  is  only  a  superficial  process  often  associated  with  oxidation. 
More  effective,  however,  is  the  hydration  due  to  the  circulation  of  ground-water 
in  the  pores  of  the  rock.  Hydration  causes  the  expansion  of  the  minerals,  form- 
ing stresses  in  the  rock,  which  lead  to  its  crumbling.  As  the  most  important 
single  example  of  hydration  is  the  changing  of  feldspars  to  kaolin  or  clay,  we 
will  consider  this  and  the  related  process  of  laterization  more  in  detail. 

Kaolinization.  —  We  have  seen  that  feldspars  are  apt  to  be  traversed  by  nu- 
merous fine  fissures  due  to  the  influence  of  heat  and  cold.  Moisture  entering 
these  in  the  presence  of  carbon  dioxide  changes  the  feldspar  to  clay  or  kaolin, 
which  may  be  seen  in  a  thin  slide  as  a  clouding  of  the  otherwise  transparent 
mineral,  along  the  sides  of  the  cleavage  lines.1  Complete  change  to  kaolin  (clay) 
occurs  in  time,  according  to  the  following  chemical  reaction : 

2  KAlSi3O8  +   2  H20     +    C02      =      H4Al2Si209  +  4  SiO2   +   K2CO3. 
Orthoclase        Water          Carbon  Kaolin  Silica        Potassium 

feldspar  dioxide  or  clay  carbonate 

In  this  change  from  feldspar  to  kaolin  or  pure  clay,  there  is  a  decrease  in  volume 
of  54.44  per  cent,  owing  to  the  removal  of  the  silica  as  quartz  and  of  the  potas- 
sium carbonate.  Kaolin  is  the  chief  substance  of  which  clay  is  produced, 
clay  being  commonly  a  variable  mixture  of  kaolin  and  impurities,  such  as  quartz- 
flour,  iron  oxides  etc.  We  see  here  the  process  by  which  this  common  substance 
originates  as  the  result  of  the  weathering  of  feldspars. 

Laterization.  —  In  moist,  tropical  regions  the  feldspars  are  not  as  a  rule  changed 
to  clay,  which  is  a  hydrous  silicate  of  alumina,  but  to  hydrous  oxides,  chief 
among  which  is  the  mineral  Hydrargillite  (Al2Os  '  3  H2O),  or  to  a  mixture  of 
oxides,  forming  bauxite.  There  is  usually  much  iron  oxide  liberated,  especially 
if  the  decaying  rock  is  basic,  such  as  basalt,  etc.  This  iron  oxide  stains  the 

1  This  clouding  may  be  due  to  the  development  of  sericite  rather  than  kaolin. 


404 


The  Formation  of  Clastic  Material 


product  a  deep  red  or  brown,  and  may  also  form  iron  nodules  or  concretions. 
The  combined  product  is  called  laterite.  It  is  a  very  widespread  product  of 
rock  decay  in  tropical  regions  and  has  been  known  to  extend  to  a  depth  of 
300  feet  in  Brazil.  In  the  process  of  laterization,  the  silica  is  separated  out  and 
accumulates  in  separate  areas  as  quartz  (agate  or  chalcedony). 

A  comparison  of  the  more  important  compounds  of  a  fresh  igneous  rock 
(dolerite,  see  p.  107)  and  its  decomposition  product,  brings  out  the  change  and 
shows  the  difference  between  kaolinization  and  laterization. 


IMPORTANT  COMPOUNDS 

PRESENT  IN 
FRESH  ROCK 
(DOLERITE), 
ENGLAND 

PRESENT  IN 
KAOLIN- 
IZATION 
PRODUCT 

PRESENT  IN 
FRESH  ROCK 
(DOLERITE), 
INDIA 

PRESENT  IN 
LATERIZATION 
PRODUCT 

Silica  (SiO2)    
Aluminum  oxide  (A12O3)     . 
Iron  oxide  (Fe2O3)    -     -     . 

49-3% 

17-4% 
2-7% 

47-0% 

i8-5% 
14-6% 

50.4% 

22.2% 

9-9% 

0-7% 
50-5% 
23-4% 

Corrosive  Work  of  the  Atmosphere 

This  is  of  little  importance,  as  a  rule,  in  rock  destruction,  though 
very  marked  in  the  case  of  snow  and  ice,  where  the  surfaces  are 
often  strongly  pitted.  The  corrosive  effect  of  the  atmosphere  in 
this  case  manifests  itself  largely  by  a  process  of  direct  evaporation. 
Entire  banks  of  snow  and  ice  may  thus  be  removed  by  evaporation 
without  passing  through  a  liquid  state.  The  moisture  in  the  air 
may  also  have  a  corrosive  effect  upon  easily  soluble  deposits,  such 
as  salt  and  gypsum  beds,  or  even  limestones,  but  in  general  it  would 
be  difficult  to  distinguish  this  from  the  work  of  rain  and  other  waters 
upon  these  rocks.  Corrosion  by  hot  gases  and  steam  is  not  an  un- 
common process  in  volcanic  districts. 


Destructive  Work  of  the  Wind 

Deflation.  —  Air  in  motion,  i.e.  wind,  often  exerts  a  powerfully 
destructive  effect  upon  rocks.  This  is  especially  the  case  where 
material  has  previously  been  loosened  by  weathering,  so  that  the 
wind  merely  picks  up  the  loose  material  and  carries  it  away.  The 
removal  of  such  material  by  the  wind  is  called  deflation,  and  to  it 
is  probably  due  the  most  extensive  effect  of  wind  erosion.  Of 
course,  material  previously  deposited  in  a  region,  either  that  trans- 
ported by  the  wind  from  elsewhere  or  that  brought  by  running 
water  or  by  ice,  may  also  be  removed  by  the  wind,  and  this  too  is 


Destructive  Work  of  the  Atmosphere          405 

deflation.  Many  exposed  uplands  are  thus  kept  free  from  loose 
material,  which  is  removed  as  fast  as  weathering  produces  it. 
The  great  elevated  plateau  of  southern  Germany,  known  as  the 
Swabian  Alp,  is  in  this  manner  kept  almost  bare  of  soil,  so  that 
agricultural  activities  are  strongly  interfered  with.  So-called  stony 
deserts  or  Hammadas  are  produced  by  the  removal  by  wind  of 
all  fine  products  of  disintegration,  leaving  only  the  coarser  frag- 
ments, which  are  worn  and  polished  by  the  corrasive  effects  which 
this  finer  material  has  as  it  is  swept  along  by  the  wind. 

Corrasion.  —  This  is  the  second  important  mode  of  wind  erosion, 
and  this  method  has  already  been  compared  to  the  work  of  the  arti- 
ficial sand  blast.  By  means  of  the  sand  swept  along  by  the  wind, 
rock  surfaces  are  polished  and  grooved,  such  grooves  often  being 
.very  marked  and  in  parallel  alignment  upon  the  surfaces  of  some 
rocks,  such  as  limestones. 

Wind-worn  grooves  of  this  type  are  found  in  the  limestone  plateau  of  the 
Libyan  desert  in  northern  Africa.  They  extend  in  a  north-northwest  by 
south-southeast  direction,  and  vary  in  depth  up  to  a  meter.  They  were  cut 
by  the  sand-laden  winds,  which  for  thousands  of  years  have  swept  across  this 
surface  with  little  or  no  variation  of  direction. 

Such  wind-channeled  surfaces  may  become  of  considerable  importance  to 
the  student  of  earth  history,  when  he  finds  them  tfn  older  rock  surfaces  which 
were  subsequently  covered  and  buried  by  material  deposited  upon  them  and 
also  consolidated  into  rock.  They  become  visible,  of  course,  only  when  the 
later  deposited  material  is  removed  again  by  erosion  or  by  quarrying,  and  it 
will  then  be  found  that  the  old  wind  channels  are  filled  in  by  this  younger  rock 
material,  a  part  of  which  will  commonly  remain  after  the  removal  of  the  gen- 
eral covering  mass.  The  presence  of  this  material  will  show  that  the  grooves 
or  channels  are  not  of  recent  origin,  but  were  formed  before  the  younger  rock 
material  was  deposited.  Such  conditions  clearly  show  that  after  the  formation 
of  the  older  rock,  a  period  of  land  succeeded,  with  strong  wind  activities  such 
as  are  found  chiefly  in  deserts,  and  that  therefore  this  interval  of  erosion  marks 
a  considerable  period  between  the  epochs  of  deposition  of  the  older  and  of  the 
younger  rock.  An  ancient  example  has  been  found  in  Michigan,  where  lime- 
stone surfaces,  of  Upper  Silurian  age,  are  grooved  in  this  manner  and  are 
covered  by  limestones  of  Middle  Devonian  age,  which  fill  these  grooves  as 
well.  This  indicates  strong  wind  activity  during  the  interval  between  the 
periods  of  deposition  of  these  two  rocks  (the  Lower  Devonian  interval),  and 
some  of  the  old,  well-rounded  sand  grains  which  were  active  in  the  production 
of  these  grooves  are  still  found  in  some  of  them. 

In  central  Asia,  surfaces  of  argillaceous  rock  are  similarly  sculptured,  but 
here  the  sides  of  the  grooves  are  often  strongly  fluted.  Such  surfaces  are  called 
locally  yardangs  (Fig.  338),  and  they  too  are  recognizable  when  found  along  the 
contact  of  two  older  rock  series,  and  tell  a  similar  story. 


406 


The  Formation  of  Clastic  Material 


Good  illustrations  of  the  corrasive  activities  of  sand-bearing 
winds  are  found  in  the  San  Bernardino  Pass  in  southern  California, 
where  the  telegraph  poles  along  the  pass  are  greatly  damaged  by 


FIG.  338.  —  Map  of  a  desert  area  with  yardangs,  and  cross  sections  of  the 
same  on  a  larger  scale.     (After  Sven  Hedin;  from  Kayser's  Lehrbuch.) 

those  blasts,  so  that  they  have  to  be  protected  by  piles  of  rock  and 
supplemental  pieces  placed  on  the  windward  side.  In  the  Trans- 
Caspian  deserts  the  telegraph  wires  stretched  along  the  railroad 
line  were  so  affected  by  the  sand  blasts  that  they  became  dimin- 
ished by  half  their  diameter,  and  had 
to  be  replaced  after  eleven  years. 
Even  the  fine  dust  of  the  city  streets, 
blown  across  tombstones  in  old  ceme- 
teries, will  in  time  efface  their  inscrip- 
tions. 

Some  of  the  most  striking  effects 
of  wind  corrasion  are  seen  in  pebbles 
exposed  to  more  or  less  constant 
sand-bearing  winds,  such  as  are  found 
in  desert  regions,  but  also  on  exposed 
portions  of  the  coast.  Upon  such 
pebbles,  smooth,  flat  faces  will  be 
cut,  several  of  these  intersecting  gen- 
erally in  well-marked  angles.  Such 
faceted  pebbles  or  dreikanter  (so- 
called  because  of  the  usual  presence 
of  three  edges)  are  of  common  oc? 


FIG.  339.  —  Eolian  carved 
pebble  — ' '  Einkanter. ' '  If  the 
small  face  on  the  lower  side 
were  enlarged  the  usual  type  of 
"dreikanter"  would  result. 
Marthas  Vineyard,  Mass. 
Gardner  collection  of  photo- 
graphs, No.  542.  (Courtesy  of 
Geological  Department,  Har- 
vard University.) 


currence.    Sometimes  only  two  facets  are  cut,  these  meeting  in  an 
edge;  then  the  pebble  is  called  an  einkanter  (Fig.  339). 

Abrasion  of    Sand  Grains.  —  The  sand  grains  carried   by  the 
wind  are  themselves  affected  by  the  impact  against  the  rock  sur- 


FIG.  340.  —  Erosion  monuments  of  white  sandstone,  with  harder  beds  ce- 
mented by  iron  oxide.  One  of  these  cuts  the  large  monument  near  the  middle, 
and  forms  the  capstones  of  the  smaller  ones.  These  beds  were  formerly  con- 
tinuous. Monument  Park,  Colorado.  (Darton,  photo ;  from  U.  S.  G.  S.) 


FIG.   341  a.  —  Erosion    forms   of   a    jointed    sandstone    (Quader    Sandstein) 
Bastei,  Saxon  Switzerland,  on  Elbe  River.     (After  Kayser,  Lehrbuch.) 


408 


The  Formation  of  Clastic  Material 


faces  as  well  as  against  one  another.  A  result  of  this  is,  that  the 
grains  of  softer  rocks  such  as  limestones,  and  those  of  cleavable 
minerals,  such  as  feldspar,  are  often  completely  ground  to  dust, 
which  is  carried  far  away,  while  the  grains  of  the  harder  minerals, 
such  as  quartz,  have  their  angles  worn  off  and  their  surfaces 
more  or  less  pitted  and  given  the  appearance  of  ground  glass. 
Grains  which  have  been  repeatedly  worn  in  this  way  may  in 


FIG.  341  b.  —  Erosion  pillars 
(Three-finger  tower)  in  sandstone, 
due  to  weathering  and  deflation 
along  joint  cracks  in  the  rock. 
Note  the  size  of  the  men  for  com- 
parison. Bastei,  on  the  River 
Elbe,  Saxony. 


FIG.  342  a.  —  Erosion  forms  in  jointed 
sandstone,  Cedar  Breaks,  Utah.  (Photo 
by  F.  J.  Pack.) 


time  approach  a  perfectly  spherical  form  (millet-seed  sands),  by 
which,  as  well  as  by  other  characters,  their  origin  as  wind-blown 
sands  can  be  recognized.  Sand  grains  are,  however,  rounded 
in  other  ways,  though  on  the  whole  less  perfectly.  (See  Figs.  361  a, 
b,p.  440,  370  a,  b,  p.  452.) 

Combined  Deflation  and  Corrasion.  —  Where  materials  are 
loosened  by  alternate  heat  and  cold,  they  can  be  removed  from  the 
exposed  surface  by  wind  and  at  the  same  time  act  as  tools  for  the 


Destructive  Work  of  the  Atmosphere          409 


corrasion  of  the  rock.  By  such  means  extensive  masses  of  rock  are 
removed  in  the  course  of  time,  and'  the  material  carried  elsewhere, 
where  it  is  redeposited. 

Rock  masses  are  com- 
monly cut  by  crevices, 
which  are  called  joints, 
and  which  penetrate 
them  in  all  directions. 
Wind  erosion  is  most 
active  in  these  crevices, 
widening  and  enlarging 
them.  In  this  manner, 


FIG.  342  b. — Weird  sculptur- 
ing by  the  wind  in  Tertiary 
rocks  of  the  Uinta  Bad  Lands, 
north  of  White  River,  Utah. 
The  harder  portions  have  been 
etched  in  relief.  (American 
Museum  Natural  History.) 

pillars  of  rock  may  become 

separated  from  the  original 

mass,  and  these  pillars  may 

be  carved  by  the  wind  into 

fantastic    shapes;    for  the 

weaker  portions  of  the  rock 

will    be    cut    away    more 

rapidly,  leaving  the  harder 

portions     in     relief     (Fig. 

342  b).     Exceptionally  fine 

examples  of  such  pillars  are 

found  in  Monument  Park, 

Colorado  (Fig.  340),  in  the 

famous  Bastei  region  along 

the  river   Elbe   in   Saxony 

(Figs.     341  a,    b),     in    the 

semiarid  regions  of  the  west 

(Figs.  342  a,  b),  in  Egypt, 

and  elsewhere.     Sometimes 

only  isolated  tables  or  buttes 


FIG.  343.  —  Erosion  monument,  Utah. 
The  spire  rises  2000  feet  above  the  base  in 
the  foreground.  (Photo  by  F.  J.  Pack.) 


4io 


The  Formation  of  Clastic  Material 


of  rock  will  arise  from  an  otherwise  flat  surface  (Figs.  343,  344) 
and  represent  the  last  remnant  of  a  formerly  continuous  layer 
which  has  been  almost  completely  removed  by  wind  work 


FIG.  344.  —  A  butte  of  horizontal  red  sandstone  capped  by  white  gypsum, 
and  surrounded  by  talus  slopes.  The  strata  once  extended  widely  over  the 
region.  Northeast  of  Cambria,  Wyoming.  (Photo  by  \Darten,  U.  S.  G.  S. 
Courtesy  of  D.  W.  Johnson.) 


DESTRUCTIVE  WORK  OF  THE  HYDROSPHERE 
Rain-Water 

Erosive  Work  of  Rain.  —  Falling  rain  forms,  in  a  measure,  a 
transition  from  the  atmosphere  to  the  hydrosphere,  originating  in 
the  former  and  becoming  a  part  of  the  latter  as  it  reaches  the  earth's 
surface.  Rain  erosion  is,  on  the  whole,  confined  to  soft  rocks,  such 
as  salt  beds,  and  to  unconsolidated  clays  and  sands,  from  which  it 
carves  pillars,  buttresses,  and  other  erosion  forms  (Figs.  345  a,  b). 
Its  effect  is  partly  mechanical,  through  impact,  followed  up  by 
the  rivulets  into  which  the  rain-water  unites,  and  partly  chemical, 
effecting  solution  or  corrosion  on  salt,  gypsum,  and  limestone  beds 
(Figs.  346  a,  b).  Cold  rain-water  falling  upon  highly  heated  rocks 
will  greatly  aid  in  shattering  them. 


Destructive  Work  of  the  Hydrosphere          411 


Disposition  of  Rain-water.  —  The  water  which  falls  upon  the 
earth  as  rain  is  disposed  of  in  three  ways.  A  part  of  it  again  evap- 
orates, this  being  most  pro- 
nounced where  there  is  much 
vegetation  to  retain  it  for  a 
time.  A  second  part  runs 
off,  following  the  slope  of  the 
land.  This  is  most  abundant 
in  regions  of  little  or  no  vege- 
tation, on  surfaces  composed 
of  hard,  impervious  material, 
and  on  steep  slopes.  This 
run-off  marks  the  beginning 
of  river  formation.  A  third 
part  finally  sinks  into  the 
ground,  and  becomes  the 
ground-water.  This  is  most 
abundant  where  the  surface 
is  composed  of  loose,  porous 

material,     and  where    its 

_                                              .  FIG.  345  a.  —  Earth  pillars  protected 

flatness  prevents  much   run-  by  a  cap  of  rock  and  due  to  rain  erosion, 

off.  Colorado.     (After  Hayden.) 


Rivers  and  the  Products  of  Their  Erosion 

Origin  of  Rivers.  —  As  we  have  seen,  the  run-off  of  the  surface 
waters  follows  the  slope  of  the  land.  At  first,  if  the  slope  is  a  ho- 
mogeneous one,  a  sheet-flood  may  result,  i.e.  a  broad,  shallow  sheet 
of  water,  without  definite  boundaries,  runs  down  the  slope.  Soon, 
however,  because  the  water  washes  away  loose  material,  some  por- 
tion of  the  surface  will  be  excavated  somewhat  more  deeply  than 
other's,  and  more  water  will  be  concentrated  in  this  deeper  portion. 
This  leads  to  increased  removal  of  the  loose  material  along  that 
line,  and  a  gully  is  formed,  which  is  gradually  deepened  and  wid- 
ened into  a  stream  bed.  At  first  it  is  a  dry  gully,  carrying  only 
the  run-off  after  rains.  But  as  soon  as  the  gully  is  deepened  suffi- 
ciently to  tap  the  surface  of  the  ground-water,  springs  are  formed 
along  its  sides,  and  a  more  or  less  permanent  stream  comes  into 
existence.  In  arid  regions  where  the  surface  of  the  ground-water 
lies  deep,  large  valleys  or  arroyos  (called  wadis  in  Africa)  may  be 


412 


The  Formation  of  Clastic  Material 


cut  by  the  run-off  without  reaching  the  ground- water  level.  These 
arroyos  will  therefore  carry  water  only  in  rainy  periods,  being  dry 
for  the  remainder  of  the  season. 


,  •• 


FIG.  345  b.  —  Earth  pillars  at  Bozen,  in  the  Tyrol,  due  to  rain  erosion  of 
glacial  till.  The  larger  rocks  often  form  protecting  caps  of  the  pillars.  (After 
Walther.) 

Erosive  Work  of  River-water.  —  The  erosive  power  of  river 
water  varies  as  the  square  of  the  velocity  of  that  water.  This 
will  be  appreciated,  when  it  is  realized  that,  if  the  speed  of  the  river 


Destructive  Work  of  the  Hydrosphere         413 

is  doubled,  it  will,  in  the  same  time  period,  hurl  twice  as  many 
sand-grains  as  before,  against  an  exposed  rock  surface  in  its  bed ; 
but  it  will  also  throw  each  grain  with  twice  the  force  of  its  former 


FIG.  346  a.  —  Typical  exposure  of  limestone  showing  peculiar  weathering 
forms  due  to  solution.  (Photo  by  Collier,  U.  S.  G.  S.  Courtesy  D.  W.  John- 
son.) 

speed.  Thus  the  rock-surface  will  be  eroded  four  times  as  fast  as 
before  the  doubling  of  the  speed.  The  velocity  of  the  water  is 
determined  partly  by  the  slope  of  the  river  bed,  and  partly  by  the 
volume  of  the  water  passing  a  given  point  within  a  given  time. 


FIG.  346  b.  —  Characteristic  surface  features  on  the  Sentis  in  Switzerland 
produced  by  solution  on  exposed  limestone  surfaces.  These  are  called  lapiaz 
or  rasdes  by  the  French  and  karren  by  the  Germans.  (After  A.  Heim.) 


414  The  Formation  of  Clastic  Material 

Thus  in  steeper  portions  of  a  river-bed,  a  greater  velocity  exists 
and  hence  more  erosion  is  accomplished.  Again,  if  the  volume  of 
river-water  is  increased  after  a  rainfall,  its  velocity  and  erosive 
power  are  increased. 

The  erosive  work  of  river- water  comprises  denudation,  or  the  re- 
moval of  loose  material  produced  by  weathering,  etc. ;  corrasion, 
or  the  actual  scraping  or  rubbing-off  of  material  from  the  rock  over 
which  it  flows ;  abrasion  of  the  pebbles  and  sand  grains  which  it 
moves  along ;  quarrying  by  undermining  of  the  banks  or  at  water- 
falls and  the  subsequent  breaking  down  of  the  undermined  rock, 
and  chemical  corrosion  or  solution. 

Denudation  is  chiefly  confined  to  the  early  stages  of  the  river 
when  the  water  first  runs  off  on  the  surface  of  the  mantle  rock,  and 
by  removing  part  of  this,  produces  its  gully.  In  mature  rivers,  too, 
the  weathering  of  the  banks  provides  material:  which  the  river  re- 
moves, and  so  do  lateral  branches,  which  deposit  their  own  debris 
upon  the  floor  of  the  main  valley,  whence  the  main  stream  must 
remove  it  by  denudation. 

Corrasion,  or  the  mechanical  wearing  away  of  the  rock  of  the 
river  bottom  and  sides,  is  perhaps  the  most  important  erosive  ac- 
tivity of  streams.  This  is  accomplished  by  the  tools  which  the 
river-water  carries  along,  these  tools  being  the  sand  or  pebbles  ob- 
tained by  denudation  or  by  corrasion  farther  up  stream.  If  the 
sand  and  fragments  carried  along  by  the  river  are  largely  quartz 
or  a  similar  hard  mineral,  while  the  bed  and  banks  are  composed 
of  softer  rock,  the  erosive  effects  will  be  most  marked  in  the  latter, 
though  the  fragments  themselves  will  be  gradually  worn  to  rounded 
pebbles,  and  the  quartz  grains  will  likewise  be  rounded.  This  wear- 
ing of  the  pebbles  and  sand  is  called  abrasion. 

Abrasion  is  also  accomplished  by  mutual  attrition,  or  the  impact  of  fragment 
upon  fragment,  as  they  are  moved  along  by  the  water.  If  the  loose  material 
is  of  soft  rock,  it  will  be  rapidly  destroyed  by  such  abrasive  activities,  and  the 
average  size  of  the  material  will  become  smaller  as  we  proceed  downstream. 
This  is  well  illustrated  by  observations  made  on  the  river  Mur  in  Austria,  where 
at  the  city  of  Graz  the  average  size  of  the  rock  fragments  in  the  stream  was  224 
cubic  centimeters.  Farther  downstream  the  average  size  diminished  at  the 
following  rates,  there  being  here  no  new  material  supplied  along  the  course : 
At  Graz,  224  cc.  56  km.  below  Graz,  81  cc. 

10  km.  below  Graz,     184  cc.  71  km.  below  Graz,      60  cc. 

26  km.  below  Graz,    132  cc.  83  km.  below  Graz,      50  cc. 

43  km.  below  Graz,     1 17  cc.  101  km.  below  Graz,      33  cc. 

1 20  km.  below  Graz  (at  Unter  Manthdorf),  21  cc. 


Destructive  Work  of  the  Hydrosphere         415 

The  average  distance  necessary  for  rock  fragments  of  different  hardness  and 
consistency  to  be  carried  by  rivers  before  they  become  completely  destroyed 
has  been  determined  for  some  rocks  to  be  as  follows : 

Sandstone  (Rhaetic  formation)  av.  wt.  40  grams,  15  km. 

Clay-slate,  av.  wt.  24  gr.  42  km. 

Compact  limestone  (Ordovician  Orthoceras  limestone)  av.  wt.  61  gr.  64  km. 

Granular  limestone,  av.  wt.  40  gr.,  85  km. 

Granite,  av.  wt.  36  gr. ,  2  78  km. 

The  vast  superiority  of  resistance  of  granite  over  that  of  the  other  rocks  is  at 
once  apparent.  A  similar  differential  destruction  of  the  minerals  derived  from 
the  disintegration  of  rocks  is  effected  by  the  abrasive  process.  As  an  example 
may  be  cited  the  sands  of  some  Scottish  rivers  (Eastern  Moray)  which  are 
derived  from  the  destruction  of  the  fundamental  gneiss.  In  the  river  Spey, 
the  percentage  of  feldspar  at  one  point  (Cromdale)  was  18  and  of  mica  i,  while 
farther  down-stream  (at  Orton),  the  feldspar  made  up  only  12  per  cent  of  the 
sand.  In  the  river  Findhorn,  near  and  parallel  to  the  Spey,  42  per  cent  of 
feldspar  was  found  in  the  sand  at  one  point  (above  Dulsie  Bridge),  while  lower 
down  (between  Forres  Bridge  and  the  sea)  the  percentage  was  reduced  to  21. 
This  more  rapid  destruction  of  the  feldspar  is  in  large  part  due  to  the  ready 
cleavability  of  this  mineral.  The  fine  particles  resulting  from  this  destruction 
are  carried  away  by  the  current. 

Sand  grains  carried  by  river- water  are  rounded  by  mutual  attri- 
tion and  by  contact  with  larger  rock  masses.  The  problem  is  a 
complex  one,  but  in  general  it  may  be  said  that  the  rounding  is  less 
effective  than  in  the  case  of  wind-blown  sand.  Moreover,  it  is 
generally  true  that  the  smaller  grains  will  be  less  well  rounded  than, 
the  larger,  and  very  small  grains  may  not  be  rounded  at  all  (except 
by  solution),  while  grains  of  the  same  size  will  readily  be  rounded 
by  abrasion  in  air.  This  is  one  of  the  means  by  which  river  sand 
may  be  distinguished  from  wind-blown  sand,  but  it  must  be  used 
with  caution  because  of  the  many  modifying  factors.1 

Quarrying,  by  undermining  the  banks,  is  an  effective  way  of 
widening  the  river  valley  and  of  supplying  new  material  for  pebbles 
and  sand.  In  river  gorges  the  bank  on  the  outer  or  convex  side 
of  the  current  is  commonly  kept  vertical  or  overhanging  by  this  un- 
dermining process,  because  the  principal  current  of  the  river  always 
makes  more  pronounced  curves  than  the  river  as  a  whole,  and  so 
flows  close  to  the  foot  of  the  concave  bank  on  the  convex  side  of  the 
river,  as  illustrated  in  the  following  diagram  (Fig.  347  a).  On  the 

1  For  a  fuller  discussion  see  Grabau,  Principles  of  Stratigraphy,  pp.  253-257,  with 
literature  references,  and  a  recent  paper  by  J.  J.  Galloway,  which  attacks  the  problem 
from  the  experimental  point  of  view  (Am.  Journal  of  Science,  vol.  xlvii,  pp.  270-280). 


416 


The  Formation  of  Clastic  Material 


concave  side  of  the  river  (convex  bank)  a  talus  slope  is  generally 
formed  from  the  accumulation  of  the  products  of  weathering 
(Fig.  347  b). 

The  manner  in  which  a  stream  changes  from 
a  straight  to  a  winding  or  meandering  course 
may  be  briefly  set  forth. 

In  a  straight  channel  of  uniform  cross-section, 
occupied  by  running  water,  the  greatest  velocity 
of  current  is  found  in  the  center  and,  with  still 
air,  just  below  the  surface  of  the  water.  This  is 
readily  shown  by  experiments  with  floats  in 
mill-races,  or  other  artificial  canals.  The  reason 
for  this  is  the  fact  that  the  center  of  the  stream 
is  free  from  friction  which  affects  the  sides  and 
bottom  -of  the  stream,  while  the  surface  film  of 
water  is  affected,  though  to  a  much  less  degree, 
by  friction  against  the  atmosphere.  If  an  ob- 
P  _  jy  struction  is  placed  in  a  channel  on  one  side,  the 

showing   the  "stronger  me-    main  velocity  current  is  at  once  deflected  away 

andering  of  the  current  as    from  this  obstruction  and  obliquely  towards  the 

compared  with  the  river  as    opposite  side,  against  which  it  may  impinge,  and 

a  whole.  from  which  it  may  in  turn  be  again  deflected. 

This  time  the  deflection  will  be  in  the  opposite 

direction,  and  the  current  will  again  cross  the  channel  obliquely  in  its  down- 

stream course.     Thus  a  single  obstruction  may  throw  the  main  current  into  a 

zigzag     or    winding     course, 

which   causes   it   to   impinge 

alternately  against  the  right 

and  left  banks  in  its  onward 

flow.1       When     the    current 

strikes  the  bank  it  will  remove 

loose  material  by  its  impact, 

this  being  especially  marked, 

where    the   banks   consist  of 

unconsolidated    material. 

When  the  current  rolls  and 

sweeps    along    pebbles    and 

sand,    these    will    be   hurled 

against  the  bank  at  the  point 


of  impingement,  and  such 
river  tools  will  perform  their 
chief  corrasive  work  at  this 
place.  Thus  the  banks  be- 
come  indented  alternately  on 


FlG  34?  &.  _  Gorge  of.  the  Genesee  River 
below  portage,  showing  the  vertical  bank  (260 
feet  high)  on  the  outer  or  convex  curve  of  the 
river  (concave  bank)  and  the  talus-covered 
slope  on  the  opposite  side.  (Photo  by  author.) 


1  The  right  and  left  bank  of  a  stream  are  those  on  our  right  and  left  hand,  respect- 
ively, when  we  face  downstream. 


Destructive  Work  of  the  Hydrosphere          417 


opposite  sides,  and  this  causes  a  further  increase  in  the  deviation  of  the  current 
from  the  original  straight  line.  Meanwhile  a  part  or  the  whole  of  the  material 
removed  from  the  bank  at  the  point  of  impingement  is  dropped  again  in  the 
slacker  water  below  the  point  of  erosion,  or  in  general  opposite  the  point  where 
the  current  next  strikes  the  bank.  Thus  erosion  and  deposition  alternate  on 
the  same  side.  As  the  result  of  the  building  of  a  new  sandbank  on  one  side, 
and  the  indentation  of  the  old  bank  on  the  opposite  side,  the  stream  itself  will 


FIG.  348.  —  Diagram  illustrating  the  development  of  meanders  in  a  river. 
a,  a  straight  channel,  the  main  current  has  been  deflected  by  an  obstruction 
above,  and  has  assumed  a  winding  course ;  b,  the  entire  river  has  begun  to  me- 
ander by  cutting  into  the  banks  at  one  place  and  building  sandbanks  on  the-same 
side  next  below;  c,  extreme  meandering  course,  two  of  the  meanders  nearly 
touch,  and  the  formation  of  an  oxbow  on  the  left  is  imminent ;  d,  completion 
of  the  oxbow  or  cut-off. 

assume  a  winding  course,  as  shown  in  diagram  6,  Fig.  348.  The  continuation 
of  this  process  will  result  in  changing  the  original  straight  form  of  the  river 
into  a  series  of  curves  or  meanders,  as  they  are  called,  from  the  river  Meander 
(now  called  Mendere)  which  flows  into  the  yEgean  Sea  near  Miletus  (Palatia) 
in  Asia  Minor  and  in  which  such  curves  are  very  pronounced  (Fig.  348  c.  See 
also  Fig.  604).  The  continuation  of  this  process  may  result  in  the  cutting  off 
of  a  very  pronounced  meander  which  then  remains  behind  as  an  "oxbow" 
frequently  forming  a  crescentic  lake  (Fig.  348,  diagram  d.  See  also  Fig.  605). 

Undermining  is  also  carried  on  actively  at  the  foot  of  a  waterfall, 
where  the  spray  from  the  falling  water  attacks  the  cliff  behind  it. 
This  is  the  origin  of  the  Cave  of  the  Winds  at  Niagara,  where  a  hard 
limestone  forms  the  top  of  the  fall  and  a  softer  shale,  or  mud-rock, 
the  lower  part.  This  softer  rock  is  worn  away  by  the  spray,  both 
by  the  impact  and  by  freezing,  while  the  harder  limestone  projects. 


4i 8  The  Formation  of  Clastic  Material 

At  intervals  this  overhanging  mass  bre'aks  down,  and  large  blocks 
of  limestone  accumulate  at  the  foot  of  the  cliff  (Fig.  349).  The 
several  recorded  falls  of  Table  Rock  are  illustrations  of  such  sudden 
changes  after  a  long  period  of  undermining.  Waterfalls  also  per- 
form effective  erosion  upon  the  river-bottom  at  their  base,  espe- 
cially if  the  volume  of  water  is  great.  This  impact  of  the  water  and 
the  grinding  work  of  the  rock  fragments,  which  it  sets  in  motion, 


FIG.  349.  —  The  American  Falls  at  Niagara,  as  seen  from  the  Canadian 
side;  showing  the  numerous  blocks  of  limestone  which  have  fallen  from  the 
edge  of  the  cliff,  because  the  soft  shale  beneath  them  has  been  eroded  by  the 
spray.  The  force  of  the  water  is  insufficient  to  destroy  these  rocks  or  to  use 
them  in  eroding  the  bottom  as  is  the  case  at  the  Horseshoe  Falls.  (Photo  by 
author.  See  also  Figs.  666-669.) 

result  in  deep  excavation  of  the  bottom.  The  "  swimming  hole  " 
at  the  foot  of  a  small  waterfall  is  a. familiar  example.  The  Horse- 
shoe Falls  at  Niagara  have  excavated  the  river-bottom  to  a  depth 
of  150  to  200  feet,  or  more  than  the  height  of  the  falls  above  the 
river-level  at  their  base.  As  these  falls  have  been  slowly  receding 
for  a  long  time,  the  gorge  has  been  deepened  to  this  extent  through- 
out a  distance  of  over  three  miles  by  this  process.  (See  further, 
Chapter  XXIII.) 

There  are  many  special  erosion  features  of  rivers,  which  will  be 
discussed  under  the  sculpturing  of  the  earth's  surface.  Of  these, 


Destructive  Work  of  the  Hydrosphere          419 

pot-holes  may  be  mentioned  in  this  connection  (Fig.  350  a).     These 
are  local  excavations  in  the  river  bed,  formed  wherever  an  eddy  is 


FIG.  350  a.  —  Pot-holes  at  Harris  Salt  Springs,  worn  in  granitic  rock  by  eddies 
in  the  bed  of  a  stream,  Tuolumne  River,  Cal.  (Photo  by  Turner,  from  U.  S. 
G.  S.) 

produced  which  sets  up  a  whirling  motion  of  the  sand  and  pebbles. 
The  holes  cut  by  this  whirling  motion  are  round  in  section  and  have 


FIG.  350  b.  —  Pot-holes  above  the  present  level  of  the  stream  that  formed 
them.     (After  Geikie.) 


420  The  Formation  of  Clastic  Material 

vertical  sides.    They  may  vary  up  to  20  feet  or  more  in  diameter, 
and  may  be  of  equal  or  greater  depth.     They  are  most  frequently 


FIG.  351.  —  Watkins  Glen,  N.  Y.,  a  narrow  gorge  largely  formed  of  confluent 
pot-holes,  cut  in  shaly  sandstones  of  upper  Devonian  age. 

found  in  the  beds  of  mountain  torrents  and  near  waterfalls  (Fig. 
3506).     A  series  of  confluent  pot-holes  of  large  size  may  produce 


Destructive  Work  of  the  Hydrosphere          421 

a  deep  and  narrow  gorge,  as  is  the  case  in  Watkins  Glen  in  southern 
New  York  (Figs.  351,  352). 


FIG.  352.  —  One  of  the  lafger  pot-holes  in  Watkins  Glen,  N.  Y. 

Corrosion.  —  Solution  or  chemical  corrosion  is  on  the  whole  of 
little  importance  in  rivers,  though  it  is  accomplished  wherever 
limestones  or  other  soluble  rocks  are  encountered.  Most  of  the 
material  held  by  river-water  in  solution  is,  however,  furnished  to  it 


422  The  Formation  of  Clastic  Material 

by  the  underground  water  which  issues  as  springs  along  the  river 
border. 

Underground  Water 

Types  of  Underground  Water.  —  The  water  which  sinks  into 
the  ground  forms  a  part  of  the  underground  or  subsurface  water. 
The  quantity  of  such  water  depends  on  the  climate,  the  porosity 
of  the  rock  or  soil,  the  surface  slope  of  the  land,  and  other  factors. 
As  the  water  sinks  into  the  ground,  it  reaches  a  level  at  which  its 
further  progress  through  the  rock  and  soil  becomes  so  slow  that  it 
may  be  regarded  as  stationary.  This  forms  the  zone  of  saturation, 
and  the  water  there  forms  the  true  ground-water.  The  upper  level 
of  this  zone  is  called  the  water-table  or  ground-water  level,  and  it 
varies  in  depth  in  the  same  locality  with  the  season,  and  for  longer 
periods,  with  the  character  of  the  rock,  the  amount  of  rainfall,  etc., 
and  in  different  localities  with  the  climate  etc.  (p.  441).  It  is 
not  a  level  surface,  but  varies  with  the  contour  of  the  land  and 
other  factors.  It  is  this  lowest  average  level  of  the  water-table 
which  must  be  reached  by  wells  to  be  permanently  supplied  by 
unfailing  springs. 

The  spaces  in  which  this  ground- water  flows  are  partly  large  fissures  and 
partly  minute  interstices  between  the  grains.  These  interstices  vary  with  the 
character  of  the  rock  and  to  some  extent  with  its  age.  The  larger  openings 
also  vary  with  the  character  of  the  rock  and  the  disturbances  to  which  it  has 
been  subjected.  They  comprise  the  planes  of  bedding  between  sedimentary 
rocks,  the  joints  and  other  fissures  which  traverse  it,  solution  cavities,  etc.  In 
the  larger  fissures  the  ground-water  is  chiefly  controlled  by  gravitation,  but  in 
the  smaller  interstices  within  the  rock  the  water  is  subject  to  the  force  of  ad- 
hesion, which  is  the  mutual  attraction  of  rock  and  water,  in  minute  quantities. 
This  force  of  adhesion  is  most  pronounced  in  rocks  with  the  smallest  interstices. 
When  the  interstices  are  so  fine  that  their  walls  will  attract  all  the  water  they 
can  hold,  or  if  interstices  are  few  and  far  between  or  are  absent  altogether,  the 
rock  is  called  impervious  to  water  under  ordinary  conditions,  that  is,  it  will  not 
permit  water  to  pass  through  it.  This  of  course  implies  that  no  cracks  or  fissures 
of  subsequent  origin  are  present.  Other  rocks  are  more  or  less  pervious  or  per- 
meable to  ground-water  under  ordinary  conditions,  the  degree  of  permeability 
depending  on  the  porosity  of  the  rock  and  on  the  amount  of  fissuring  which  it 
has  suffered. 

That  solid  appearing  rocks  also  will  take  up  water  can  easily  be  shown  by 
immersing  them  in  water,  and  noting  the  difference  in  weight  between  the  dry 
and  water-soaked  rock.  The  water  occupies  the  spaces  between  the  rock 
particles.  Such  spaces  may  constitute  as  much  as  thirty  per  cent  or  over 
(in  some  cases  60  per  cent)  of  the  volume  of  the  rock,  or  they  may  form  less 


Destructive  Work  of  the  Hydrosphere          423 

than  one  per  cent  of  its  volume.  The  greatest  porosity  is  found  in  rocks  com- 
posed of  grains  of  uniform  size  and  spherical  form,  especially  if  the  arrangement 
of  the  grains  is  such  that  they  rest  vertically  one  upon  the  other.  Rocks  made 
up  of  rounded  and  uniform  grains  are  best  produced  from  wind-blown  sands, 
and  hence,  where -such  rocks  are  part  of  the  bedded  rock  series  of  the  earth  s 
crust  they  form  good  media  for  the  transmission  and  storage  of  underground 
water  provided  the  changes  in  character  and  position  which  they  have  under- 
gone 'are  not  unfavorable  to  such  processes.  Unconsolidated  sands  and 
pebble  beds  are  of  course  among  the  best  of  media  for  the  transmission  and 
storage  of  underground  water,  but  if  these  are  surface  deposits,  they  do  not 
afford  good  reservoirs  for  well  waters,  since  they  also  admit  the  surface  drainage. 
The  rate  of  flow  of  underground  water  varies  greatly,  being  especially  in- 
fluenced by  the  porosity  of  the  rock.  Though  locally  it  may  be  relatively  rapid, 
the  average  rate  of  flow  is  probably  not  much  over  a  mile  a  year. 

As  stated  above,  the  name  ground-water  is  applied  to  the  water 
in  the  zone  of  saturation,  that  is,  the  water  below  the  water-table. 
This  water  is  also  called  phreatic  water,  a  term  derived  from  the 
Greek  word  for  well,  because  it  is  the  water  which  supplies  wells 
and  springs.  The  water  in  the  rock  and  soil  above  the  water-table, 
held  there  temporarily  by  molecular  attraction  against  gravity, 
but  slowly  sinking  to  the  level  of  the  water-table,  constitutes  a  part 
of  the  vadose  water,  which  is  the  name  applied  to  all  the  subsurface 
water  circulating  through  the  soil  or  rocks  above  the  water-table. 

The  best  conditions  for  the  storage  of  well-water  are  found  when  a  horizontal 
or  gently  inclined  pervious  bed  is  covered  by  an  impervious  one,  for  this  wi  1 
keep  out  the  surface  drainage.  It  is  of  course  necessary  that  all  wells  sunk 
through  such  an  impervious  bed  should  be  properly  protected  above  the  level 
of  that  bed,  to  prevent  the  entrance  of  surface  waters.  The  entrance  of  the 
water  into  the  pervious  bed  takes  place  at  its  outcrop,  which  may  b 
miles  removed  from  the  region  where  the  water  is  tapped. 

The  surface  of  the  ground-water  or  the  water-table  is  not  a  level  one.     It 
rises  inland  from  the  sea,  and  conforms  somewhat  to  the  irregularities  of  the 
country  but  is  much  less  diversified.     The  porosity  of  the  rock,  the  amount 
rainfall  in  the  region,  the  presence  or  absence  of  much  vegetation,  especially 
forests,  etc.,  are  among  the  factors  which  influence  the  position  of  the  watei 
table  in  a  given  region. 

Where  valleys  are  cut  below  the  surface  of  the  ground-water  level,  tl 
water  will  issue  along  their  sides  in  the  form  oi.  hillside  springs.    Where  t 
ground-water  level  and  that  of  the  surface  of  the  land  coincide,  swamps  are 
produced ;    where  the  land-surface  is  depressed  below  the  normal  wate       /el, 

lakes  are  formed. 

Where  porous  or  permeable  layers  enclosed  by  impervious  ones  crop  c 
along  the  mountain  sides  or  at  a  considerable  elevation,  the  water  entem 
them  will  gradually  find  its  way  to  lower  levels,  but  because  of  the  elevation 


424 


The  Formation  of  Clastic  Material 


.      ^3    tn    O  .t} 

s     g  -  .2 


of  the  point  of  entrance,  the  water  thus  stored  in  the  porous  rock  will  be  under 
great  hydrostatic  pressure.    If  now  wells  are  sunk  in  the  low  country  to  this 

porous  rock,  the  water  will  rise,  because  of 
the  pressure  it  is  under,  and  overflow  at  the 
surface,  or  if  the  country  in  which  the  well  is 
sunk  is  much  lower  than  the  head  of  water  at 
the  intake,  a  gushing  well  may  result.  Such 
wells  are  called  artesian  wells.  Their  rela- 
tionship to  the  intake  near  the  mountains 
-is  shown  in  the  diagram  (Fig.  353).  If  the 
rock  of  the  low  country  is  intersected  by 
dislocation-planes  or  faults  (see  Chapter 
XIX),  the  water  may  rise  along  such  planes 
and  form  artesian  springs. 

Beside  the  two  types  of  subsurface 
waters  mentioned,  there  are  two 
others  to  which  reference  has  already 
been  made  in  previous  chapters. 
These  are  the  connate  waters,  or  the 
waters  originally  included  in  the  sedi- 
ments deposited  in  water,  either  salt 
or  fresh,  and  the  magmatic  or  juvenile 
waters,  which  are  formed  as  emana- 
tions from  deep-lying  igneous  mag- 
mas. These  are,  however,  of  little 
inportance  in  rock  destruction. 

Destructive  Work  of  Underground 
Water.  —  The  only  kind  of  destruc- 
tive work  which  underground  water 
performs  is  chemical,  comprising  so- 
lution and  hydration,  and  indirectly, 
oxidation  and  carbonation  by  carry- 
ing oxygen  and  carbon  dioxide.  Oxi- 
dation, hydration  and  carbonation 
have  already  been  described. 

Solution.  —  This  is  most  effective 
in  limestone  regions,  where  caverns 
are  dissolved  out  by  the  circulating 
ground-water  above  the  water-table, 
i.e.  within  the  vadose  belt.  Solution 

of  limestones  is  accomplished  by  the  aid  of  carbon  dioxide  which 
the  water  carries,  and  in  the  resulting  solution,  an  extra  mole- 


(L»    pr 

-5  8 


ll*sf.a 


<fi 


iff 

e  &  = 

.3-0  1 

"  -a  E 


I  8-5  j  I 


Destructive  Work  of  the  Hydrosphere         425 

cule  of  that  gas  is  locked  up  for  each  molecule  of  calcium  car- 
bonate. On  the  re-deposition  of  the  calcium  carbonate,  this  extra 
molecule  of  CO2  is  again  liberated.  Caverns  are  most  readily  dis- 
solved out  along  the  bedding  planes  of  the  limestone  and  along 
the  cracks  or  joints  which  intersect  it.  After  the  cavern  has  been 
dissolved  out  it  is  slowly  filled  up  again  by  stalactite  and  stalag- 
mite growths,  though  this  may  go  on  in  one  part  of  -a  cavern 
while  solution  enlarges  another.  If  the  water-table  rises  so  that 
the  ground-water  will  drown  the  cave,  its  walls  may  become  lined 
with  crystals  of  calcite,  often  of  enormous  size,  or  with  other 


FIG.  354.  —  Diagram  of  a  cavern,  showing  different  levels  of  the  under- 
ground stream.  (After  G.  C.  Matson,  U.  S.  G.  S.)  AB,  Connecting  funnels 
between  the  upper  and  lower  cavern.  The  latter  is  undergoing  active  solu- 
tion, the  ground-water  level  having  been  lowered  to  it,  while  deposition  of 
stalactites  and  stalagmites  goes  on  in  the  upper  level,  where  pillars  have  also 
been  formed.  A  sink  hole  is  seen  on  the  left  of  the  diagram,  and  several  others 
filled  with  soil  are  indicated.  (See  also  Figs.  184  and  185,  pp.  263,  264.) 

deposits  (copper  salts,  etc.).  The  crystal  caves  of  Missouri  are 
examples.  The  characteristic  deposits  in  limestone  caves  have 
already  been  discussed  (pp.  262-265). 

On  the  surface  of  the  country  where  caves  abound  there  are  com- 
monly found  more  or  less  circular  depressions  called  sink-holes  or 
swallow-holes  (Fig.  354),  which  form  entrances  for  the  waters  to 
the  cave  and  were  dissolved  by  the  waters  thus  entering  the  under- 
ground drainage.  Other  irregular  hollows  are  formed  by  the  break- 
ing or  incaving  of  the  roof  of  the  cavern.  The  Natural  Bridge  of 
Virginia  (Fig.  355)  is  the  last  remnant  of  a  cavern  roof. 

Other  rocks  besides  limestones  are  subject  to  solution  by  the  sub- 
surface waters.  Among  these  rock  salt  and  gypsum  are  the  most 
conspicuous.  Even  quartz  is  dissolved,  especially  when  the  sub- 


426  The  Formation  of  Clastic  Material 

surface  waters  contain  much  humic  acid  from  plant  decay,  and  it 
is  indeed  this  dissolved  silica,  which  is  carried  by  the  streams  into 


FIG.  355.  —  Natural  Bridge,  Virginia.      A  remnant  of  a  former  cave  roof. 
The  rock  is  lower  Ordovician  limestone.      (Courtesy  of  D.  W.  Johnson.) 

the  sea,  that  constantly  supplies  the  material  from  which  the  radio- 
larians  and  the  diatoms  construct  their  silicious  cases. 
The  rate  of  solution  varies  greatly,  and  is  strongly  influenced  by 


Destructive  Work  of  the  Hydrosphere          427 

the  temperature  of  the  ground-water.  When  this  temperature 
approaches  the  freezing  point,  the  solvent  action  of  the  water  is 
practically  lost.  The  warm  springs  of  Bath,  England,  with  a  mean 
temperature  of  120°  F.,  discharge  annually  a  quantity  of  dissolved 
sulphates  of  calcium  and  sodium,  and  chlorides  of  sodium  and  mag- 
nesium, sufficient  to  make  a  square  column  of  mineral  matter  nine 
feet  in  diameter  and  140  feet  high.  The  great  deposits  of  litho- 
graphic limestone  at  Solnhofen  in  Bavaria  are  reduced  by  solu- 
tion one  meter  in  thickness  in  every  72,000  years,  and  the  similar 
extensive  limestones  of  the  Nittany  Valley  in  Pennsylvania,  one 
meter  in  every  30,000  years.  The  materials  removed  by  solution 
each  year  throughout  the  entire  globe  have  been  estimated  at 
96  tons  per  square  mile,  of  which  calcium  carbonate  constitutes 
50  tons,  calcium  sulphate  20  tons,  sodium  chloride  8  tons,  silica  7 
tons,  alkaline  carbonate  and  sulphates  6  tons,  and  oxide  of  iron  i  ton 
(T.  Mellard  Reade),  leaving  4  tons  for  other  substances. 

Indirect  Formation  of  Clastic  Rock  Due  to  Underground  Solu- 
tion. —  When  the  roof  of  a  cave  breaks  down,  the  rock  comprising 
it  is  broken  into  large  and  small  angular  fragments,  which  are 
piled  together  in  a  confused  heap.  Such  breaking  down  of  the  sur- 
face rock  is  frequently  seen  in  limestone  regions,  and  is  even  more 
marked  where  gypsum  and  salt  have  been  extracted.  Thus  in 
many  regions  of  western  England,  where  the  salt  (Permian)  is  mined 
by  the  solution  process,  extensive  cavings  have  occurred,  often  de- 
stroying villages  and  parts  of  small  towns.  Because  of  the  angular- 
ity of  the  material  resulting  from  such  a  break,  it  has  a  distinctive 
character,  and  the  name  brecciation  is  applied  to  the  process,  and 
brecciated  material  to  the  result.  When  reconsolidated  it  becomes 
a  brecciated  rock,  or,  for  short,  a  breccia.  Such  cavern  breccias 
are  not  uncommon  among  the  rocks  of  the  earth's  crust.  Other 
breccias,  such  as  fault  breccias,  volcanic  breccias,  etc.,  have  al- 
ready been  referred  to.  (See  further  Chapter  XVIII.) 


Destructive  Work  of  the  Waves 

The  waves  of  the  sea  and  of  lakes  exert  a  powerfully  destructive 
effect  upon  the  margins  of  the  land  on  which  they  border.  This  de- 
structive effect  is  due  in  part  to  the  mere  impact  and  lifting  power 
of  the  water  and  its  prying  power  where  it  enters  crevices,  and  in 
part  it  is  due  to  the  work  of  tools  which,  in  the  form  of  sand  and 


428  The  Formation  of  Clastic  Material 

rock  fragments,  are  hurled  by  the  waves  against  the  cliff,  or  moved 
along  over  a  rock  surface. 

The  force  of  the  waves  in  great  storms  is  often  very  surprising.  Measure- 
ments on  the  north  Scottish  coast  have  shown  the  force  of  impact  to  be  611 
pounds  per  square  foot  in  summer  and  2086  pounds  per  square  foot  in  winter, 
while  exceptional  storms  may  produce  an  impact  of  waves  nearly  three  times 
that  amount.  The  greatest  recorded  pressure  was  6083  pounds  per  square  foot 
during  a  heavy  westerly  gale  near  the  end  of  March,  1845.  Measurements 
recorded  from  the  French,  Italian,  and  north  African  coasts  show  ranges  from 
600  to  more  than  3000  pounds  per  square  foot.  As  an  illustration  of  the  power 
of  the  waves  to  lift  rock  material  may  be  cited  the  breakwater  in  the  harbor  of 
Wick  on  the  northeast  coast  of  Scotland.  This  breakwater  consisted,  above 
the  foundation,  of  three  large  blocks,  weighing  80  to  100  tons  each,  across  which 
a  huge  concrete  monolith,  weighing  over  800  tons,  was  cast  in  place  and  .firmly 
anchored  to  the  blocks  by  iron  chains.  The  total  mass  weighed  1350  tons,  and 
yet  during  the  great  storm  of  December,  1872,  it  was  lifted  by  the  waves  and 
hurled  into  the  inner  harbor,  a  distance  of  10  to  15  meters,  the  monolith  and 
three  foundation  stones  remaining  anchored  together  and  being  moved  as  a 
single  mass. 

The  erosive  work  of  waves  is  confined  to  ablation  processes,  and 
these  are  chiefly  restricted  to  quarrying  and  corrasion  and  abrasive 
work.  There  is,  in  addition,  a  certain  amount  of  solution  going  on, 
as  well  as  prying  by  the  freezing  of  the  spray  and  the  compression 
of  air  in  crevices,  but  these  are  of  minor  importance. 

Wave  Quarrying.  —  By  the  constant  impact  of  waves  against 
the  cliffs,  rock  fragments  are  loosened  and  lifted  from  their  position. 
This  is  especially  the  case  where  the  rock  of  the  cliff  is  traversed 
by  cracks  or  fissures.  On  the  Massachusetts  coast,  the  basaltic 
dikes  commonly  have  a  rude  columnar  structure,  the  columns  ex- 
tending across  the  dike  from  wall  to  wall.  Wave  impact  loosens 
these  columns,  and  they  are  then  lifted  out  by  the  waves  and  car- 
ried away,  and  thus  the  shoreward  ends  of  these  dikes  are  marked 
by  deep  parallel-sided  chasms  (see  Fig.  135,  p.  192).  On  the  Irish 
coast,  the  columns  of  basalt  which  form  the  Giant's  Causeway 
(Fig.  1 20,  p.  176)  are  similarly  removed  by  the  waves,  and  on  the 
island  of  Staffa  on  the  Scottish  coast,  such  removal  has  produced 
the  famous  FingaPs  Cave  (see  Fig.  122,  p.  178).  Other  examples 
are  found  on  the  coast  of  the  Bay  of  Fundy  and  elsewhere,  and 
such  removal  of  columns  produces,  usually,  a  steep  or  vertical  cliff. 
When  granite  cliffs  dip  into  water  of  moderate  depth,  steep  cliffs 
are  seldom  produced,  especially  if  the  granite  shows  few  fissures 
or  structure  planes  which  permit  such  quarrying  operations.  The 


Destructive  Work  of  the  Hydrosphere          429 


waves  have  comparatively  little  effect  upon  granite,  which,  because 

of  its  hardness,  is  not  easily  corraded.     The  upper  parts  of  the  cliff, 

therefore,  being  subject  to  the  attack  of  the  weather,  will  retreat 

more   rapidly   than  the 

lower,    and    a    sloping 

granite    headland    will 

project     into     the    sea 

(Fig.     356  a).       If    the 

granite    is    fine-grained 

and     is     traversed     by 

many  joint  fissures,  the 

quarrying  operations  of 

the  waves  are  of  course 

facilitated. 

Rocks  which  are  char- 
acterized by  bedding 
planes  may  be  quarried 
into  large  blocks  by 
waves.  Such  effects  are 
seen  in  the  limestone 
and  chalk  cliffs  of  the 
English  and  French 


FIG.  356  a.  —  Granite  headland,  Marble- 
head,  Mass.,  showing  the  sloping  surface  which 
is  due  to  the  more  effective  attack  of  the 
weather  on  the  upper  part  of  the  cliff  as  com- 
pared with  wave  erosion  which  is  effected 
mainly  in  the  joint  fissures.  (Photo  by 
author.) 


coasts  (Fig.  201,  p.  279, 

and  Fig.  713),  especially 

upon  the  North  Sea  coast  of  England.     Many  of  the  vertical  cliffs 

of  softer  rocks  are,  however,  produced  by  corrasion  at  their  base 

and  by  the  falling  of  the  masses  of  rock  thus  undermined. 

Corrasion  and  Abrasion.  —  When  the  waves  hurl  sand  and  rock 
fragments  against  the  base  of  a  cliff,  this  becomes  subject  to  wear 
by  corrasion.  The  sand  and  rock  fragments  at  the  same  time  suffer 
erosion  by  a  process  of  abrasion.  Cliffs  of  soft  rock  are  especially 
attacked  in  this  manner,  and  where  the  wave  work  is  more  or  less 
constant  at  their  base,  they  are  apt  to  present  a  vertical  face  (Fig. 
16,  p.  31).  The  rate  of  wear  varies,  of  course,  with  the  force  and 
constancy  of  the  waves  and  the  degree  of  softness  or  corradability 
of  the  rock,  as  well  as  with  the  hardness  of  the  tools  employed  by 
the  waves. 

The  rock  fragments  employed  as  tools  by  the  waves,  whether 
broken  off  by  the  waves  themselves  or  supplied  by  the  weathering 
of  the  cliff,  suffer  wear.  This  is  most  pronounced  upon  the  angles 


430 


The  Formation  of  Clastic  Material 


and  corners  of  the  fragments.  In  this  manner  angular  blocks  are 
rapidly  worn  into  rounded  pebbles  and  boulders  (Figs.  356  b,  c). 
Sand  grains  suffer  in  a  similar  manner,  and  the  softer  or  more 

cleavable  minerals 
among  them  are 
rapidly  destroyed, 
so  that  the  remain- 
ing sand  is  largely 
or  wholly  composed 
of  quartz  grains. 

On  shores  charac- 
terized neither  by 
rock  cliffs  nor  by 
glacial  material, 


FIG.  356  b.  —  A  sand  and  cobble-stone  beach  with 
rock  ledges  in  the  background. 


such  as  drumlin 
moraines,  etc.  (see 
Fig.  447),  pebbles 

and  boulders  are  relatively-  uncommon,  though  some  of  them  may 
be  transported  for  considerable  distances  by  shore  currents.  Even 
where  the  pebbles  are  not  hurled  against  a  cliff,  but  are  merely 
rolled  back  and  forth  by  the  waves,  mutual  impact  will  effect  a 


FIG.  356  c.  —  Pebbles  worn  from  bricks  by  the  waves,-  Nahant,  Mass.  These 
fragments  illustrate  three  stages  in  pebble  making.  The  first  is  rectangular, 
with  only  the  corners  and  edges  worn  off;  the  second  is  sub-rounded  and  the 
third  fully  rounded,  though  still  retaining  the  elongated  form  of  the  original. 
(F.  H.  Lahee  photo,  from  Gardner  collections  of  photographs,  No.  7401.  Cour- 
tesy Geol.  Dept.,  Harvard  University.) 

wearing   off   of  angles  and  corners,  and  rounded  pebbles  result. 
By  continued  wear  these  will  be  reduced  to  smaller  and  smaller 
size  and  may  eventually  be  reduced  to  fine  dust  or  rock-flour. 
Sands,  which  make  up  most  of  the  material  of  shores  not  bounded 


Destructive  Work  of  the  Pyrosphere          431 

by  cliffs,  also  have  their  grains  worn  round  by  mutual  abrasion. 
Such  rounding,  however,  affects  primarily  the  larger  grains,  which 
from  their  size  and  weight  come  more  constantly  in  contact  with 
one  another ;  the  smaller  grains,  being  more  readily  held  in  sus- 
pension by  the  agitated  waters,  suffer  less  abrasion  and  so  remain 
more  generally  in  an  angular  state.  Moreover,  the  wet  sand  of  a 
beach  commonly  forms  a  compact  mass,  owing  to  the  water  be- 
tween the  grains,  so  that  a  gently  sloping  sand  beach  is  often  nearly 
as  hard  surfaced  as  a  rock  floor.  Here  the  waves  have  little  effect, 
and  the  rounding  of  the  grains  by  mutual  abrasion  is  greatly  re- 
tarded (Fig.  357). 


FIG.  357.  —  Daytona  Beach,  Florida.     A  hard-packed  sand  beach,  extending 
for  many  miles.     A  favorite  beach  for  automobile  races. 

On  the  whole,  the  grains  of  beach  sands  may  be  regarded  as  much 
less  well  rounded,  than  those  worn  by  wind,  this  being  especially 
true  of  the  smaller  grains.  Nevertheless,  there  are  many  modify- 
ing influences,  and  caution  must  be  used  in  the  employment  of  this 
criterion  for  the  determination  of  the  origin  of  a  given  mass  of 
sand  grains. 

DESTRUCTIVE  WORK  OF  THE  PYROSPHERE 

In  volcanic  eruptions  of  the  explosive  type,  both  the  semi-fluid 
and  the  solid  lava  and  the  solid  rock  of  non- volcanic  origin  may  be 
shattered.  The  size  of  the  fragments  thus  produced  may  range 
from  large  blocks  to  the  finest  dust,  the  finer  material  being  as  a 
rule  more  abundant.  When  lava,  which  has  not  yet  become 
thoroughly  hardened,  is  shattered  by  an  explosion  during  the 
eruption,  these  f ragmen tal  lava  masses  are  hurled  into  the  air  and 
may  harden  in  the  process.  The  larger  blocks  will  fall  back  to  the 


432  The  Formation  of  Clastic  Material 

surface  in  the  neighborhood  of  the  volcano,  and  form  rounded,  elon- 
gated, or  otherwise  shaped  volcanic  bombs  (Fig.  58,  p.  116).  Such 
bombs  generally  show  rounded  surfaces  from  the  molding  process 
which  the  mass  of  lava  underwent  in  its  passage  through  the  air,  this 
being  often  accompanied  by  a  gyrating  or  spinning  movement. 
When  solidified  lavas  are  shattered,  large  angular  fragments  are  pro- 
duced, which  may  form  great  accumulations  and  be  subsequently 
bound  together  to  form  volcanic  agglomerates.  Other  rocks  shat- 
tered by  these  explosions  will  also  produce  angular  fragments, 
though  of  course  all  such  fragments  may  subsequently  be  rounded 
by  becoming  water-worn.  The  finer  products  of  shattering  of  still 
viscid  lavas  are  known  as  lapilli,  and  they  also  show  rounded  or 
smooth  surface  characters.  Fine  fragments  resulting  from  the 
shattering  of  solid  rocks,  on  the  other  hand,  will  be  sharply  angular. 
The  finest  volcanic  dust  is  carried  into  the  upper  atmosphere  and 
may  be  transported  by  the  air-currents  over  wide  areas  before  it 
settles  again.  Such  material  is  generally  recognized  only  by 
microscopic  examination,  which  reveals  the  fact  that  it  is  largely 
or  wholly  of  volcanic  origin.  (See  Fig.  488.) 

DESTRUCTION  OF  ROCKS  BY  MOVEMENTS  OF  THE  EARTH'S  CRUST 

When  through  some  disturbance  in  the  earth's  crust,  either  vol- 
canic or  under  the  influences  of  stresses  and  strains,  a  fracture  arises, 
movement  of  the  rock  masses  along  such  a  fracture  will  result  in  the 
grinding  and  fragmentation  of  the  contiguous  rock  walls.  Such 
movements  may  be  back  and  forward,  or  they  may  be  mainly  in 
one  direction,  the  moving  mass  producing  a  displacement  of  the 
crust  or  &  fault  (Figs.  542 , 543 ,  Chapter  XIX) .  Such  disturbances  are 
commonly  manifest  on  the  surface  as  earthquakes,  and  may  be  shown 
by  actual  displacements,  either  horizontally  or  vertically,  of  the 
adjoining  masses  of  rock  and  of  the  soil  and  other  structures  upon  it. 
(See  further,  Chapter  XXI.)  Within  the  fissure  the  shattered  ma- 
terial from  the  walls  will  accumulate,  commonly  in  the  form  of 
coarse  and  fine  fragments  thoroughly  intermingled.  The  fragments 
will  be  angular  and  will  consist  of  the  material  of  the  adjoining 
walls  of  the  fracture.  Such  a  mixture  of  angular  fragments  and 
fine  rock,  sand,  and  rock-flour  is  called  a  fault  rubble,  and  when 
consolidated  forms  &  fault  breccia  (Fig.  32,  p.  80).  Fault  breccias 
may  occur  in  vertical  or  inclined  fissures,  or  they  may  lie  in  hori- 


Rock  Destruction  by  Glaciers 


433 


zontal  beds  between  two  masses  of  rock,  one  of  which  has  been 
pushed  over  or  under  the  other,  or  both  of  which  may  have 
moved  past  each  other.  Its  angular  character,  and  the  fact  that 
the  material  consists  only  of  the  broken  fragments  derived  from  the 
enclosing  rocks,  distinguish  such  a  fault  breccia  from  other  rubble 
rocks. 

ROCK  DESTRUCTION  BY  GLACIERS 

Ice  is  a  rock,  though  only  a  temporary  one  in  most  climates,  and 
the  movement  over  other  rock  of  an  ice  mass,  such  as  that  of  a  gla- 
cier, is  analogous  to  the  movement  described  in  the  preceding  sec- 
tion. One  may  of  course  consider  a  glacier  as  a  frozen  river,  but 
the  erosive  effects  of  moving  ice  are  much  more  like  those  of  moving 
rock  masses  than  they  are  like  those  of  running  water. 

Glacial  erosion  is  wholly  mechanical,  and  is  accomplished  by 
plucking  and  sapping  or  quarrying  and  by  grinding  or  corrasion  and 


FIG.  358.  —  Limestone  polished,  furrowed,  and  scratched  by  the  modern 
glacier  of  Rosenlaui,  Switzerland.  (After  Agassiz,  from  Lyell.)^  aa,  white 
streaks  or  scratches  caused  by  small  grains  of  flint  frozen  into  the  ice ;  bb,  fur- 


rows. 


abrasion  (see  Chapter  XIV).  In  addition  to  this  a  newly  formed 
glacier  will  perform  denudation  or  the  removal  of  the  material  which 
had  accumulated  by  weathering  and  otherwise  on  the  surface  over 
which  it  moves,  and  prior  to  its  formation  or  advent.  This  is  the 
most  important  erosive  work  of  a  glacier  in  the  early  stages,  while 
later,  quarrying  (plucking  and  sapping)  and  corrasion  become  the 
chief  erosive  work  of  the  ice.  In  general,  the  loose  material  en- 


434  The  Formation  of  Clastic  Material 

countered  is  removed  by  becoming  incorporated  in  the  basal  portion 
of  the  ice  through  the  freezing  of  the  moisture  which  saturates  this 
material.  In  quarrying,  the  ice  mass  freezes  to  projecting  rocks  or 
in  fissures,  surrounding  such  a  projection,  and  in  its  movement  tears 
or  plucks  the  rock  away.  In  the  performance  of  corrasion  the  rock 
material,  frozen  firmly  into  the  base  of  the  ice,  scratches  and  pol- 
ishes the  rock  floor  over  which  it  moves.  If  the  motion  is  uni- 
form, series  of  parallel  scratches  are  formed,  which  indicate  the 
direction  of  ice  movement  (Figs.  358,  359,  360  a). 


FIG.  359.  —  A  glaciated  surface  of  gneiss,  Bronx  Park,  N.  Y.  The  bands 
extending  across  the  surface  from  the  lower  left-hand  side  are  the  gneissic 
bands ;  the  glacial  striae  run  nearly  at  right  angles  to  these.  (Photo  by  Willis ; 
from  U.  S.  G.  S.) 

The  fragments  which  the  ice  uses  as  tools  are  themselves  affected 
by  this  corrasive  process.  The  sands  are  ground  and  crushed  to 
fine  rock  flour,  which  is  one  of  the  most  characteristic  types  of  ma- 
terial produced  by  glacial  erosion.  This  must  be  distinguished 
from  clay  or  the  similarly  fine  material  due  to  atmospheric  decom- 
position. Under  the  ice  there  is  little  or  no  opportunity  for  chem- 
ical decomposition,  and  even  after  this  rock  flour  is  carried  away 
by  glacial  streams,  which  it  renders  turbid  or  milky,  and  is  de- 
posited elsewhere,  it  may  not  undergo  decomposition  except 
near  the  surface.  Rock  flour,  therefore,  though  resembling  clay 


Rock  Destruction  by  Glaciers 


435 


in  fineness,  is  distinct  from  it.  Its  minute  grains  may  consist  of 
a  variety  of  minerals,  according  to  the  rock  from  which  it  was 
derived. 

The  larger  rock  fragments  are  ground  and  polished  upon  their 
surfaces,  partly  by  being  dragged  over  the  rock-bottom  and  partly 
by  the  movement  of  the  sand-laden  ice  over  them.  If  the  rock 
fragments  were  originally  flat,  they  will  be  polished  and  striated  only 
on  the  two  larger  faces,  the  others  remaining  more  or  less  angular 
(Fig.  360  b).  If  the  fragments  are  of  approximately  equal  dimen- 
sions in  all  directions,  they  may  have  turned  over  frequently,  and 


FIG.  360  a.  —  Photograph  of  a 
pebble  from  Permian  glacial  deposits 
near  Jaquariakyva,  Parana,  South 
Brazil.  This  is  a  fragment  of  the 
old  pre-Permian  rock  floor,  showing 
its  glacial  striae.  (Courtesy  of  Prof. 
J.  B.  Woodworth.) 


FIG.  360  b.  —  A  small  glaciated 
boulder  from  the  Pleistocene  till. 
About  one  half  natural  size.  (Photo 
by  B.  Hubbard.) 


all  sides  may  be  polished  and  striated.  Unless  subsequently  worn 
by  water,  such  striated  rock  fragments  constitute  a  characteristic 
feature  of  the  deposits  .formed  by  glaciers  and  will  lead  to  the 
identification  of  such  deposits  even  after  consolidation  (Fig.  419). 
It  should,  however,  be  remarked  that  in  talus  slides  and  under 
certain  other  conditions,  rock  fragments  may  be  polished  and  stri- 
ated, but  they  will  never  be  so  perfect  nor  so  abundant  as  those 
produced  by  glacial  activities. 

Glacially  produced  debris  never  rests  where  formed,  but  always 
suffers  more  or  less  transportation,  partly  by  the  ice  itself  and  partly 
by  the  streams  formed  from  the  melting  of  the  ice.  This  will  be 
more  fully  discussed  in  the  next  chapter. 


436  The  Formation  of  Clastic  Material 

ROCK  DESTRUCTION  BY  ORGANISMS 

If  we  except  man,  who  is  by  far  the  most  powerful  agent  active 
in  the  destruction  of  the  rocks  of  the  earth's  crust,  we  must  confess 
that  organisms  play  but  a  minor  role  in  the  destruction  of  rocks. 
True,  there  are  certain  bacteria  which  appear  to  be  very  active 
agents  in  bringing  about  the  weathering  of  rock  surfaces,  and  the 
decay  of  plants  furnishes  both  carbon  dioxide  and  humic  acids, 
which  are  powerful  agents  in  rock-solution,  but  aside  from  this 
the  destructive  work  of  organisms  is  slight.  Plants,  whose  roots 
grow  in  fissures  in  the  rock,  will  by  continued  growth  force  the  walls 
of  the  fissures  apart  and  so  perform  a  certain  amount  of  superficial 
destruction  (Fig.  33,  p.  81).  Great  herds  of  animals,  too,  perform 
a  certain  amount  of  surface  destruction  by  pounding  the  rock  to 
powder  under  their  hoofs,  along  the  paths  of  their  migration  and 
especially  around  drinking  pools.  Such  destructive  work  performed 
by  herds  .of  cattle  in  Hereroland,  former  German  Southwest  Africa, 
has  been  described  as  follows  by  Pechuel  Loesche : 

"  In  extensive  manner  these  animals  aid  in  the  leveling  of  many  land  areas. 
As  the  dryness  increases,  the  herds  of  grazing  cattle  become  more  numerous 
around  the  last  of  the  sparsely  distributed  water  bodies.  Thousands  and  tens 
of  thousands  of  the  large  and  the  small  animals  overrun  for  miles  the  surround- 
ing country  for  days,  weeks,  and  months.  Through  countless  hoof-beats  the 
ground  is  loosened  and  so  furnishes  enormous  masses  of  dust,  while  at  the  same 
time  all  inequalities  are  trampled  down  and  destroyed.  The  inclined  surfaces 
would  be  furrowed  by  numerous  rain  water  gullies  if  these  were  not  constantly 
destroyed  by  the  hoofs  of  the  roaming  animals  and  .if  it  were  not  for  the  fact 
that  rain  water  is  constantly  guided  along  the  paths  formed  by  the  animals 
going  to  and  from  the  water  in  long  lines,  ranged  one  behind  the  other.  Fur- 
thermore, the  cover  of  dust  prevents  to  an  astonishing  degree  the  penetration 
of  the  short  heavy  downpour  of  rain  into  the  deeper  strata." 

This  loosening  and  pounding  of  rock  to  dust  becomes  an  effect- 
ive agent  in  the  lowering  of  the  surface,  because  the  finer  particles 
will  be  constantly  removed  by  the  winds,  and  it  is  not  too  much 
to  attribute  to  the  destructive  work  of  great  herds  of  animals  in 
semi-arid  regions  the  principal  role  in  the  lowering  of  the  land  over 
large  areas  and  the  production  of  gently  inclined  planes  free  from 
furrows,  above  which  project  remnants  of  the  older  surface  in  the 
form  of  butte-like  hills  or  ridges.  Such  "  island  hills,"  as  they  are 
called,  abound  in  the  great  Kalahari  desert  region  of  Central  Africa. 
The  work  of  burrowing  land  animals,  such  as  the  moles,  prairie 


Rock  Destruction  by  Organisms  437 

dogs,  etc.,  also  aids  to  a  certain  extent  in  the  further  destruction 
of  unconsolidated  material. 

When  we  come  to  the  smaller  land  animals  we  must  note  espe- 
cially the  worms  and  the  ants  and  termites  which  produce  a  super- 
ficial loosening  and  rearrangement  of  the  unconsolidated  material 
'of  the  earth's  crust,  but  perform  relatively  little  destructive  work 
in  the  solid  rocks.  The  many  galleries  produced  by  ants  and  ter- 
mites admit  the  air  to  the  soil  and  so  favor  decomposition,  while 
organic  matter  carried  down  by  these  animals  furnishes  acids  for 
the  solution  of  rock  particles. 

In  the  sea,  too,  animals  effect  a  certain  amount  of  rock  destruc- 
tion. Thus  many  fish,  feeding  upon  growing  coral  polyps,  crop  off 
the  coral  ends  with  the  polyps  and  grind  the  coral  to  powder,  which 
eventually  passes  from  them  as  lime  flour.  Calcareous  algag  or 
nullipores  are  also  ground  up  in  this  manner.  Crabs,  and  other 
crustaceans  also,  break  up  shells  of  mollusks  and  echinoderms  on 
which  they  feed,  and  so  produce  a  not  inconsiderable  quantity 
of  shell  fragments  which  will  enter  into  the  construction  of  new 
limestones.  Other  marine  animals  active  in  this  way  are  the  echi- 
noids,  which  destroy  rocks  to  a  certain  extent,  by  drilling  holes  in 
them ;  but  their  work  as  rock  breakers  is  of  moderate  significance. 


CHAPTER  XVI 

TRANSPORTATION,   SORTING,  AND  DEPOSITION  OF 
CLASTIC   ROCK   MATERIAL 

ONLY  the  clastic  material  due  to  weathering  may  remain  in  the 
place  where  it  is  produced  or  suffer  only  slight  shifting  of  locality, 
before  becoming  reconsolidated  into  a  rock,  which  then  will  be  un- 
assorted in  respect  to  material,  and  essentially  without  structure. 
Such  old  soils,  subsequently  changed  to  rock,  are  found  in  some 
localities,  but  for  the  most  part  the  product  of  weathering  is  trans- 
ported and  redeposited  elsewhere,  and  in  this  process  undergoes 
more  or  less  sorting,  while  new  structures  incident  to  these 
modifications  are  impressed  upon  it. 

AGENTS  or  TRANSPORTATION  AND  SORTING  AND  REGIONS 
OF  DEPOSITION 

Transportation.  —  The  chief  agents  of  transportation  of  clastic 
material  are  wind,  and  water  currents,  both  those  of  rivers  and  those 
of  the  sea.  Glaciers,  however,  and  floating  icebergs  also  transport 
clastic  material,  though  much  of  that  transported  at  first  by 
glaciers  is  subsequently  carried  farther  sometimes  by  water,  and 
sometimes  by  wind.  There  are  of  course  minor  agents  of  trans- 
portation, among  which  may  be  mentioned  other  floating  structures, 
such  as  vegetation,  etc.,  to  which  various  kinds  of  rock  material 
are  attached ;  snow  and  landslides,  and  animals  which  carry 
rock  material  either  attached  externally  or  in  their  stomachs. 
Man  is  of  course  the  most  effective  agent  of  transportation,  calling 
to  his  aid  the  resources  of  wind,  water  currents,  steam,  electricity, 
and  animal  energy  ;  but  he  works,  as  yet,  upon  a  far  smaller  scale 
than  do  the  less  efficient  rivers,  winds,  waves,  and  ice. 

Sorting.  —  Both  wind  and  running  water  accomplish  sorting 
of  material,  and  the  waves  of  the  sea  and  lakes  are  also  active  in 
this  respect.  Glaciers  and  icebergs,  on  the  other  hand,  transport 
material  without  sorting  it  in  any  way. 

438 


Transporting  and  Sorting  by  Winds  439 

Deposition.  —  Deposition  of  clastic  material  takes  place  in  the 
sea  (marine  deposits),  in  lakes  and  ponds  (lacustrine  deposits),  and 
on  dry  land  (terrestrial  deposits).  The  last  two  types  are  often 
designated  continental  as  distinct  from  marine  deposits.  Inter- 
mediate between  the  lake  deposits,  on  the  one  hand,  and  those  of 
dry  dust  and  sand,  on  the  other,  are  the  deposits  formed  on  river 
flood-plains  and  on  alluvial  fans,  while  between  the  terrestrial  and 
marine  deposits,  the  sea-border  delta  sediments  form  a  transition 
type. 


TRANSPORTING  AND  SORTING  BY  WINDS 

Whatever  the  origin  of  clastic  material,  if  it  is  light  enough  and 
small  enough  in  grain,  it  can  be  transported  by  wind.  As  we  have 
seen,  the  removal,  by  wind,  of  loose  material  from  a  surface  where  it 
has  been  produced  is  called  deflation,  and  this  is  the  beginning 
of  eolian  transport.  Such  transport  is  accomplished  either  by 
carrying  the  material  in  suspension  as  in  sand  and  dust  storms,  or  by 
rolling  and  pushing  it  along  over  the  surface  without  lifting.  Only 
fine  material  such  as  dust  can  be  retained  in  suspension  for  any 
length  of  time,  and  thus  be  carried  far.  The  volcanic  dust  pro- 
duced by  the  explosion  of  Krakatoa,  in  1883,  referred  to  in  Chap- 
ter VII  (p.  137),  was  carried  by  the  upper  air  currents  repeatedly 
around  the  earth  before  settling.  But  even  pebbles,  8  mm.  in 
diameter  or  larger,  may  be  picked  up  by  a  strong  wind  and  carried 
a  short  distance.  Much  larger  sand  grains  and  pebbles  may  of 
course  be  rolled  along  the  surface  than  could  be  lifted  by  wind 
of  the  same  strength. 

Both  the  material  which  is  carried  in  suspension  and  that  which 
is  rolled  along  by  the  wind  become  assorted  according  to  size  and 
weight  of  grain,  the  latter  varying,  of  course,  for  particles  of  the 
same  size  according  to  their  mineral  character.  The  smaller 
grains  are  picked  out  and  carried  farthest ;  the  larger  ones  settle 
out  earlier.  The  lighter  minerals  too  are  carried  far,  leaving  the 
heavier  behind.  Thus  after  repeated  transportation  by  wind,  the 
material  will  become  well  sorted,  only  grains  of  about  the  same 
size  and  of  the  same  mineral  character  accumulating  at  any  given 
locality.  In  this  process  of  transportation  the  grains  will  also  be 
more  or  less  worn  and  rounded  by  mutual  abrasion,  as  already  de- 
scribed (p.  406). 


440  Deposition  of  Clastic  Rock  Material 


When  an  older  sand  deposit  consists  of  well-rounded  grains  of 
uniform  size  and  of  only  one  mineral  species,  the  supposition  is  very 
strong  that  such  a  deposit  represents  the  work  of  the  wind  at  a 
former  time.  A  rock  formed  of  such  material,  and  known  as  the 
Sylvania  sandstone  of  Silurian  age,  occurs  in  Ohio  and  Michigan. 
The  grains  are  wonderfully  well  rounded  (millet  seed  type) ;  they 
are  of  essentially  uniform  size  in  each  locality,  though  those  of  one 
locality  may  differ  from  those  of  another,  as  is  shown  in  the  two 
photographs  (Fig.  361  A,  B)  which  represent  grains  from  different 
localities,  enlarged  to  the  same  degree.  They  consist  wholly  of 


FIG.  361.  —  Microphotographs  of  Sylvania  sand  grains,  enlarged  about  n 
times.  Ay  from  National  Silica  Co.,  Monroe  county,  Mich.,  near  top;  B, 
from  Rockwood  pits,  four  feet  down,  American  Silica  Co.  The  grains  are  well 
rounded  and  of  nearly  uniform  size,  but  differ  in  size  in  the  different  localities. 
(W.  H.  Sherzer,  photo;  from  Grabau  and  Sherzer,  Monroe  Formation  of 
Michigan.) 

quartz  without  any  foreign  admixture.  This  fact  and  the  ready 
separation  of  the  grains  makes  the  rock  valuable  for  glass  manu- 
facture. This  rock  has  been  interpreted  as  formed  of  wind- trans- 
ported, wind-sorted,  and  wind-worn  sand  grains,  and  the  larger 
structural  characters  of  the  deposit  bear  out  this  hypothesis.  (See 
Fig.  374,  p.  455.) 

Dust  and  Sandstorms  (Fig.  362).  —  All  deserts  are  subject  to 
violent  sand  and  dust  storms,  when  the  strong  wind  picks  up  the 
dry  material  from  the  surface  and  whirls  it  along  in  a  cloud,  which 
may  appear  like  a  solid  moving  wall.  After  such  a  storm  the  air 
will  be  hazy  with  dust  for  a  long  time,  and  indeed  in  some  desert 
regions  the  air  is  seldom  free  from  suspended  dust.  As  the  wind 
dies  down,  the  coarsest  material  settles  first,  the  finest  being  long 


Transporting  and  Sorting  by  Winds  441 

held  aloft  and  carried  far.  Sandstorms  may  be  very  destructive  to 
life,  and  as  a  result  of  their  occurrence  great  changes  in  the  aspect 
of  the  country  are  produced.  To  this  constant  motion  of  the  sand 
and  dust,  as  much  as  to  the  absence  of  moisture,  is  due  the  general 
lack  of  vegetation  in  the  sandy  desert  (Figs.  363  a,  b). 


FIG.  362.  —  Sandstorm  in  the  Sahara;    from  nature.      (After  Ratzel.) 

Distances  to  which  Sand  and  Dust  are  Transported.  —  The  dis- 
tance over  which  material  is  transported  depends  not  only  upon  the 
size  and  weight  of  the  particles  and  the  strength  and  continuity 
of  the  wind,  but  also  upon  the  initial  position  of  the  material. 
When  dust  is  projected  high  into  the  air,  as  by  volcanic  explosions, 
the  force  of  the  wind  is  wholly  employed  in  carrying  it,  and 
hence  such  substances  will  be  transported  farther  than  those  picked 
up  by  the  wind  directly.  As  already  noted,  the  fine  dust  of  Kra- 
katoa  was  repeatedly  carried  around  the  world,  and  its  suspension  in 
the  upper  air  caused  everywhere  the  brilliant  sunsets  for  which  the 
period  following  the  eruption  was  noted.  The  coarser  ashes  fell 
inches  deep  at  a  distance  of  a  thousand  miles  from  the  volcano,  and 
some  even  fell  in  Holland.  Volcanic  dust  from  Iceland  has  fallen 
repeatedly  in  Scandinavia,  Holland,  and  northern  Britain.  Dust 


442  Deposition  of  Clastic  Rock  Material 


from  the  Sumatran  volcano,  Tomboro,  has  fallen  a  thousand  miles 

away,  and  dust  from  the  Mexican  volcano,  Colima,  fell  in  February 

and  March,  1903,  at 
points  more  than  200 
miles  northeast  of  the 
volcano. 

The  ash  from  the 
eruption  in  1835  of  the 
Nicaraguan  volcano, 
Cosequina,  covered  an 
area  of  1,500,000  square 
miles,  and  even  reached 
Jamaica,  more  than  750 
miles  away.  Dust  from 
the  eruption  of  Mont 
Pelee  in  1812  is  said  to 
have  reached  the  Azores 
on  the  other  side  of  the 
Atlantic,  and  dust  from 

Vesuvius  has  been  observed  to  fall  in  Greece,  in  France,  and  in 

Austria. 
Dust  of  non-volcanic  origin  is  also  carried  to  great  distances. 

Some  known  to  have  been  derived  from  the  Sahara,  was  repeatedly 


FIG.  363  a.  —  View  of  a  portion  of  the  Desert 
of  Sahara. 


FIG.  363  b.  —  General  view  in  the  arid  region,  western  North  America. 


Deposition  of  Wind-blown  Sands  443 

carried  across  the  Alps  and  fell  in  such  distant  regions  as  England 
and  North  Germany,  2000  miles  away.  Dust  from  Australia  some- 
times reaches  New  Zealand,  1500  miles  away,  and  yellow  dust 
from  the  interior  of  China  has  fallen  on  the  decks  of  vessels  south- 
west of  Japan,  having  been  carried  at  least  a  thousand  miles  from 
its  source. 

Volume  of  Dust  Falls.  —  The  quantity  of  dust  transported  during 
a  single  storm  may  be  very  great.  In  1863  a  rain  of  dust  fell  on  the 
Canary  Islands  which  was  estimated  to  have  a  weight  of  6,500,000 
tons.  The  great  dust  storm  of  March  9-12, 1901,  brought  1,960,000 
tons  of  dust  to  Europe  and  1,650,000  tons  to  North  Africa,  cover- 
ing a  total  area  of  300,000  square  miles  of  land  surface  and  170,000 
square  miles  of  ocean.  It  was  estimated  that  this  dust  traveled, 
in  part  at  least,  a  distance  of  2500  miles.  It  is  thus  clear  that  wind- 
transported  material  plays  an  important  role  in  nature. 

DEPOSITION  OF  WIND-BLOWN  SANDS 

Wind-blown  material  may  be  deposited  in  water  or  upon  dry 
land.  Only  the  latter  has  distinctive  characters,  for  the  former 
is  incorporated  with  other  water-laid  (lacustrine  or  marine)  elastics. 
The  most  characteristic  types  of  wind-borne  sand  deposits  are  the 
dunes.  These  are  found  on  most  coasts,  but  also  abound  inland, 
especially  in  desert  regions,  where  they  often  assume  considerable 
proportions  and  cover  wide  areas. 

Types  of  Sand  Dunes 

Three  main  types  of  sand  dunes  are  recognized :  (a)  the  conical 
hill,  (b)  the  linear,  and  (c)  the  curved  or  crescent-shaped  type, 
known  as  the  barchane  (Fig.  368).  Between  the  last  two  are  often 
combination  types  which  have  some  of  the  characters  of  both.  The 
linear  type  is  most  common  on  the  coast,  where  a  uniform  and 
constant  supply  of  sand  is  furnished ;  while  the  crescent-shaped  or 
barchane  type  is  most  characteristic  of  the  interior,  the  river  flood- 
plains  of  arid  regions,  and  the  deserts.  According  to  location, 
dunes  may  be  classed  as  (a)  coastal,  (b)  river-bottom  and  flood- 
plain,  and  (c)  inland  or  desert  dunes. 

Coastal  Dunes.  —  These  are  chiefly  of  the  linear  type  and  ex- 
tend in  general  at  right  angles  to  the  prevailing  wind.  They  al- 
ways have  an  asymmetrical  cross-section,  the  opposite  sides  sloping 


444  Deposition  of  Clastic  Rock  Material 


EAST  5EA 


at  decidedly  different  angles.  The  side  toward  the  coast,  exposed 
to  the  direct  force  of  the  wind,  generally  has  a  slope  of  only  about 
10° ;  the  angle  of  the  opposite  or  leeward  slope  is  often  as  high 
as  30°.  Sand  grains  are  rolled  up  the  windward  side,  and  passing 
the  crest  of  the  dune,  roll  down  upon  the  lee  slope.  Thus  the 

dune  slowly  advances  in  the 

direction  in  which  the  pre- 
vailing wind  blows,  which, 
being  from  the  sea,  causes 
the  dunes  to  move  inland 
at  a  very  variable  rate. 

The  advancing  sand  dunes 
bury  whatever  lies  in  front 
of  them.  The  dunes  formed 
on  the  barrier  beaches  of 
our  coasts  advance  over  the 
salt  meadows  formed  in  the 
lagoon  behind  the  bars  (see 
p.  330).  In  long-inhabited 
coastal  regions,  the  dunes 
may  cover  buildings  and 

FIG.  364  a.  —  Map  of  the  region  between 
Danzig  and  Memel  on  the  East  Sea  or 
Baltic,  Eastern  Germany,  showing  the 
Kurische  Nehrung  (or  bar)  and  Kurische 
Haff  (or  lagoon)  north  of  Kdnigsberg, 
and  the  Frische  Nehrung  and  Frische 
Haff  south  of  Konigsberg.  These  are 
typical  examples  of  dune  covered  sand 
bars  enclosing  lagoons. 


even  villages.  The  dunes 
of  the  North  Sea  coast  of 
Denmark  and  Schleswig- 
Holstein,  which  cover  an 
area  of  165,557  acres,  and 
rise  in  places  to  a  height  of 
nearly  a  hundred  feet,  fur- 
nish a  good  illustration  of 

this  phenomenon,  especially  as  the  coast  here  is  also  cut  back 
rapidly  by  the  sea,  and  the  region  is  believed  to  be  subsiding.  In 
1757,  it  was  found  necessary  to  tear  down  the  church  of  the  village 
of  Rantum  on  the  island  of  Sylt,  on  the  west  coast  of  Schleswig- 
Holstein,  because  it  was  reached  by  the  advancing  dunes.  In 
1791  or  1792,  the  entire  dune  chain  had  passed  over  the  church 
ruins,  these  then  lying  on  the  shore,  which  had  advanced  to  this 
point.  Sixty  years  later,  the  site  of  the  church  was  700  feet  from 
shore,  the  depth  of  water  at  that  point  being  12  feet.  The  second 
church  has  since  been  buried  by  the  advancing  dunes. 

On  the  east  shore  of  the  Baltic  (East  Prussia)  are  two  long  sand 


Deposition  of  Wind-blown  Sands 


445 


bars,  the  Kurische  and  the  Frische  Nehrung,  which  separate  large 
lagoons,  the  Kurische  and  the  Frische  Haff,  from  the  Baltic  (Fig. 
364  a).  These  bars  are  covered  with  extensive  sand  dunes,  some 


Kircht  von  Kunzen. 


Kurisches  Haff 


Stelle  ier  vertandeten  Kirche, 


Ruine  dtr  Kirche  von  Kunzen. 


FIG.  364  b.  —  Diagrams  showing  the  advance  of  the  coastal  dunes  over 
the  village  and  church  of  Kunzen  on  the  Kurische  Nehrung  (bar),  from  the 
east  shore  of  the  Baltic  Sea  to  the  Kurische  Haff  (barachois).  (i)  At  the 
beginning  of  the  nineteenth  cerltury,  when  the  dunes  had  just  reached  the 
church  of  Kunzen.  (2)  Conditions  in  1839,  when  the  church  was  buried  by  sand. 
(3)  Conditions  in  1869,  when  the  dunes  had  passed  beyond  the  village  site, 
uncovering  again  the  ruins  of  the  church.  (After  Behrend ;  from  Ratzel.) 

of  which  reach  a  height  of  sixty  meters  or  more,  being  among  the 
most  imposing  dunes  of  the  European  coast.  Between  the  years 
1809  and  1869  the  dunes  of  the  Kurische  Nehrung  (bar)  in  their 


FIG.  365  a.  —  Tower  of  the  buried  church  of  Eccles,  Norfolk,  England,  1839. 
The  inland  slope  of  the  hill  of  blown  sand  is  shown  in  this  view,  with  the  light- 
house of  Hasborough  northwest  of  the  tower  in  the  distance.  (See  Fig.  365  6.) 
(From  Lyell's  Principles.) 


446  Deposition  of  Clastic  Rock  Material 

advance  buried  the  church  of  the  village  of  Kunzen  by  passing 
over  it  and  later  resurrected  it  by  migrating  further  inland  (Fig. 
364  b).  An  analogous  phenomenon  has  been  observed  on  the 
coast  of  Norfolk,  England,  as  shown  in  the  next  two  figures 
(Figs.  365  a,  b). 

The  largest  dune  area  of  the  European  coast,  and  one  of  the 
largest  coastal  dune  areas  known  anywhere,  is  on  the  western  shores 
of  France,  along  the  Bay  of  Biscay.  The  dunes  extend  almost 


FIG.  365  b.  —  Eccles  Tower  as  it  appeared  after  the  storm  of  November, 
1862.  From  a  drawing  by  Rev.  S.  W.  King,  taken  from  nearly  the  same 
position  as  Fig.  365  a.  (From  LyelPs  Principles.) 

without  interruption  for  a  length  of  240  km.  and  lie  in  parallel 
rows  up  to  ten  in  number,  over  a  coastal  strip  from  four  to  eight, 
and  in  some  cases  10  km.  wide,  covering  a  total  area  of  120,000 
hectares  (296,520  acres) .  In  height  the  dunes  range  up  to  90  meters, 
making  them  the  highest  known  coastal  dunes,  though  some  in- 
land or  desert  dunes  which  have  advanced  toward  the  coast  in  North 
Africa  reach  twice  this  height. 

Behind  the  great  Biscayan  dune  belt  lies  the  low,  swampy  tract 
known  as  the  Landes.  The  dunes  travel  inland  at  a  rate  ranging 
from  15  to  105  feet  per  year,  burying  not  only  the  swamp  lands, 
but  forests,  farms,  vineyards,  and  even  villages.  The  church  at 
Lege  was  taken  down  at  the  end  of  the  seventeenth  century  and 
rebuilt  two  and  a  half  miles  farther  inland.  One  hundred  and  sixty 


Deposition  of  Wind-blown  Sands  447 

years  later  it  had  to  be  removed  again,  because  the  sands  had  once 
more  reached  it.  This  gave  an  average  rate  of  advance  for  the 
dunes  of  81  feet  per  year.  The  sea,  too,  advances,  cutting,  back 
the  coast  in  places  at  the  rate  of  not  less  than  2  meters  per  year. 

The  dunes  on  the  Atlantic  coast  of  North  America  are  mostly 
of  inferior  size  and  formed  in  connection  with  off-shore  sand  bars. 
At  the  extreme  end  of  Cape  Cod,  however,  where  the  waves  have 
built  the  series  of  bars  which  form  the  foundation  of  the  Province- 
town  dune-lands  (see  Fig.  714),  great  sand  dunes,  some  of  them 


FIG.  366  a.  —  Dune  of  barchane  form  overwhelming  trees  on  the  Province- 
lands  of  Cape  Cod.  The  top  branches  of  a  tree  protrude  from  the  crest  of  the 
dune.  (D.  W.  Johnson,  Shore  Processes  and  Shoreline  Development;  John 
Wiley  &  Sons.) 

exceeding  3o«or  40  meters  in  height,  have  been  formed  by  the  wind. 
These  dunes  slowly  advance  southward  and  westward,  burying 
forests  and  even  buildings  in  their  advance  (Fig.  366  a).  Their 
movement  has,  however,  been  checked  to  some  extent  by  the 
planting  of  grasses  and  shrubs  on  the  windward  slopes.  Between 
the  years  1826  and  1838,  some  $28,000  were  spent  in  this  work  of 
arresting  the  dunes.  The  dunes  of  the  Virginia  and  Carolina  coasts 
are  also  of  considerable  magnitude,  and  they,  too,  bury  forests  and 
whatever  else  lies  in  the  path  of  their  advance  (Fig.  366  b). 

In  the  mastery  of  sand  dunes  by  means  of  skillfully  planted 
vegetation  France,  Belgium,  Holland,  Germany,  and  other  countries 


448  Deposition  of  Clastic  Rock  Material 

have  far  outstripped  America,  because  the  lands  in  the  old  world 
coastal  district  are  far  more  valuable  on  account  of  the  denser 
populations. 

On  the  shores  of  the  Great  Lakes,  dunes  are  likewise  well  de- 
veloped. The  most  extensive  dune  area  lies  on  the  eastern  and 
southern  shores  of  Lake  Michigan,  and  some  of  the  dunes  there  rise 
to  the  height  of  a  hundred  feet,  especially  in  the  Dune  Park  area 
of  Indiana.  A  dune  near  Muskegon,  Michigan,  has  a  height  of 
several  hundred  feet,  and  from  its  rate  of  advance  has  become 
known  as  "  Creeping  Joe."  Many  of  these  dunes  are  grassed  over 


FIG.  366  b.  —  Shore  dunes  near  Cape  Henry,  Virginia,  migrating  inland  over 
the  forest.  (D.  W.  Johnson,  Share  Processes  and  Shoreline  Development;  John 
Wiley  &  Sons.) 

and  even  wooded,  but  they  are  still  recognizable  by  their  form. 
Others  are  in  active  motion,  burying  and  killing  forests  and  again 
uncovering  them  in  their  advance.  In  many  cases  only  the  very 
tops  of  the  trees  now  project  above  the  surface  of  the  dune.  The 
sand  of  these  dunes  is  chiefly  of  glacial  origin. 

River-bottom  and  Flood-plain  Dunes.  —  In  semi-arid  regions 
the  rivers  partly  dry  away  during  the  summer  season,  leaving  broad 
flood-plains  and  river-bottoms  covered  with  the  sands  brought 
down  by  the  rivers.  These  sands  are  then  heaped  into  dunes  by 
the  winds  and  travel  across  the  country,  sometimes  from  one  river- 
bottom  to  another.  Such  dunes  are  extensively  developed  along 


Deposition  of  Wind-blown  Sands 


449 


the  river  borders  in  the  semi-arid  regions  of  southern  Russia ;  as, 
for  example,  along  the  Dnieper,  the  Don,  and  the  Volga,  where 
they  occur  in  belts  sometimes  as  wide  as  30  kilometers,  and  have 
a  length  of  150  kilometers  or  more.  The  dunes  are  seldom  more 
than  five  to  seven  meters  in  height,  though  some  reach  a  height  of 
12  meters  or  more. 


FIG.  367.  —  Outline  map  of  the  drainage  region  of  the  Oxus  and  Jaxartes 
rivers  (Amu-darya  and  Syr-darya)  and  the  deserts  of  Kyzyl-Kum  and  Kara- 
Kum. 

As  typical  examples  of  river-bottom  dunes  may  be  cited  those  of 
the  rivers  Oxus  and  Jaxartes  in  Turkestan,  Central  Asia,  both  of 
which  flow  into  the  Aral  Sea.  "  These  streams  bring  vast  quan- 
tities of  sand  and  mud  from  the  Tian  Shan,  Great  Pamir,  and  Hindu- 
Kush  mountains,  in  which  they  arise,  and  spread  them  over  the 
low  ground  of  their  flood-plains,  which  range  in  width  up  to  ten 
kilometers.  The  thickness  of  the  deposit  made  by  the  Oxus  was 
found  to  be  23  meters  at  Tschard-schui.  The  rivers  rise  three 
meters  from  March  to  July,  and  overflow  the  flood-plains,  deposit- 
ing sandy  sediment.  As  the  water  of  the  Jaxartes  falls,  the  hot 


450  Deposition  of  Clastic  Rock  Material 

northern  winds  soon  dry  the  deposit  and  carry  away  all  the  finer 
dust  particles,  leaving  only  the  pure  quartz  sand,  which  is  heaped 
into  dunes.  These  wander  southward  across  the  Kyzyl-Kum 
desert  [Fig.  367],  sometimes  at  a  rate  of  20  meters  during  a  stormy 
day,  but  generally  the  sand  masses  move  at  an  average  rate  of  six 
miles  per  year.  Reaching  the  Oxus,  these  sands  are  incorporated 
in  its  sediment,  and  the  operations  of  sorting  the  deposits  on  the 
flood-plain  of  this  river  are  repeated,  and  the  sands  are  again  heaped 
into  dunes  which  wander  southward  across  the  Trans-Caspian  or 
Kara-Kum  desert  until  they  reach  the  borders  of  the  Caspian  Sea. 
The  activities  of  the  streams  are  unceasing,  and  the  supply  of 
material  in  the  mountains  in  which  they  rise  is  practically  inex- 
haustible. Thus  there  is  a  constant  succession  of  sand  dunes 
wandering  southward  across  these  deserts,  and  layer  upon  layer 
of  sand  accumulates,  each  showing  the  characteristic  eolian  struc- 
tures and  helping  to  build  up  a  deposit  of  pure,  unf ossilif erous  sand 
of  almost  unlimited  thickness." 1  Large  sand  dunes  are  also  found 
on  western  American  river-bottoms,  such  as  those  of  the  Columbia 
(Fig.  368)  and  Snake  rivers  in  Oregon  and  Washington. 

The  dunes  of  these  river  bottoms  and  intervening  deserts  are*  both 
of  the  linear  and  the  crescent-shaped  type.  The  linear  dunes  are 
generally  symmetrical  in  section  and  extend  parallel  to  the  direc- 
tion of  the  wind,  where  this  is  strong  and  the  supply  of  sand 
large.  They  may,  indeed,  be  regarded  in  many  cases  as  the 
extended  horns  of  the  crescent  or  barchane  type. 

Desert  Dunes.  —In  deserts  the  barchane  or  crescent  type  of  dune 
is  most  typical.  When  best  developed,  this  presents  a  crescent 
form  with  the  convex  side  to  the  wind  and  its  surface  a  gentle  slope 
(Fig.  368).  The  lee  side  is  steep  and  abrupt,  with  a'  concave  out- 
line, from  the  ends  of  which  the  horns  extend  in  the  direction  of 
advance.  By  an  elongation  of  these  horns,  when  much  sand  is 
supplied  and  the  winds  are  strong,  long,  parallel  ridges,  united  at 
intervals  by  cross  ridges,  may  be  produced,  their  direction  running 
parallel  to  that  of  the  wind. 

The  great  desert  dune  districts  are  found  to-day  in  Asia,  Africa, 
and  Australia.  Dunes  cover  only  one  ninth  of  the  total  area  of 
the  Sahara,  but  even  this  aggregates  a  total  of  18,000  geographical 
square  miles.  Arabia  is  par  excellence  the  land  of  desert  dunes, 
nearly  one  third  of  the  entire  surface,  or  not  less  than  15,000 

1  A.  W.  Grabau,  Principles  of  Stratigraphy,  p.  561^ 


Deposition  of  Wind-blown  Sand 

geographical  square  miles,  being  covered  with  sand.  Almost  the 
whole  southern  area  is  occupied  by  the  terrible  Desert  of  Roba-el- 
Khali,  or  the  desert  Dehna,  which  is  wholly  covered  by  eolian 
sands,  and  is  without  the  relief  of  oases.  This  desert  is  150  geo- 
graphical miles  in  length  and  80  in  width.  In  the  northern  part  of 
the  peninsula  lies  the  Nefud  Desert,  where  the  sands  are  red  and 
water  practically  absent. 


FIG.  368.  —  Sand  dunes  of  the  barchane  type  in  the  valley  of  the  Columbia 
River,  near  Biggs,  Oregon.  River  terraces  shown  in  the  background.  (Photo 
by  Gilbert,  U.  S.  G.  S.  Courtesy  of  D.  W.  Johnson.) 

Sandy  deserts  exist  in  many  parts  of  Asia,  and  the  whole  in- 
terior of  Australia  is  a  desert,  many  parts  of  which  are  covered  with 
drifting  sands.  Sandy  deserts  also  abound  in  southwestern  North 
America,  especially  in  southern  California  and  Arizona.  Almost 
one  fourth  of  the  state  of  Nebraska,  or  about  18,000  square  miles,  is 
covered  with  drifted  sands,  forming  the  "  Sand  Hills  "  region 
with  its  many  lakes,  from  some  of  which  potash  salts  are  ex- 
tracted. These  lakes  are  the  result  of  a  change  in  climate  from 
dry  to  moist,  in  consequence  of  which  the  old  dunes  are  mostly 
covered  with  vegetation,  though  many  fresh  hollows  or  "  blow- 
outs "  occur.  In  the  central  plains  'of  Hungary,  too,  are  many 
old  sand  dunes  now  covered  with  vegetation,  but  readily  recogniz- 
able by  their  form. 


452 


Deposition  of  Clastic  Rock  Material 


The  sands  of  many  deserts  are  derived  from  the  disintegration  pf 
older  sandstones ;  as,  for  example,  the  sands  of  the  Libyan  Desert, 
which  are  derived  from  the  Nubian  sandstone,  and  from  a  younger 


69.  —  The  edge  of  the  Libyan  Desert. 


rock  from  which  the  Sphinx  is  carved.  This  sand  (Fig.  369) 
has  been  carried  in  places  for  a  hundred  miles  from  its  source,  and 
often  rests  upon  old  limestone  surfaces,  the  weathered-out  fossils 
of  which  it  encloses  in  its  basal  portion.  From  long  transport 


FIG.  370.  —  Microphotographs  of  sand  grains  from  the  Libyan  Desert, 
enlarged  about  n  times.  A,  finer  sand  only  partly  rounded,  due  to  constant 
accession  of  new  material ;  B,  coarser  grains  (mechanically  separated),  showing 
pronounced  rounding  and  relative  uniformity  of  size.  (W.  H.  Sherzer,  photo ; 
from  Grabau  and  Sherzer,  Monroe  Formation  of  Michigan.) 


Deposition  of  Wind-blown  Sands 


453 


the  sand  is  well  sorted  according  to  size  of  grain  and  purity  of  ma- 
terial, and  from  wear  the  grains  are  well  rounded  (Fig.  370,  B). 
Where  much  fresh  material  is  furnished  by  local  disintegration  of 
rock,  a  certain  amount  of  angularity  of  the  smaller  grains  is  still 
seen  (Fig.  370,  A).  The  more  perfectly  rounded  grains  are  similar 
to  those  of  the  Sylvania  sandstone  of  Silurian  age  (Fig.  361,  p. 
440). 

Structure  of  Sand  Dunes 

Ripple  Marks.  —  We  have  seen  that  the  wind  carries  the  sand  up 
the  gentler  side  of  the  dune,  and  that  it  rolls  down  the  steeper  side. 
The  progress  of  the  sand  on  the  windward  side  is  commonly  shown 


FIG.  371.  —  Typical  sand  d 
California,  Colorado  Desert. 
D.  W.  Johnson.) 


nes  with  ripple  marks  in  desert  region  of  southern 
(Photo  by.  Mendenhall,  U.  S.  G.  S.     Courtesy 


by  ripple  marks  of  an  asymmetric  character  (Fig.  371),  these  being 
in  reality  secondary  dunes  of  minute  size  upon  the  surface  of  the 
larger  ones.  The  following  diagram  shows  types  of  ripple  marks 
observed  in  desert  sands  (Fig.  372  a). 

Cross-bedding  of  Dunes.  —  With  different  velocities  of  the  wind, 
particles  of  different  size  will  be  deposited  upon  the  slopes,  and 
thus  layers  of  different  texture  will  be  formed,  which  may  appear 
distinctly  in  a  section  of  the  dune.  These  layers  have  a  gentle 


454  Deposition  of  Clastic  Rock  Material 


slope  on  one  side  and  a  steeper  one  on  the  other,  and  at  the  base  of 
the  dune  the  gentler  sloping  layers  will  gradually  pass  into  a 


FIG.  372  a.  —  Various  types  of  ripple  marks  in  Lop-Nor  Desert,  Turkestan. 
(After  Sven  Hedin,  Scientific  Results  of  a  Journey  in  Central-  Asia.) 


FIG.  372  b.  —  Three  cross-sec- 
tions of  dunes  near  Ostend, 
Belgium,  showing  Eolian  cross- 
bedding.  (From  Kayser's  Lehr- 
buch.} 


horizontal  position.     If  now  by  a  change  in  the  direction  or  force 
of  the  wind  a  part  of  the  dune  is  cut  away  again,  the  remaining 


Deposition  of  Wind-blown  Sands 


455 


basal  portion  will  present  layers  which  slope  in  two  directions 
and  will  be  abruptly  truncated  above  by  a  horizontal  or  oblique 
erosion  plane.  A  second  dune  deposited  above  this  remnant  may 
become  so  placed  that  its 
layers  slope  in  the  opposite 
direction,  or  at  a  different 
angle  from  those  beneath  it. 
Thus  will  be  formed  a  type 
of  cross-bedding,  as  it  is 
called,  in  which  there  are 
successive  divisions  of  the 
mass,  separated  by  erosion 
lines,  and  with  the  layers  of 
each  division  dipping  in  one 
or  more  directions  without 
reference  to  the  dip  of  the 
layers  in  other  sections.  The  Ff-  373- -  Thin  section  of  quartz 
J  ,  sandstone  shown  under  the  microscope, 

preceding  illustration  repre-  The  original  rounded  quartz  grains  are 
sents  actual  cross-sections  of  surrounded  by  new  quartz  (secondary  en- 
dunes  near  Ostend,  and  shows  ^gement)  forming  a  quartrite,  Haupt- 

buntsandstem,     near     Heidelberg;     en- 

the  result  of  such  repeated  larged  12  diameters.  (After  Rosenbusch.) 
partial  destruction  of  older, 

and  deposition  of  newer,  dunes,  and  the  wind  or  eolian  cross- 
bedding  thus  formed  (Fig.  372  b). 

Ancient  Deposits  with  Eolian  Cross-bedding.  —  Many  ancient 
sandstones  show  a  cross-bedding  of  this  type,  from  which  it  may  be 
inferred  that  they  represent  older  dune  deposits  now  consolidated. 
As  such  dunes  are  at  present  most  characteristic  of  and  most  ex- 
tensive in  desert  areas,  we  are  led  to  conclude  that  the  deposits  in 
question,  if  at  all  extensive,  were  formed  under  similar  circumstances. 

For  corroborative 
evidence,  the  char- 
acter of  the  grains 
must  be  examined, 
since  in  typical 
eolian  deposits 
these  should  be 


FIG.  374.  —  Cross-bedding  in  Sylvania  sandstone. 


more  or  less  well  rounded,  assorted  according  to  size,  and  essentially 
of  the  same  mineral  substance.  The  original  roundness  of  the 
grains  may  be  obscured  toy  the  subsequent  deposition  around  them 


Deposition  of  Clastic  Rock  Material 


of  mineral  matter  of  the  same  character,  so  that  they  appear  angu- 
lar. Such  secondary  enlargement  of  the  grains,  as  it  is  called,  can 
generally  be  detected  under  the  microscope  (Figs.  373,  472). 


FIG.  375.  —  Eolian  cross-bedding  in  Mesozoic  sandstone,  Little  Meadow 
district,  Utah.  (Gardner  Collection  of  Photographs.  Courtesy  'of  the  Geo- 
logical Department  of  Harvard  University.) 

Eolian  cross-bedding  is  well  shown  in  the  Sylvania  sandstone, 
above  referred  to,  as  having  the  characteristic  grains  of  eolian 
deposits  (Fig.  374).  Eolian  cross-bedding  is  strikingly  shown  in 

the  Jurassic  sand- 
stone which  forms 
the  White  Cliff  of 
the  Colorado  Plateau 
and  in  other  sand- 
stones of  western  as 
well  as  eastern  North 
America.  This 
seems  to  point  to  the 
eolian  origin  of  these 
rocks  (Figs.  375, 

FIG.   376. — Large  boulder  of  sandstone   show-     37   /• 
ing    Eolian    cross-bedding,    Mt.    Pisgah,    Greene         Possible  Sources  of 
County,  N.  Y.  Error.  —  It    should, 

however,  be  borne  in  mind  that  only  when  such  cross-bedding  is 
developed  on  a  large  scale  and  over  a  wide  extent  can  it  be  regarded 
as  indicative  of  eolian  deposition.  When  restricted  to  a  narrow 


Dust  Deposits 


457 


area,  it  may  be  due  to  the  cross-bedding  produced  in  sand  bars 
along  the  coast.  As  will  be  shown  later,  this  is  of  limited  extent 
only.  Cross-bedding  of  this  type,  but  with  the  successive  divisions 


FIG.  377.  —  Cross-bedding  of  the  Eolian  type  in  the  Orange  sand  or  La 
Fayette  Formation,  Mississippi  Central  Railroad,  Oxford,  Miss.  (After 
Hilgard.) 

measuring  inches  rather  than  feet  in  thickness,  can  also  be  pro- 
duced by  successive  current  ripples  such  as  may  be  formed  upon 
the  bottom  of  a  shallow  water  body.  Cross-bedding  on  a  large 


FIG.  378.  —  Eolian  type  of  cross-bedding  in  ancient  limestones  (Mississip- 
pian),  formed  of  uniform  lime-sand  grains  (calcarenyte)  south  of  St.  Louis, 
Mo.  Scale,  i  inch  =  3^  feet.  (From  Principles  of  Stratigraphy.} 

scale  cannot,  however,  be  produced  in  this  manner.  The  preceding 
illustrations  show  such  cross-bedding  on  a  large  scale,  in  uncon- 
solidated  older  sands  (Fig.  377),  and  in  ancient  limestones 

(Fig.  378). 

DUST  DEPOSITS 

Mode  of  Deposition.  —  Dust-laden  wind  sweeping  across  a 
steppe  land,  where  herbaceous  vegetation  grows  in  a  scattered 
manner,  will  have  its  velocity  checked  when  it  comes  in  contact 
with  this  vegetation,  and  a  part  of  its  dust  load  will  be  dropped. 
This  will  accumulate  around  the  plants,  which  thus  become  buried 
in  the  dust  deposits  essentially  in  the  position  of  growth.  §  Succes- 
sive accumulations  of  dust  will  raise  the  surface  of  the  plain  until 
the  plants  die  and  new  ones  take  their  place.  In  this  manner, 
thick  deposits  of  dust  may  be  built  up. 


458  Deposition  of  Clastic  Rock  Material 

When  dust-laden  winds  reach  a  rainy  district  they  are  washed 
clean  by  the  rains,  and  similar  deposits  of  dust  accumulate,  but 
these  are  likely  to  be  more  or  less  modified  by  running  water  or 
become  incorporated  in  pond  or  lake  sediments. 

Character  of  Deposits.  —  Unmodified  dust  deposits,  owing  to  their 
uniformity  and  fineness  of  grain,  lack  as  a  rule  all  evidence  of 
bedding  or  deposition  in  layers  —  i.e.,  stratification.  They  will 
form  a  more  or  less  homogeneous  mass  of  uniform  structure 
throughout. 

The  Loess  an  Example  of  a  Dust  Deposit 

In  the  eastern  part  of  China,  extensive  deposits  of  fine  dust-like 
material  have  been  formed,  to  which  the  name  loess  is  applied. 
This  dust  is  believed  to  have  been  carried  by  the  wind  from  the 
great  deserts  of  the  interior,  especially  the  desert  of  Gobi,  and  it 
has  accumulated  to  such  an  extent  that  in  some  sections  its  thick- 
ness is  one  or  even  two  thousand  feet.  This  loess  has  a  very  high 
fertility,  and  although  its  surface  has  been  cultivated  for  thousands 
of  years  without  the  application  of  artificial  fertilizers,  it  is  still 
productive,  owing  perhaps  largely  to  the  constant  addition  of  new 
material.  Its  extent  appears  to  coincide  with  the  limits  of  Chinese 
agriculture.  Loess  has  the  property  of  maintaining  a  vertical  face 
whenever  a  section  is  cut,  due  largely  to  the  presence  in  it  of  verti- 
cal tubes  which  are  more  or  less  filled  with  mineral  matter,  and 
which  cause  the  vertical  scaling  off  of  layers ;  thus  walls  of  loess 
200  feet  in  height  are  produced.  Where  roads  have  been  worn  into 
it  by  constant  travel  and  where  the  material  loosened  thereby 
has  been  removed  by  wind,  these  roads  lie  in  canyon-like  depres- 
sions with  vertical  walls  often  of  considerable  height.  In  some 
places  in  China,  caverns  are  dug  into  the  base  of  such  a  wall  and 
occupied  as  dwelling  places  by  the  humbler  natives. 

Loess  is  also  found  in  the  United  States,  the  material  having 
been  supplied  by  glacial  streams  during  the  ice  age  of  a  former 
period.  During  the  drier  periods  between  ice  advances  (inter- 
glacial  periods),  this  dust  has  been  taken  up  by  the  winds  and  de- 
posited in  favorable  localities.  Such  deposits  frequently  include 
the  shells  of  land  mollusks,  such  as  snails  (Helix,  Fig.  256,  p.  316), 
and  occasionally  those  of  river  border  types.  (See  Figs.  252-255, 
pp.  315-316.)  In  it  are  also  found  the  peculiar  little  concretionary 
masses  which  characterize  the  Chinese  and  many  European  loess 


Dust  Deposits  459 

deposits  and  which  have  become  known  as  loess  dolls  (Loess- 
pilpchen  or  Loessmannchen),  and  which  were  apparently  formed  at 
subsequent  periods  by  the  concentrative  work  of  ground-water 

(Fig.  477  d,  p.  573)- 

Loess  contains  a  small  percentage  (4  or  5  per  cent)  of  iron  oxide 
generally  in  the  hydrous  form.  Being  uniformly  disseminated 
through  the  mass,  it  gives  the  loess  a  yellowish  color,  and  where 
streams  cut  into  such  deposits,  their  waters  are  colored  by  the  fine 
sediment  carried  along.  The  Yellow  River  of  China  (Hoang-Ho) 
owes  its  color  and  name  to  such  loess  sediment  carried  by  it,  and 
the  color  of  the  Yellow  Sea  is  likewise  derived  in  this  manner. 

We  can  picture  to  ourselves  what  would  happen  if  such  loess 
deposits  became  buried  by  other  sediments  laid  down  upon  them, 
and  consolidated  into  a  rock.  Such  a  rock  would  be  of  uniform 
grain  and  show  no  bedding  structure  (stratification),  except  per- 
haps at  considerable  intervals.  Moreover,  from  the  gradual 
effect  of  aging  and  of  heat,  the  water  of  the  iron  hydrate  would  be 
driven  off,  and  the  color  of  the  deposit  would  change  from  yellow 
to  a  uniform  red.  Thus  a  bright-red  or  brick-red  rock  of  uniform 
fine  grain,  without  bedding  planes,  would  be  produced.  Ancient 
rocks  of  this  character  are  not  unknown,  and  they  have  sometimes 
been  interpreted  as  altered  loess  deposits  of  a  former  time.  Such 
rocks  are  of  course  as  a  rule  free  from  the  remains  of  organisms, 
except  that  those  of  terrestrial  animals  and  the  seeds  and  other 
wind- transported  parts  of  plants  may  become  included  in  them. 
The  plant  remains  will  tend,  by  decomposition,  to  furnish  sub- 
stances which  locally  change  or  remove  the  iron  so  that  white  or 
greenish  spots  and  streaks  may  mark  the  otherwise  red  rock. 

The  Black  Earth  of  Russia.  —  Over  considerable  areas  in  Russia, 
the  surface  of  the  earth  is  covered  with  a  loess-like  deposit,  the 
upper  part  of  which,  for  20  feet  or  more,  is  colored  black  by  a 
thorough  admixture  of  organic  matter  (Fig.  379).  This  Black 
Earth  or  Tchernozom  or  Tschernosem,  as  it  is  called,  appears  to 
represent  the  accumulation  of  fine  dust  among  actively  growing 
herbaceous  vegetation,  the  decay  of  which  furnished  the  organic 
matter  of  the  black  earth,  of  which  it  sometimes  forms  ten  or 
more  per  cent.  This  organic  matter  renders  the  soil  exceedingly 
fertile  and  capable  of  growing  large  crops  continually  without 
manure.  A  similar  black  soil,  called  the  Regur  or  black  cotton  soil, 
occurs  in  India.  Such  soils  when  buried  under  other  deposits  and 


460 


Deposition  of  Clastic  Rock  Material 


compacted,  become  black  carbonaceous  shales,  such  as  are  found 
throughout  the  southern  states  in  the  late  Palaeozoic  series  (Chat- 
tanooga Shale),  though  other  interpretations  have  also  been  pro- 
posed for  them. 


FIG.  379.  —  Map  showing  the  distribution  of   the   Black   Earth  or  Tscher- 
nosem  in  Russia.     (After  Glinka.) 

TRANSPORTING  AND  SORTING  BY  STREAMS 
Transportation  of  Sediments 

Streams  are  among  the  most  effective  agents  of  transportation. 
Besides  carrying  substances  in  solution,  they  carry  fine  particles 
in  suspension,  and  push  and  roll  larger  ones  along  their  bottoms. 
In  this  manner  material  may  be  transported  by  streams  for  hun- 
dreds of  miles  before  it  finally  comes  to  rest. 

Conditions  of  Transportation.  —  The  materials  which  streams 
carry  are  the  products  of  rock  weathering  and  of  mechanical  wear, 
partly  produced  by  the  stream  itself,  and  partly  supplied  by  rain 
water,  by  rills,  and  by  wind.  The  power  of  the  stream  to  carry,  roll, 
or  push  along  material  varies  with  the  velocity,  and  in  any  given 
section  of  the  stream  this  velocity  varies  as  the  cube-root  of  the 
volume.  Thus  a  stream  swollen  to  eight  times  its  original  size  will 


Transporting  and  Sorting  by  Streams          461 

have  its  velocity  doubled,  and  therefore  its  carrying  power  is  in- 
creased. This  in  general  varies  as  the  sixth  power  of  the  velocity, 
and  therefore,  a  stream  the  velocity  of  which  has  been  doubled  will 
be  able  to  carry  sixty-four  times  the  quantity  of  material  which  it 
could  transport  before. 

This  may  be  demonstrated  in  the  following  manner  (Fig.  380).  Given,  a 
current  which  can  just  move  along  a  cube  of  rock  (a)  of  a  certain  size.  If 
the  velocity  were  doubled,  twice  as  much  water  as  before  would  strike  the  cube 
in  a  given  time,  and  with  twice  the  force.  Therefore,  such  a  current  would 
move  along  four  such  cubes  placed  end  to 
end.  If  1 6  such  prisms,  each  of  four  cubes 
placed  end  to  end,  were  piled  together,  a  cube 
would  be  produced,  and  as  each  of  the  six- 
teen prisms  would  be  subjected  to  the  same 
impact  of  water  which  would  move  it  sepa- 
rately, it  is  apparent  that  when  united  into 
a  cube  (b),  the  entire  mass  will  be  moved  as  a 
body,  for  the  area  against  which  the  water 
impinges  has  been  enlarged  16  times.  It  is  of 
course  evident  that  this  holds  only  for  the  case 
here  outlined,  namely  a  cube  64  times  as  large 
as  the  original  cube.  A  rock  of  any  other 
shape  would  not  be  moved  in  the  same  de- 
gree, and  as  few  rock  fragments  approach  a 
cubical  form,  the  formula  can  be  applied  only 
in  a  general  way.  Nevertheless,  it  is  evident 
that  with  increase  in  velocity  an  enormous  in- 
crease in  carrying  power  results,  and  this  explains  why  streams  after  rains 
become  heavily  laden  with  debris  and  can  perform  destructive  work  of  almost 
unthought-of  magnitude,  besides  carrying  away  huge  amounts  of  material  in 
a  short  time  and  moving  along  very  large  blocks  of  rock. 

In  general,  a  river  current  flowing  at  the  rate  of  one  fifth  of  a  mile 
an  hour  can  carry  along  fine  clay ;  one  running  at  the  rate  of  half 
a  mile  an  hour  transports  sand ;  at  the  rate  of  a  mile  per  hour,  the 
current  can  roll  along  medium  sized  gravel ;  while  at  ten  miles  an 
hour  it  can  roll  along  pebbles  of  the  size  of  an  egg.  There  is,  how- 
ever, much  variation  according  to  the  nature  of  the  bottom,  whether 
covered  with  sediment  or  free  from  it,  and  whether  the  finer  ma- 
terial is  stirred  up  by  eddies,  etc.,  or  has  to  be  picked  up  by  the 
current  itself.1 


FIG.  380.  —  Diagrams  illus- 
trating the  increased  carrying 
power  of  streams  with  in- 
creased velocity.  (After  Le 
Conte.) 


1  For  citation  of  detailed  measurements  see  A.  W.  Grabau,  Principles  of  Stratigraphy, 
pp.  248-251. 


462  Deposition  of  Clastic  Rock  Material 

Volume  of  Material  Transported  by  Rivers.  —  The  total  amount 
of  material  transported  by  rivers  is  surprisingly  great,  and  also 
varies  much  for  different  rivers.  Taking  some  of  the  large  rivers 
of  the  earth,  we  find  that  the  Mississippi,  with  a  flow  of  17,500 
cubic  meters  of  water  per  second,  carries  211,300,000  cubic  meters 
of  material  per  year;  while  the  La  Plata,  with  a  flow  of  19,820 
cubic  meters  of  water  per  second,  carries  only  44,000,000  cubic 
meters  of  material  per  year.  Again,  the  Hoang-Ho  or  Yellow  River 
of  China,  with  a  flow  of  only  3,285  cubic  meters  of  water  per  second, 
carries  the  enormous  quantity  of  472,500,000  cubic  meters  per 
year,  its  waters  being  turbid  with  sediment.  The  Mississippi 
River  actually  carries  more  than  400,000,000  tons  of  sediment  into 
the  Gulf  of  Mexico  each  year,  or  more  than  a  million  tons  a  day. 
The  exact  volume  of  material,  according  to  the  measurement  of 
Humphreys  and  Abbot,  is  7,471,411,200  cubic  feet  (211,273,000 
cubic  meters),  a  mass  sufficient  to  cover  an  area  of  one  square  mile 
to  a  depth  of  268  feet.  The  amount  carried  to  the  sea  by  all  the 
rivers  of  the  earth  in  one  year  has  been  estimated  to  be  about 
forty  times  this  quantity. 

While  the  coarser  material  is,  as  a  rule,  pushed  or  rolled  along 
the  river-bottom,  the  sands  generally  assume  the  form  of  low  banks 
alternating  in  position  on  the  two  sides  of  the  stream.  They  are 
generally  of  triangular  form,  their  bases  lying  against  the  river 
bank,  and  between  them  and  the  opposite  bank  lies  the  deeper 
channel  with  the  main  current.  (See  map,  Fig.  381,  at  Delta,  and 
Fig.  605,  at  Georgetown  Bend.)  Sand  is  removed  on  the 
upstream  and  deposited  on  the  downstream  side  of  the  sand  bank, 
which  thus  slowly  wanders  down  stream.  This  may  go  on  at  the 
rate  of  200  to  400  meters  per  year  in  some  cases  (Rhine),  and  in 
others  (Loire)  from  less  than  two  meters  per  day  in  summer  to  more 
than  1 8  meters  per  day  in  winter,  which  is  the  season  of  floods.  In 
all  cases  the  movement  of  the  sands  is  much  slower  than  that  of 
the  water,  and  the  amount  of  water  passing  a  given  point  may  be 
a  thousand  times  more  than  the  amount  of  waste  shifted  past  the 
same  point. 

Sorting  of  Sediments  by  Rivers 

River- transported  material  is  subject  to  assortment  by  the 
destruction  of  the  softer  materials  in  the  transport,  so  that  after  a 
while  only  the  most  resistant  material  will  remain.  A  much 


FIG.  381.  —  Flood-plain  of  the  Mississippi  River,  near  Vicksburg,  Miss.  The 
highland  to  the  east  rises  150  to  250  feet  above  it.  Note,  (i)  the  meandering 
course;  (2)  its  change  since  the  interstate  boundary  was  fixed;  (3)  recently 
abandoned  channels :  Paw  Paw  Chute  and  Old  Channel ;  (4)  Wilton  Bayou ; 
(5)  ox-bow  cut-off  at  De  Soto  Island;  (6)  crescent  lake:  Long  Lake; 
(7)  artificial  channels :  Yazoo  River ;  Diversion  Canal  and  Grant's  attempted 
diversion  near  Delta ;  (8)  cut  banks  near  Vicksburg ;  (9)  artificial  banks  or 
levees  near  Delta;  (10)  deposition  on  inner  side  of  curve  south  of  Delta; 
(n)  "made  land"  or  islands  of  deposition;  (12)  concentric  ridges  and  silted 
hollows  near  Wilton  Bayou,  representing  lateral  migration  of  the  channel; 
(13)  swampy  lowland  recently  abandoned  and  imperfectly  silted.  (Military 
Geology.} 

463 


464  Deposition  of  Clastic  Rock  Material 

worked-over  series  of  river  deposits  may,  indeed,  have  been  sub- 
jected so  thoroughly  to  this  searching  out  of  destructible  material 
that  nothing  but  quartz  pebbles  and  sand  will  remain.  Observa- 
tion on  Scottish  rivers  has  shown  that  the  percentage  of  feldspar 
in  the  sand  derived  from  the  crystalline  rocks  of  the  Highlands 
slowly  decreases  downstream  owing  to  its  progressive  destruction. 
(See  p.  415.) 

In  general,  great  masses  of  pure  quartz-sand  and  pebbles,  whether  uncon- 
solidated  or  bound  together  to  form  a  solid  rock  mass,  may  be  regarded  as  the 
product  of  prolonged  and  repeated  working  over  by  water.  While  in  most  cases 
that  work  is  probably  performed  by  rivers  which  carry  the  material  a  long  way, 
it  may  also  be  accomplished  by  the  waves  and  shore  currents  of  the  sea-coast, 
and  in  many  cases  it  may  be  a  combination  of  both. 

RIVER  DEPOSITS 

The  deposits  of  clastic  material  formed  by  rivers  may  be  located 
upon  land,  in  standing  bodies  of  water  (lakes,  ponds),  or  in  the  sea. 
The  last  will  always  be  subject  to  more  or  less  reworking  by  the 
ocean  waves  and  currents,  which  will  impress  characteristic  fea- 
tures upon  them.  They  are,  therefore,  properly  classed  as  marine 
sediments.  Deposits  formed  in  large  lakes  also  have  certain  dis- 
tinctive characters,  which  require  separate  treatment.  Those 
deposits,  which  were  built  upon  the  margins  of  lakes  and  of  the  sea, 
that  is,  the  deltas,  are  properly  considered  as  special  types  of 
river  deposits.  According  to  the  place  and  manner  of  deposition, 
we  may  recognize  the  following  types  :  (i)  Alluvial  fans  and  plains, 
(2)  flood  plain  deposits,  (3)  playas  and  (4)  deltas. 

Alluvial  Fans  and  Plains 

General  Forms  and  Character.  —  When  a  river  issues  from  the 
mountains,  where  the  slope  of  its  bed  is  steep  and  hence  its  velocity 
and  carrying  power  great,  and  descends  to  the  plains  or  low  ground 
at  the  foot  of  the  mountains,  where  as  a  result  of  the  decrease  of 
slope  its  velocity  and  carrying  power  become  suddenly  diminished, 
it  is  forced  to  drop  a  part  of  the  load  which  it  has  carried  in  its 
mountain  course,  and  in  this  manner  an  alluvial  fan  is  built  up 
(Figs.  382  a  and  b).  This  will  generally  have  the  form  of  a  low 
half-cone  resting  against  the  high  ground  in  the  back,  and  sloping 
outward  at  a  low  angle  in  all  directions  from  the  point  where  the 
river  issues  (Fig.  382  b).  The  angle  of  surface  slope  depends  upon 


River  Deposits 


465 


the  velocity  of  the  issuing  stream,  the  amount  and  size  of  the 
material  which  it  carries,  the  size  of  the  fan  at  any  given  time, 
and  on  other  factors.  It  may  be  as  high  as  20°  or  even  30°  in  small 
fans,  but  in  general,  the  angle  is  much  lower,  especially  in  large 
fans,  where  the  beds  appear  nearly  horizontal.  As  the  fan  grows, 
the  stream  breaks  up  into  a  number  of  diverging  terminal  fringes 
or  distributaries,  each  of  which  may  build  a  separate  lobe  of  the 
main  fan,  whose  outer  margin  will  thus  become  lobate  or  scalloped. 
The  paths  of  these  distributaries  are  generally  marked  by  channels 
cut  during  seasons  when  the  amount  of  material  brought  by  the 


FIG.  382  a.  —  Diagram  of  alluvial  cones,  showing  surface  and  underground 
structures.  (Drawn  by  F.  K.  Morris.)  In  this  case  the  original  plain,  F,  on 
which  the  alluvial  cone  is  built,  and  the  mountainous  mass,  C,  are  separated 
by  a  fault.  Note  the  progressive  overlapping  of  the  successive  beds  of  the  fan 
as  shown  in  the  section. 


stream  is  smaller  and  when,  therefore,  some  of  the  energy  of  the 
flowing  water-currents  is  expended  in  erosion.  When  several 
streams  issue  near  together  from  a  mountain  region,  their  separate 
fans  may  in  time  become  confluent,  forming  a  more  or  less  continu- 
ous deposit  along  the  mountain  front.  When  this  is  broad  and  flat 
it  is  called  an  alluvial  plain. 

The  area  covered  by  an  alluvial  fan  may  vary  in  size  from  a 
few  square  feet  to  thousands  of  square  miles,  while  confluent 
plains  may  cover  hundreds  of  thousands  of  square  miles. 

Alluvial  Fans  as  Sources  of  Water.  —  The  steeper  alluvial  fans 
at  the  mountain  front  commonly  furnish  a  ready  supply  of  water 
in  their  deeper  layers.  This  water  sinks  into  the  sands  near  the 


466 


Deposition  of  Clastic  Rock  Material 


head  of  the  fan  and  slowly  makes  its  way  through  the  sediments. 
The  sloping  character  of  the  layers  also  tends  to  supply  the  water 
with  a  sufficient  "  head  "  to  make  it  available. 


FIG.  382  b.  —  Small  alluvial  fan,  showing  distributaries,  Utah.     (Photo   by 

F.  J.  Pack.) 

Overlaps  of  the  Successive  Beds.  —  As  the  alluvial  fan  grows,  the 
upper  layers  will  extend  farther  than  the  lower  ones,  and  overlap 
them  around  the  radius  of  the  fan.  Thus  in  the  outer  zone  of  the 
fan  only  the  highest  layers  will  be  present,  resting  directly  upon  the 
old  surface  upon  which  the  fan  is  built  or  upon  deposits  formed  in 
front  of  it.  As  we  approach  the  head  of  the  fan,  borings  in  it  would 
reveal  the  presence  of  successively  earlier  and  earlier  layers  at  the 
bottom.  This  is  illustrated  in  the  following  diagram  (Fig  382  c), 


FIG.  382  c  —  Diagram  showing  the  progressive  overlapping  of  the  successive 
divisions  of  an  alluvial  fan,  away  from  the  source  of  supply. 

which  represents  a  radial  section  of  such  a  fan,  the  oldest  layer 
being  lettered  a,  the  youngest ,  /. 


River  Deposits  467 

Modern  Examples  of  Alluvial  Fans  and  Plains 

Among  the  many  modern  examples  of  great  alluvial  fans  those  of  the  Merced 
River  of  California,  the  Cooper  River  of  South  Australia,  the  Yellow  River 
(Hoang-Ho)  of  China,  and  the  Indo-Gangetic  Plain  may  be  given. 

The  Merced  River  Fan.  —  The  Merced  River  rises  in  the  Sierra  Nevadas  and 
carries  a  large  amount  of  waste  down  their  western  slope.  As  the  river  reaches 
the  great  open  California  Valley,  which  lies  between  the  Sierra  Nevadas  and  the 
Coast  Ranges,  it  drops  its  material  and  builds  an  alluvial  fan,  which  has  at  pres- 
ent reached  a  radius  of  about  40  miles.  The  material  of  this  fan  consists  of 
coarse  gravel  near  the  mountains,  and  of  fine  silt  in  its  outer  portion.  The 
slope  of  the  fan,  on  the  whole,  is  a  very  gentle  one,  on  account  of  which  the  river 
is  easily  diverted  at  the  point  of  issuance,  and  may  follow  different  directions 
across  the  fan  at  different  times. 

The  Merced  fan  is  only  one  of  a  number  of  such  fans  built  by  the  rivers  which 
flow  from  the  Sierra  Nevadas  into  the  Great  Valley.  These  fans  are  so  large, 
their  slopes  so  low,  and  their  confluence  so  complete,  that  it  is  difficult  to  recog- 
nize the  individual  convexity  of  each  without  the  aid  of  surveying  instruments. 
Similar,  though  smaller,  fans  descend  from  the  eastern  slopes  of  the  Coast  Range 
which  bounds  the  valley  on  the  west,  and  these  two  sets  meet  Jn  the  center  of 
the  valley  to  form  a  broad,  flat-floored  trough. 

The  Delta  of  the  Cooper  River,  Australia.  —  The  Cooper  River  is  one  of  the 
many  intermittent  streams  which  rise  in  the  mountains  of  Queensland  and  flow 
westward  to  the  lake  district  of  South  Australia  during  seasons  of  flood.  It 
enters  Lake  Eyre,  a  saline  body  of  water  1 2  meters  below  sea-level.  This  river 
has  built  a  delta  or  alluvial  plain  of  large  size,  but  as  it  is  not  built  in  a  perma- 
nent body  of  standing  water,  its  character  lies  midway  between  that  of  an 
alluvial  fan  and  a  true  delta.  The  area  of  this  delta-plain  is  more  than  twice 
that  of  the  Nile  Delta,  its  length  being  nearly  185  miles  and  its  width  over  170 
miles.  The  surface  of  the  delta-plain  is  dissected  by  numerous  dry  channels 
through  which  the  water  flows  after  periods  of  rain. 

Alluvial  Plain  of  the  Hoang-Ho.  —  The  Hoang-Ho  or  Yellow  River  of  China 
leaves  the  mountains  at  a  point  about  300  miles  from  the  present  sea-shore, 
and  over  this  distance  it  has  built  a  very  gently  sloping  alluvial  plain,  which 
spreads  out  in  the  form  of  a  triangle,  the  base  of  which,  along  the  present  coast, 
extends  for  about  400  miles  south  from  Pekin  to  the  great  plain  of  the  Yangtze- 
Kiang,  including  and  surrounding  the  rocky  headland  of  Shantung.  The  head 
of  the  plain,  where  the  river  leaves  the  mountains,  is  only  400  feet  above  sea- 
level,  hence  there  is  only  an  average  fall  of  i^  feet  per  mile,  making  a  surface  of 
such  gentle  slope  that  it  appears  in  all  respects  horizontal  (Fig.  383).  This 
very  gentle  slope  is  due  to  the  fact  that  the  material  of  which  this  plain  is  built 
is  mostly  fine  silt  derived  from  the  loess  of  the  interior.  It  is  the  yellowish  color 
of  this  material,  due  to  the  hydrated  iron  in  it,  which  has  given  the  Yellow  River 
its  name  (see  ante,  p.  459).  This  fine  material  is  deposited  in  a  series  of  nearly 
horizontal  layers,  one  above  the  other,  forming  thus  a  regular  succession  of  beds 
or  strata,  and  giving  the  deposits  a  regular  stratified  character. 

Because  of  the  gentle  slope,  the  river  is  easily  diverted  from  its  course  when 
swollen,  and  a  slight  change  at  the  head  may  produce  a  marked  alteration  of 


468  Deposition  of  Clastic  Rock  Material 

the  distributing  streams  over  the  surface.  The  main  mouth  of  the  stream  has 
been  repeatedly  shifted,  the  extent  being  as  much  as  200  miles.  The  plain  is 
intensively  cultivated,  and  there  are  many  lakes,  ponds,  and  swamps  in  which 
vegetable  deposits  accumulate,  forming  peat  beds,  which  in  time  may  be  con- 
verted into  coal.  When  the  river  breaks  its  banks,  inundations  of  vast  extent 
result,  and  such  deposits  of  peat  become  buried  by  the  silt.  In  1887  such  a 
flood  covered  an  estimated  area  of  50,000  square  miles  of  immensely  fertile 


FIG.  383.  —  View  of  the  flat  alluvial  plain  of  the  Hoang-Ho  in  eastern  China. 

and  densely  inhabited  land.  At  least  a  million  people  were  drowned,  and  an 
even  greater  number  succumbed  to  the  famine  and  diseases  which  resulted  be- 
cause of  the  flood. 

The  Indo-Gangetic  Plain.  —  The  streams  descending  from  the  southern 
slopes  of  the  Himalaya  Mountains  in  northern  India,  and  the  ranges  extending 
westward,  carry  large  quantities  of  clastic  material  produced  by  the  disintegra- 
tion of  the  rocks  during  the  dry  season  of  the  year  and  their  decomposition  during 
the  moist  season  (Fig.  384).  Along  the  foot  of  the  mountains  numerous  alluvial 
fans  of  coarse  material  and  steep  grade  are  built,  while  the  finer  material,  carried 
forward  into  the  lowland  of  northern  India,  becomes  incorporated  in  the  great 
alluvial  plain  which  is  traversed  by  the  two  major  streams  of  north  India,  the 
Ganges,  with  its  tributary  the  Brahmaputra  on  the  east,  and  the  Indus  on  the 
west.  This  Indo-Gangetic  Plain,  as  it  is  called,  has  an  area  of  about  300,000 
square  miles,  varying  in  width  from  90  to  nearly  300  miles.  It  entirely  separates 
the  lower  peninsula  of  India  from  the  Himalayas  on  the  north,  and  forms  the 
richest  and  most  densely  populated  district  of  India.  The  highest  portion 
of  this  plain  rises  only  924  feet  above  the  sea-level,  the  average  slope  of  the  sur- 
face being  of  similar  degree  to  that  of  the  Hoang-Ho  plain  (Fig.  385).  Borings 
into  this  plain,  which  have  penetrated  to  nearly  1000  feet  below  sea-level, 
have  shown  that  the  material  is  essentially  the  same  throughout.  This  means, 
of  course,  that  the  region  is  sinking  as  the  deposits  are  being  formed,  otherwise 
the  beds  now  a  thousand  feet  below  the  sea  could  not  have  been  formed  above 
that  level  as  are  the  modern  beds,  which  these  older  ones  resemble  in  all  respects. 
This  belt  of  country  parallel  to  the  Himalayas  thus  constitutes  a  modem  ex- 
ample of  a  geosyndine  of  deposition,  a  structural  feature  of  the  earth's  crust  to 
which  we  shall  refer  again  in  subsequent  pages. 


River  Deposits 


469 


The  material  of  which  this  great  deposit  is  composed  varies  greatly  in  char- 
acter. Along  the  sloping  borders,  especially  in  the  north,  gravels  are  com- 
mon, but  away  from  these,  fine  material  prevails,  pebbles  being  scarce  at  a 
distance  of  more  than  20  or  30  miles  from  the  hills.  This  finer  material  consists 
of  sands  more  or  less  well  assorted,  of  clays,  and  of  other  substances.  Beds 
of  wind-blown  or  eolian  sand  of  great  thickness  are  found  in  some  regions.  In 
some  sections  the  shells  of  river  and  pond  mollusks  are  common  in  the  clays, 
and  in  other  sections,  as  along  the  banks  of  the  Jumna  River,  the  bones  of  many 
land  and  river  animals  are  embedded  in  the  sediments,  among  these  being  the 
remains  of  elephant,  hippopotamus,  ox,  horse,  antelope,  crocodile,  and  various 


FIG.  384.  —  Khyber  Pass  in  the  Himalayas,   showing  the  extensive  forma- 
tion of  rock- waste. 

fish.  Peat  beds  are  also  forming  in  many  sections,  and  older  peat  beds,  buried 
by  later  sediments,  have  been  found  at  a  depth  of  20  to  30  feet  below  the  sur- 
face in  the  borings.  Bones  of  terrestrial  mammals  and  crocodiles,  etc.,  have 
been  found  at  considerable  depths,  but  nowhere  are  there  any  traces  of  marine 
organisms,  showing  that  throughout  the  period  of  deposition  of  these  sediments 
the  region  stood  sufficiently  above  sea-level  to  prevent  marine  waters  from 
entering. 

At  various  levels  beds  of  earthy  limestones  or  layers  of  calcareous  concre- 
tions, called  kankar,  are  found,  these  representing  the  lime  which  was  separated 
out  from  the  river- water  under  the  semi-tropical  heat  of  the  sun  (see  ante,  p.  260). 
Such  lime,  derived  from  the  solution  of  older  limestones  in  the  upper  river 
courses,  is  not  uncommonly  deposited  in  regions  of  semi-aridity  in  various 
parts  of  the  world. 

On  the  eastern  side  of  the  plain,  where  the  Ganges  enters  the  sea,  it  builds  a 
normal  delta,  and  here  some  of  the  beds  enclose  the  remains  of  marine  organisms. 
Similar  conditions  exist  upon  the  western  side,  where,  moreover,  extensive  sea- 


470  Deposition  of  Clastic  Rock  Material 

border  marshes  and  salt-pans  exist,  and  where  salt  deposits  are  included  in  the 
series  of  stratified  sediments  which  are  forming  there  (ante,  p.  233).  Salt  lakes 
and  playas  also  exist  inland,  where  climatic  conditions  are  favorable  for  the 
evaporation  of  salt-bearing  waters. 

The  thickness  of  the  deposits  of  the  Indo-Gangetic  Plain  is  not  known,  but 
it  probably  exceeds  several  thousand  feet.  It  rests  in  part  upon  an  older  de- 
posit of  precisely  the  same  character,  a  portion  of  which,  near  the  mountains,  has 
been  bent  or  tilted  by  a  comparatively  recent  uplift  or  rising  of  the  Himalayas. 
These  uplifted  ends  have  been  more  or  less  dissected  by  the  modern  streams, 
so  that  the  character  of  the  deposits  is  ascertainable.  Thus  it  is  seen  that 
the  material  of  this  older  alluvial  plain  is  similar  to  that  which  forms  the 


FIG.  385.  - —  Flood-plain  of  the  Ganges;  from  Calcutta. 

modern  plain,  and  like  it  includes  the  remains  of  land  and  fresh- water  animals. 
The  remains  of  marine  organisms  are  found  only  in  its  lowest  portion, 
where  it  grades  into  the  underlying  marine  formation.  The  most  surprising 
thing  about  this  older  alluvial  fan  deposit  is  its  enormous  thickness,  which  is  in 
the  neighborhood  of  15,000  feet.  As  the  material  of  this  fan  was  laid  down 
at  a  comparatively  slight  elevation  above  the  sea,  —  judging  from  what  is  seen 
in  the  deposits  now  forming,  —  it  is  apparent  that  during  its  formation  there  must 
have  been  a  slow  but  constant  sinking  of  the  area  over  which  it  was  deposited, 
and  that  when  the  topmost  layer  was  spread  out,  the  bottom  layers  must  have 
been  more  than  ten  thousand  feet  below  sea-level,  from  which  position  they 
were  lifted  by  the  subsequent  disturbance  which  affected  this  region. 

Another  striking  feature  of  this  older  deposit  is  the  presence  in  it  of  beds  of 
red  clay,  often  of  great  thickness.  These  red  beds  were  originally  deposits  of 
yellow  sandy  clays  such  as  now  form  upon  the  modern  plain.  The  yellow  color 
is  due  to  the  oxidation  and  hydration  of  finely  disseminated  iron  in  the  sedi- 
ments, the  thorough  oxidation  of  which  is  accomplished  whenever,  during  the  dry 
seasons,  the  ground- water  level  sinks  so  low  that  air  can  penetrate  and  to  some 
extent  circulate  through  the  sediments.  The  red  color  of  the  older  deposit  is 


River  Deposits  471 

merely  a  later  stage  in  change,  the  iron  losing  its  water  with  age,  and  so  changing 
to  the  red  oxide  (hematite),  just  as  the  burning  of  bricks  from  yellow  clay  drives 
off  the  combined  water  of  the  iron  and  changes  the  color  from  yellow  to  red. 


Deposits  in  Mountain-enclosed  Basins 

Where  a  basin  lies  within  the  mountains  with  only  a  single  outlet 
or  with  none  at  all,  clastic  material  formed  by  weathering  on  the 
mountain  sides  will  be  washed  by  the  rivers  into  the  center  of  the 
basin  where  it  accumulates,  often  to  a  very  great  thickness.  If 
the  climate  is  moist,  the  basin  will  be  filled  with  water  to  the  point 
of  overflow,  forming  a  lake  or  series  of  lakes,  while  the  amount  of 
clastic  material  washed  into  this  basin  will  be  comparatively  small, 
because  weathering  will  be  interfered  with  by  the  cover  of  vegeta- 
tion which  forms  upon  the  mountain  sides.  Thus  the  clastic  de- 
posits formed  on  the  floors  of  the  glens  and  other  valleys  in  the 
Scottish  Highlands  are  never  very  extensive,  though  peat  deposits 
form  on  all  the  mountain  sides  and  on  the  valley  bottoms  as  well. 
In  regions  of  dry  climate,  however,  where  vegetation  is  scant  or 
absent  at  least  for  parts  of  the  year,  much  weathering  results,  and 
the  product  of  such  weathering  accumulates  upon  the  floor  of  the 
intermontane  basin.  A  typical  example  of  such  a  waste-filled  basin 
is  the  Vale  of  Kashmir,  which  lies  within  the  northwestern  ranges 
of  the  Himalaya  Mountains.  Its  area  equals  that  of  the  Connecti- 
cut Valley  of  the  eastern  United  States,  being  of  elliptical  form, 
100  miles  long  from  southeast  to  northwest,  and  40  or  50  miles 
broad.  The  floor  of  the  valley  is  more  than  5000  feet  above 
sea-level,  and  it  is  deeply  filled  with  waste  material  from  the 
mountain  sides,  this  material  being  coarse  around  the  margins, 
but  fine  toward  the  center,  where  it  is  free  from  pebbles.  Its 
thickness  is  probably  several  thousand  feet,  though  no  borings 
have  been  made  to  determine  this.  The  waters  of  the  various 
streams  which  bring  the  material  from  the  mountains  and  deposit 
it  upon  the  valley  floor  are  gathered  to  form  the  Jehlam  River, 
which  meanders  across  the  plain  and  escapes  from  the  basin  by  a 
deep  and  rocky  gorge. 

A  similar  waste-filled  basin  exists  in  the  Rocky  Mountains  in  the  upper  valley 
of  the  Arkansas  River,  wnich  escapes  from  the  basin  by  the  Royal  Gorge.  The 
deposits  within  this  basin  slope  from  the  mountains  toward  the  river  borders. 

Deposits  formed  at  an  earlier  period  (Tertiary)  in  a  similar  basin  in  south- 
western Wyoming,  which  is  enclosed  by  the  Wasatch,  Uinta,  and  Wind  River 


472  Deposition  of  Clastic  Rock  Material 

ranges  of  mountains,  have  now  been  thoroughly  dissected,  apparently  as  the 
result  of  a  change  in  climate,  which  brought  with  it  increased  precipitation  of 
moisture.  This  dissection  is  accomplished  by  the  Green  River  and  its  tribu- 


FIG.  386.  —  Flood-plain  of  the  river  escaping  from  the  front  of  the  Bartlett 
Glacier,  Alaska.     (Courtesy  of  Alaska  Engineering  Commission.) 


FIG.  387  a.  —  Morainal  and  river  terraces  on  east  side  of  Columbia  Gorge, 
Chelan  Ferry,  Wash.     (Photo  by  Baily  Willis,  from  U.  S.  G.  S.) 

taries,  the  waters  of  which  escape  by  a  deep  gorge  through  the  Uinta  Mountains. 
As  a  result  of  this  dissection  the  older  parts  of  the  deposit  have  been  laid  open 
to  view,  in  cliffs  sometimes  a  thousand  feet  high,  and  the  details  of  structure 


River  Deposits 


473 


and  composition  of  such  deposits  can  thus  be  studied  to  great  advantage,  whereas 
those  of  the  undissected  deposits  can  be  ascertained  only  by  borings. 

River  Flood-plain  Deposits 

When  a  river  is  heavily  laden  with  sediment,  it  is  bound  to  de- 
posit this  wherever,  by  an  expansion  of  its  bed  or  a  lessening  of  its 
grade,  the  velocity  is  de- 
creased. A  river  escaping 
from  the  foot  of  a  glacier 
usually  carries  a  large 
quantity  of  fragmental 
material,  both  coarse  and 
fine,  and  the  former  is  de- 
posited soon  after  the 
river  leaves  the  ice  front 
(Fig.  386).  During  the 
last  glacial  period,  when 

vast  ice  masses  covered     FlG  ^  b  _  old  terraces  of  the  West 
much  of  northern  North       Brattleboro,  Vt.     (Photo  by  Prof.  E.  Fisher.) 
America  and  northwest- 
ern Europe,  the  rivers  escaping  from  the  front  of  this  ice,  during 

periods     of      prolonged 

melting,  carried  large 
quantities  of  coarse 
debris  and  deposited  this 
in  their  valleys,  often 
filling  them  from  side  to 
side  to  a  depth  of  even 
a  hundred  feet  or  more. 
Subsequently  when, 
owing  to  the  melting 
away  of  the  ice,  less 
debris  was  furnished,  the 
energy  of  the  river  was 
expended  in  again  cut- 
ting away  the  old  de- 
posit. Commonly,  how- 
ever, the  deposit  was  not 


FIG.  388  a.  —  Flood-plain  of  the  Saco  River 
at  Intervale,  N.  H.,  showing  the  coarse  ma- 
terial derived  from  an  erosion  of  the  terraces. 
(Photo  by  the  author.) 


completely  removed,  but 
terraces  of  the  old  sedi- 


474  Deposition  of  Clastic  Rock  Material 

ment  were  left  on  both  sides  of  the  river  valley.     Such  terraces 
may  be  seen  along  most  of  the  larger  streams  in  the  northern 


FIG.  388  b.  —  Flood-plain  of  Connecticut  River,  in  Vermont. 

states,  often  occurring  in  several  successive  series,  each  lower  than 
the  preceding,  and  each  set  marking,  by  its  summit  level,  which  is 
in  accord  on  opposite  sides  oi  the  valley,  a  temporary  surface  of 


FIG.  389.  —  The  Ohio  River  in  flood,  showing  inundation  of  flood-plain. 


the  flood-plain  of  the  river  before  it  resumed  cutting  toward  a 
lower  level  (Figs.  387  a,  b).  Close  to  the  front  of  the  old  ice  sheet 
the  material  of  which  these  terraces  are  composed  is  very  coarse, 
boulders  and  cobblestones  of  the  size  of  a  man's  fist  often  pre- 


River  Deposits 


475 


A  MILES 


FIG.  390  a.  —  A  well-developed  river  flat,  Mississippi  Valley,  near  Prairie 
du  Chien,  Wis.  Note  the  steep  confining  bluffs  and  numerous  lagoons,  cres- 
cent lakes,  ox-bows,  cut-offs,  and  abandoned  channels,  showing  various  stages 
of  silting.  (From  Military  Geology.) 


FLOOD-PLAIN    DEPOSITS         NATURAL  LEVEES 

FIG.  390  b.  —  Block  diagram  showing  the  flood-plain  of  a  river,  with  ox-bows 
and  marginal  streams ;  and,  in  section,  the  flood-plain  deposits  and  natural 
levees.  (Drawn  by  F.  K.  Morris.) 


476  Deposition  of  Clastic  Rock  Material 

dominating  (Fig.  388  a).     Farther  down  stream  the  material  be- 
comes largely  sand,  increasing  in  fineness  with  its  distance  from 

the  original  source. 

Where  rivers  flow  at  a 
gentle  grade,  as  in  the 
case  of  the  lower  reaches 
of  most  large  rivers  (Fig. 
388  b),  the  flood-plain, 
which  is  that  part  of  the 
valley  floor  inundated 
only  at  high  water  (Fig. 
FIG.  391- T  Break  in  levee  of  the  Mississippi  g  )  ig  commonly 

River  opposite  New  Orleans,  submerging  a  ' 7  f 

plantation.     (Photo  by  Howell.)  made  UP  ot  laYers  o: 

mud,  with  many  ponds 

and  swamps  scattered  over  its  surface  (Fig.  390  a).     The  mud  is 
derived  from  the  river,  which,  on  rising,  overflows  its  banks.     If 


FIG.  392.  —  Mud  cracks,  Nile  flood-plain ;  sufficiently  wide  and  deep  to  admit 
a  man's  arm  to  the  elbow.  Abu  Simbel,  near  second  Nile  Cataract.  (After 
Hobbs.  Courtesy  of  the  American  Geographical  Society,  Broadway,  at  i52d 
St.  From  the  Geographical  Review.} 


River  Deposits 


477 


FIG.  393.  —  The  flood-plain  of  the  Nile.     Plowing  with   the   aid  of   camels. 


Evarts   VsO  ' 


4- MILES 


FIG.  394  a.  —  A  braided  stream,  Platte  River,  in  the  broad  alluvial  valley 
near  Kearney,  Neb.  A  mile-wide,  sandy  chamiel  filled  with  water  only  at 
flood  time.  Over  the  bottom  during  most  of  the  year  a  little  water,  not  diverted 
for  irrigation,  percolates  through  the  sand,  or  finds  its  way  in  a  tortuous 
course  through  a  series  of  interlacing  channels  whose  pattern  changes  with 
every  flood.  Northwest  winds  here  lift  the  sand  from  the  channel,  sweep  it 
across  a  grassy  plain,  and  pile  it  in  dunes  nearly  two  miles  south  of  the  river. 
(Kearney,  Neb.,  topographic  sheet,  U.  S.  G.  S.  From  Military  Geology.) 


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HOM  X4djn^  Pt:X: 

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478 


River  Deposits 


479 


UNCONSOLIDATED  SEDIMENTS 


Impervious  clay 


Porous  sand  and 
gravel  above  ground- 
water  table 


Porous  sand  and 
gravel  below  ground- 
water  table 


FIG.  394  c.  —  Diagrammatic  section  showing  artesian  conditions  in  Sulphur 
Spring  Valley,  a,  dry  hole,  which  if  sunk  deeper  would  strike  rock  without 
finding  water ;  b,  dry  hole  which  if  sunk  deeper  would  find  water ;  c,  shallow 
pump  well ;  d  and  e,  flowing  wells.  (Military  Geology.} 

vegetation  grows  along  the  bank,  this  will  commonly  retard  the 
current  of  the  spreading  river,  and  as  a  result,  a  considerable  amount 
of  the  river  silt  will  "settle  out  along  its  banks,  building  up  natural 
levees.  Such  natural  levees 
are  very  crAacteristic  of  the 
Mississippl^fc^pther  large 
rivers  (Figs.  v  ^o  /;,  391). 
Beyond  them  the*^ 
generally  lower  and  swampy, 
with  many  lakes,  as  in  the 
"Back  Swamps  "of  the  lower 
Mississippi  (see  map,  Fjg. 
381,  p.  463).  Because  the 
mud  settles  out  in  relatively 
small  quantities  at  each 
flooding,  the  layers  compos- 
ing the  flood-plain  will,  as  a 
rule,  be  thin,  and  a  finely 
stratified  structure  results. 
As  the  mud  dries  after  a 
flooding,  it  breaks  into 
polygonal  blocks  separated 
by  cracks,  the  depth  of 
which  depends  on  the  length 
of  exposure  and  on  other 
causes.  Such  sun-cracked 


FIG.  395.  —  Mud-cracked  surface  on 
bank  of  Little  Colorado  River,  Texas, 
formed  during  drought  of  1918.  Some 
of  the  large  cracks  are  from  4  to  10  inches 
deep,  and  a  secondary  set  of  finer  ones  has 
formed  upon  the  larger  blocks.  (Photo- 
graphed and  contributed  by  Prof.  Eliza- 
beth Fisher,  Wellesley  College.) 


480 


Deposition  of  Clastic  Rock  Material 


or  " mud-crack"  surfaces  are  very  characteristic  of  river  flood- 
plains  (Fig.  392).  Illustrations  of  mud-cracks  from  the  flood 
and  delta-plains  of  American  rivers  are  given  in  figures  395  and 

396. 

The  flood-plain  of  the  Nile  (Fig.  393)  extends  for  a  length  of 
500  miles  and  has  a  width  ranging  from  five  to  fifteen  miles,  which 
on  the  delta  increases  to  100  miles.  On  both  sides  it  is  bordered 
by  rocks  or  by  desert  sands,  while  the  banks  at  low-water  are  from 
20  to  30  feet  in  height.  The  river  overflows  its  banks  every  year, 
the  flood  beginning  in  June  and  usually  rising  25  feet  or  more 


FIG.  396.  —  Mud-cracks  on    the  delta  of  the  Colorado   River.     (Photo  by 
G.  K.  Gilbert,  from  U.  S.  G.  S.) 

at  Cairo  in  the  late  summer  or  early  autumn.  The  annual  addition 
of  the  river  silt  causes  a  slow  rising  of  the  flood-plain  at  a  rate  esti- 
mated to  equal  4^-  inches  in  a  century. 

Flood-plains  of  rivers  of  variable  flow,  where  a  large  amount  of 
material  is  brought  from  the  mountains  during  flood  time,  while 
little  or  no  water  occupies  the  channel  during  the  dry  season,  have 
special  surface  characters  and  structure.  Trie  excessive  amount  of 
silt  causes  the  river  to  break  up  into  a  series  of  interlacing  threads, 
forming  a  "  braided  "  structure  (Fig.  394  a).  The  deposits  them- 
selves consist  of  variable  and  discontinuous  layers  of  interfingering 
coarse  and  fine  material  (Fig.  394  b,  c). 


River  Deposits 


481 


Playa  Deposits 

When  rivers  end  in  desert  basins  from  which  there  is  no  outlet, 
they  form  saline  or  more  rarely  fresh  lakes  (Fig.  397)  or  they 
spread  out  after  a  period  of  flood  into  flat  and  very  shallow  playa 
lakes  which  disappear  again  by  evaporation.  Such  playa  lakes 
come  into  existence  very  suddenly  on  account  of  the  usual  flat 


FIG.  397. — Lake  Sorkul,  without  outlet,  in  Great  Pamir  Desert.  (After  Reclus.) 

bottom  of  the  basin,  and  they  are  often  of  considerable  size.  In  the 
Black  Rock  desert  of  Nevada  a  large  playa  lake  forms  every  winter, 
covering  an  area  of  450  to  500  square  miles,  but  seldom  reaching  a 
depth  exceeding  a  few  inches.  An  Old  World  playa  lake  has  been 
known  to  become  full  grown  in  twenty  minutes,  reaching  a  width  of 
10  to  15  km.  and  an  unascertained  length,  with  a  depth  ranging 
only  from  one  to  six  incjies.  Often  such  a  playa  lake  is  only  a  body 
of  very  liquid  mud,  and  as  the  water  dries  away  this  mud  settles 
down  as  a  continuous  but  thin  sheet  over  the  entire  bottom.  In  a 
few  hours  or  a  few  days  the  water  has  evaporated,  leaving  a  hard, 
dry,  and  absolutely  barren  playa  surface,  cracked  in  all  directions  as 
the  mud  contracts  in  drying.  When  the  river  water  carries  salts 


482 


Deposition  of  Clastic  Rock  Material 


in  solution,  these  remain  behind  on  evaporation  of  the  lake  water, 
either  impregnating  the  muds  or  forming  distinct  layers.  Besides 
the  mud  cracks  and  occasional  impressions  of  raindrops  left  by  a 
passing  sharp  shower,  the  footprints  of  many  animals  which  come 
to  these  waters  to  drink  may  be  impressed  upon  the  mud  surface 

and  retained  in  it  on 
drying.  If,  subse- 
quently, sands  are 
blown  across  such  a 
surface  or  washed 
there  by  a  later  inun- 
dation, they  will  not 
only  fill  the  cracks 
between  the  polygonal 
blocks,  into  which  the 
su'rface  has  been 
broken  while  drying, 
but  will  also  cover,  and 
preserve  a  relief  im- 
pression of,  the  rain- 
drops and  foot-prints. 
On  solidifying,  the 
resulting  layer  of 
sandstone  will  have 
such  relief  features 
on  its  under  side, 
while  the  hardened 
mud  retains  the  actual 

impressions.  In  the  sandstones  of  Triassic  age  found  in  eastern 
North  America  and  in  Europe,  many  such  relief  impressions 
of  footprints  of  now  extinct  gigantic  land  reptiles  (Dinosaurs)  and 
of  Stegocephalians  (Fig.  398)  are  found  (see  Chapter  XLIV).  The 
actual  impressions  are  also  present  in  the  mud-rock  beneath,  but 
these  are  not  easily  obtained  on  account  of  the  readiness  with  which 
the  mud  layers  shatter  on  quarrying.  Trails  of  insects  and  other 
organisms  are  also  formed  upon  the  play  a  surfaces,  and  sometimes 
these  are  preserved ;  but  the  actual  remains  of  animals  are  seldom 
found,  since  those  that  die  are  rarely  buried  by  the  sands  and  muds 
before  their  bones  have  completely  disintegrated. 

Playa  lakes  which  endure  for  some  months  may  become  stocked 


FIG.  398. — Tracks  (in  relief)  of  Chirotherium, 
Buntsandstein,  Hessberg,  Germany.  (From 
Haas'  Leitfossilien.)-  This  slab  represents  the 
consolidated  sand  which  was  spread  over  the 
original  surface  on  which  the  foot-print  impres- 
sions were  formed.  The  relief  structures  here 
shown  are,  therefore,  the  natural  "casts"  of  the 
original  impressions  and  represent  the  form  of 
the  animal 's  feet.  The  side  with  these  relief 
structures  is  the  under  side  of  the  slab. 


River  Deposits  483 

with  animals  whose  eggs  can  withstand  prolonged  periodic  drying, 
and  develop  only  when  the  lake  comes  into  existence.  Originally 
such  eggs  may  be  brought  by  the  wind  or  by  birds  or  otherwise, 
but  few  long-existing  lakes  are  without  them  unless  their  water  is 
very  salty.  Chief  among  such  forms  are  the  small  fresh-water 
crustaceans  of  the  genera  Estheria,  Cypris,  etc.  (Fig.  399),  the 
shells  of  which  may  cover  the  playa  surface  in  considerable  number 


F" 


F' 


FIG.  399.  —  Cypris,  a  modern  ostracod.  Female  before  sexual  maturity, 
right  valve  of  shell  removed  to  show  internal  anatomy.  A' A",  first  and  second 
pair  of  antennae ;  Ob,  upper  lip ;  Md,  mandible  with  leg-like  feelers ;  G,  cerebral 
ganglion  with  impaired  eye ;  Sm,  shell  muscle ;  MX',  MX",  first  and  second  pair 
of  maxillae;  F',  foot  for.  crawling;  F",  foot  for  cleaning;  Fu,  furka;  M, 
stomach;  D,  intestine;  L,  liver;  Ge,  genitals.  Much  enlarged.  (From  Haas.) 

after  drying,  and  may  even  form  thin  deposits  mostly  composed 
of  such  shells.  Estheria  is  known  to  live  in  playa  ponds  which 
are  dry  for. eleven  months  in  the  year.  Forms  of  this  type  are  also 
found  in  some  of  the  mud  layers  in  the  Triassic  series  of  sediments 
referred  to  above.  River  fish,  too,  may  be  swept  in  large  numbers 
into  such  temporary  lakes,  and  their  remains  may  become  buried 
in  the  accumulating  deposits. 

On  the  whole,  playa  deposits  have  many  characters  in  common 
with  the  mud  deposits  formed  on  river  flood-plains,  and  ancient 
deposits  of  one  type  may  easily  be  mistaken  for  those  of  the  other. 

Deltas 

Where  rivers  laden  with  sediments  enter  a  lake  or  the  sea,  they 
will  build  a  normal  delta,  provided  the  shore  currents  are  of  insuffi- 
cient strength  to  remove  all  the  material  brought  by  the  rivers. 
Because  of  the  absence  of  tides  in  lakes,  the  deltas  built  into-  such 
water  bodies  will  be  more  perfect  and  less  variable  than  those  built 


484  Deposition  of  Clastic  Rock  Material 


into  the  sea.  The  name  delta  is  derived  from  the  resemblance  of 
one  of  the  best-known  examples,  the  delta  of  the  Nile,  to  the  Greek 
letter  delta  (A),  though  few  deltas  have  such  a  regular  triangular 


flosett 


Port  Said 


FIG.  400.  —  Map  of  the  Nile  Delta.     (After  Kayser.) 

form  (Fig.  406).  Small  deltas,  especially  those  built  in  lakes, 
show  two  sets  of  beds,  one  sloping  lakewards  at  a  considerable 
angle  (up  to  20  degrees  or  more)  and  generally  composed  of  the  finer 
material,  and  one  set  which  is  nearly  horizontal  or  but  slightly 


FIG.  401.  —  Section  of  a  small  delta  in  a  lake,  showing  the  foresets  of  pebbles 
and  sand,  the  bottom-sets  of  clay,  and  the  topsets  of  coarse  sand  and  pebbles. 
(After  Kayser.) 

inclined,  and  is  generally  composed  of  coarser  material  and  rests 
upon  the  steeper  beds  with  an  abrupt  change  of  angle.  The  first 
series  is  called  the  "  foresets,"  the  second  the  "  topsets  "  (Figs.  401, 
402).  Deltas  built  in  the  sea  are  of  more  complicated  structure, 
though  in  general  the  two  sets  of  beds  may  also  be  recognized. 


River  Deposits  485 

On  the  surfaces  of  large  deltas,  such  as  those  of  the  Nile  and  the 
Mississippi,  many  ponds  and  more  or  less  permanent  lakes  may 
exist.  Most  of  these  will  be  fresh- water  lakes,  but  along  the  sea- 
coast  salt-water  lagoons  and  shallow  lakes  will  often  be  found.  If 
the  climate  is  dry,  some  of  these  shallow  lagoons  may  become 
natural  salt  pans,  as  is  the  case  with  some  of  them  along  the  edge 
of  the  Nile  delta.  The  main  stream,  too,  commonly  divides  into 
many  ramifying  branches  or  distributaries,  which  intersect  the  sur- 


FIG.  402.  —  Section  of  a  delta-plain  built  into  Lake  Bonneville,  Utah.  The 
finer-grained  inclined  foreset  beds  are  covered  by  horizontal  and  coarser  topset 
beds.  (Photo  by  F.  J.  Pack.) 

face  of  the  delta,  and  some  of  these  streams  may  build  independent 
lobes  or  even  long  arms  into  the  sea,  as  is  the  case  in  the  "  bird-foot 
delta  "  of  the  Mississippi  (Fig.  403). 

The  muds  of  the  delta  are  very  apt  to  bury  and  preserve,  the  re- 
mains of  fish  and  other  animals  which  live  in  the  rivers  or  in  the 
ponds  upon  the  delta  surface,  and  if  the  river  comes  from  wooded 
regions,  tree  trunks  and  branches  may  also  be  buried.  The  vege- 
tation, too,  which  grows  upon  the  surface  of  the  delta  and  in  the 
ponds  will  leave  its  remains  in  the  deposits.  Along  the  sea  margin, 
marine  animals,  especially  floating  types  and  seaweeds,  may  be 
cast  upon  the  delta  surface  during  storms,  or  may  become  stranded 
after  a  temporary  flooding  of  the  delta  by  the  sea-water.  Lakes  or 
ponds  near  the  coast  may  also  receive  such  organisms  by  overwash 


486 


Deposition  of  Clastic  Rock  Material 


from  the  sea.  Thus  a  commingling  of  the  remains  of  river,  pond, 
and  marine  organisms  is  to  be  expected  along  the  borders  of  the 
delta.  (For  illustrations  of  the  common  forms  see  ante,  pp.  310 
to  316.) 


0  -  6mEO  20  -  50  m 
6  •  lOmEZS  50  -  100m 
I  .IHlO-20iaMI  100-  200m 


FIG.  403.  —  Bird-foot  delta  of  the  Mississippi.  (After  coast  survey  chart.) 
The  various  shades. show  the  depths  of  water  in  meters  as  indicated  in  the 
legend.  (From  Ratzel,  Die  Erde.) 

Structural  and  Other  Physical  Characters  of  River  Deposits 

Among  the  important  structural  characters  found  in  most  river  deposits, 
the  following  may  be  mentioned :  (a)  stratification,  (b)  cross-bedding,  (c) 
cut-and-fill  structures,  (d)  ripple  marks,  (e)  rill-marks,  (/)  mud-cracks,  (g) 
raindrop  impressions,  (ti)  footprints,  (i)  clay  galls,  etc.  It  must  be  clearly 
understood,  however,  that  these  structures  are  not  confined  to  river  deposits, 
though  special  phases  of  them  may  be  so  restricted. 

Stratification  (Pig.  404).  — This  is  the  arrangement  of  rocks  in  layers,  each 
of  which  was  at  one  time  the  topmost  one.  The  individual  layers  or  beds  are 
called  the  strata  (singular,  stratum},  especially  if  they  differ  markedly  in  char- 
acter, as,  for  example,  a  stratum  of  clay  which  is  followed  by  a  stratum  of  sand 
or  by  one  of  peat.  Within  each  stratum  there  may  be  minor  layers  or  lamina 


River  Deposits 


487 


which  vary  slightly  in  texture  or  color  or  other  characteristics.  The  strata  of 
river-laid  deposits  vary  with  the  coarseness  and  character  of  the  material  (Fig. 
394  b,  p.  478) .  Coarse  deposits,  such  as  pebbles,  are  often  very  irregular,  varying 
from  thick  to  thin,  and  often  passing  laterally  into  sands.  Even  the  strata  of 
sand  are  not  always  regular,  but  vary  in  thickness,  and  often  thin  away  or  pinch 
out.  Such  variation  is  readily  seen  when  two  sections,  a  short  distance  apart, 
are  compared  (Fig.  405).  The  fine  deposits,  on  the  other  hand,  such  as  charac- 
terize the  flood-plains  and  playas,  are  commonly  well  stratified,  regular,  and 
occur  in  layers  of  uniform  thickness  over  wide  areas  and  generally  show  a  finely 


FIG.  404.  —  Cliff  cut  on  horizontal  well-stratified  rocks.  Note  the  heavy 
stratum  near  the  center.  Weathering  of  weaker  strata  has  left  the  harder 
ones  projecting  and  this  has  emphasized  the  stratified  appearance. 

laminated  structure.  In  this  respect,  they  are  as  well  stratified  as  are  marine 
deposits.  (Compare  Fig.  16,  p.  81,  and  Fig.  487,  p.  579,  the  former  an  ancient 
marine,  the  latter  a  flood-plain  deposit  of  the  same  age.) 

Cross-bedding.  —  This  structure  is  well  marked  in  river  deposits  which 
are  formed  by  more  or  less  torrential  currents  that  carry  forward  a  large 
amount  of  material  at  a  time.  Consequently  it  is  best  developed  in  the  coarser- 
grained  deposits,  such  as  sands  and  the  finer  pebble  beds.  Typically  it  is 
characterized  by  a  succession  of  sloping  layers  separated  by  horizontal  beds. 
The  parts  characterized  by  the  sloping  layers  may  be  as  much  as  four  or  five 
feet  in  thickness,  the  angle  of  slope  of  the  layers  being  from  20  to  30  degrees. 
At  the  bottom  these  layers  tend  to  change  toward  horizontality,  but  at  the 
top  they  are  generally  abruptly  truncated,  and  a  set  of  horizontal  beds  up  to 
several  feet  in  thickness  rests  upon  their  edges.  Above  these  horizontal  layers 
is  a  second  series  of  sloping  beds,  similar  to  the  first  and  with  the  inclination 
in  the  same  direction.  There  may  be  many  such  successions  of  sloping  and 
horizontal  beds,  but  in  all  cases  the  inclined  beds  dip  in  the  same  direction, 
which  is  that  of  the  flow  of  the  current.  Such  a  type  of  cross-bedding  is  readily 


488  Deposition  of  Clastic  Rock  Material 

distinguished  from  the  eolian  type,  in  which  the  slopes  are  in  varying  directions 
and  the  successive  divisions  are  separated  by  erosion  planes  instead  of  horizontal 
layers.  The  following  diagram  (Fig.  406)  illustrates  this  type  of  cross-bedding, 

which  may  be  taken  as  in- 

m 


fireclay 

Sand  and 
/Sandstone 


Fireclay 

Sand  and  \ 
Sandstone] 


Lignite  with  seams 

of  Fireclay,  Pebbles 

<&  Gypsum. 

Clay 


Soft, 
Sandstone 


Lignite  with  seams 

of  Sand,  Clay 

<&  Pebbles 


±i 


dicating  deposition  by  tor- 
rential rivers  whenever 
found.  An  irregular  form  of 
cross-bedding  is  also  pro- 
duced by  rivers  where  there 
is  a  confluence  of  opposing 
currents.  Lyell  described 
the  structure  of  a  sand  bank 
formed  in  the  spring  of  1828, 
where  the  opposing  currents 


soft  white 
Sandstone 


Lignite 


'  Lignite\ 

Clay 
Sand 


FIG.  406. — Diagram 
showing  the  type  of  cross- 
bedding  which  is  charac- 
teristic of  torrential  river 
deposits.  (Compare  with 
Eolian  cross-bedding,  Figs. 
372  b,  and  374~377.) 

of  the  Rhone  and  the  Arve 
met  and  neutralized  each 
other,  causing  a  retarda- 
tion of  their  motion.  Into 
this  sand  bank  a  section  was 
subsequently  cut,  which  is 
reproduced  in  the  following 
figure  (Fig.  407).  This  sec- 
tion was  about  twelve  feet 
long  and  five  feet  high. 
The  stratum  A  A  consists 
of  irregular  alternations  of 
pebbles  and  sand  in  undu- 
lating beds ;  below  thtse  are 

seams  of  very  fine  sand,  BB,  some  as  thin  as  paper,  others  about  a  quarter  of 
an  inch  thick.  The  stratum  CC  is  composed  of  layers  of  fine,  greenish  gray 
sand  as  thin  as  paper.  Some  of  the  inclined  beds  will  be  seen  to  be  thicker  at 
their  upper,  others  at  their  lower,  extremity,  the  inclination  of  some  being 
very  considerable. 


::---._-.-";:::" 

Clau 

Lignite 

^-•fc-i-aSSsS&SSfia 

Clay 

•20  feet  - 

FIG.  405.  —  Parallel  vertical  or  columnar 
sections  on  the  face  of  Pulpit  Rock,  near  Colo- 
rado Springs,  through  identical  strata,  and 
only  20  feet  apart;  illustrating  rapid  lateral 
changes  in  the  character  of  the  strata. 
(From  Crosby.) 


River  Deposits 


489 


Cut-and-fill  Structure.  —  This  is  especially  characteristic  of  the  coarser  river 
deposits,  such  as  those  formed  in  alluvial  fans  and  waste-filled  basins.  It  may, 
however,  also  occur  in  the  finer  sediments.  It  is  characterized  by  abrupt  chan- 


FIG.  407.  —  Section  of  a  sandbank  in  the  bed  of  the  Arve  at  its  confluence 
with  the  Rhone,  showing  the  stratification  of  deposits  where  currents  meet. 
(From  LyelPs  Principles.} 

nels  which  cut  off,  or  are  excavated  into,  the  older  layers,  and  are  filled  by  a  part 
of  the  next  higher  layer.  They  indicate  the  channelling  so  characteristic  of 
the  surface  of  alluvial  fans,  and  the  subsequent  filling  in  of  these  channels 
where  a  change  in  direction  of  the  current  has  occurred  and  new  deposits 
are  added. 

Ripple  Marks.  —  These  structures  are  formed  under  varying  conditions.  In 
river  deposits,  they  generally  represent  a  series  of  low,  parallel  ridges,  one  side 


FIG.  408.  —  Diagram  showing :  a,  the  formation  of  current  ripples  (after 
Darwin) ;  and  b,  plan  showing  arrangement  of  ripples  in  parallel  lines  with 
transverse  connections.  (After  Walther.) 

of  which  is  steeper  than  the  other  (Fig.  408  a).  These  are  the  current  ripples 
which  form -at  right  angles  to  a  gently  flowing  current  in  the  shallow  pools,  as 
sand  dunes  form  at  right  angles  to  the  direction  of  the  wind.  Such  ripples  are, 
however,  by  no  means  confined  to  river  deposits,  but  may  occur  in  all  strata 
laid  by  and  in  shallow  water,  and  occur  in  wind  deposits  as  well.  (See  Fig.  371, 
p.  453,  and  Fig.  453,  p.  537.) 

Rill-Marks.  —  These  are  formed  upon  the  mud  surfaces  of  flood-plains  and 
playa  surfaces,  where  a  small  stream  of  water  trickles  from  a  bank  or  bubbles  up 
from  beneath  and  runs  away,  spreading  into  numerous  distributaries,  which 
become  finer  and  finer  outwards  until  they  fade  away.  Such  spreading  rill- 
marks  are  the  reverse  of  those  most  common  on  the  shore  in  which  the  channels 


490  Deposition  of  Clastic  Rock  Material 


converge  like  the  branches  of  a  stream  system.     When  covered  by  sand,  these 

rill-marks  may  be  preserved  in  relief,  and  they  often  have  a  strong  resemblance 

to  some  form  of  plant,  for  which  fossil, 
rill  marks  have  often  been  mistaken 
(Fig.  409). 

Mud- cracks.  —  These  are  most 
characteristic  of  the  playa  and  flood- 
plain  surfaces  and  are  formed  by  con- 
traction of  the  surface  layer  of  mud 
in  drying.  As  a  result,  such  layers 
split  up  into  polygonal  blocks,  which 
generally  curve  gently  upward  at  the 
margin,  producing  a  saucer-shaped 
surface.  (See  Figs.  392,  p.  476,  and 
395-396,  pp.  479,  480.)  The  cracks  in- 
crease in  width  and  depth  with  pro- 
gressive drying,  and  this  may  go  on 
to  such  an  extent  that  their  depth 
is  measured  by  several  feet.  Ordi- 
narily, however,  the  depth  is  not  over 
one,  or,  at  most,  a  few  inches.  A 
covering  of  sand  preserves  these  mud- 
cracks,  as  it  fills  the  fissures  between 
the  blocks  (Fig.  410).  On  the  under 
side  of  the  sand  layers  when  hard- 
ened, the  polygonal  surfaces  will  be 
gently  convex  and  bounded  by  raised 

ridges  of  hardened  sand.     Mud-cracks  may  be  formed  on  tidal  flats  which 

are  exposed  for  a  long  period 

of  time.     Ordinarily,  however, 

such    flats    do   not   dry   suffi- 
ciently between  tides  to  permit 

the  formation  of  mud-cracks, 

or  if  they  are  formed,  the  mud 

does  not  harden  sufficiently  to 

withstand  the  softening  effect 

of  the  returning  tide.     Figure 

411  shows  a  photograph  of  an 

ancient    clay-rock    with    very 

narrow  mud-cracks. 

Raindrop      Impressions.  — 

These    are   also   most    typical 

on  the  mud  surfaces  of  flood- 
plains  and  playas  (Fig.    412). 

When  raindrops  strike  the  mud 

sharply,  they  leave  a  concave          FIG.  410.  —  Fossil  ripple  marks  cut   by 

impression,  around  the  margin      mud-cracks  which  have  been  filled   in   by 

of  which  the  pressed-out  mud      other  material.     (Reverse.)     (U.  S.  G.  S.) 


FIG.  409.  —  Fragment  of  a  relief 
mold  of  rill-marks  on  the  under  side 
of  the  stratum  which  covered  the 
layer  in  which  the  rill-channels  were 
originally  made.  Triassic  sandstone, 
Portland,  Conn.  (After  Newberry.) 


River  Deposits 


491 


forms  a  low  rim.  If  the  raindrop  strikes  obliquely,  as  when  driven  by  wind, 
the  impression  will  be  asymmetrical,  being  deeper  on  the  side  from  which 
the  rain  struck  and  the  wind  blew,  while 
an  asymmetrical  bordering  rim  is  also 
produced.  Raindrops  also  may  be  pre- 
served in  relief  on  the  under  side  of  a 
stratum  subsequently  deposited  upon  the 
pitted  surface. 

Footprints.  —  The  footprints  of  land 
animals  are  most  commonly  preserved  in 
the  flood-plain  and  playa  deposits  when 
these  are  subject  to  hardening  by  ex- 
posure. Footprints  of  the  camels  of  a 
caravan  in  the  Sahara  were  still  distinctly 
recognizable  fifteen  years  later  in  the 
hardened  mud.  Such  footprints  may  also 
be  preserved  in  relief  on  the  under  side 
of  a  covering  stratum.  (See  Fig.  398, 
p.  482.) 

Clay  Galls.  —  When  only  a  thin  layer 
of  mud  is  deposited  on  the  flood-plain  or 
playa,  this  may  curl  up  into  fine  spirals 
resembling  wood  shavings,  and  these  may 
be  transported  by  the  wind  and  come  to 
rest  in  sands  either  of  eolian  or  river- 
laid  origin.  On  being  wetted,  these  clay  shavings  suffer  compression  and 
eventually  form  only  a  thin  oval  plate  or  film  of  clay  on  the  sand  mass.  Such 
"  clay  galls  "  are  a  characteristic  feature  of  many  hardened  sand  deposits  of 
subaerial  origin. 

Red  Color  of  Ancient  River-laid  Deposits.  —  Many  ancient  deposits  which 
by  their  general  character  suggest  their  origin  as  river  deposits  now  have  a  red 

color,  such  as  is  seldom  if  ever  seen  in  modern 
river  sediments,  where  the  prevailing  tones  are 
dark  or  light  grays  or  blues  or  else  yellows. 
The  latter  is  most  commonly  met  with  in  al- 
luvial fans  and  plains  formed  in  regions  of  little 
vegetation,  because  of  protracted  dryness  of 
climate  for  at  least  part  of  the  year.  This 
yellow  color,  as  already  described  in  an  earlier 
section  (page  459),  is  due  to  the  oxidation  and 
hydration  of  the  disseminated  iron  in  the  sedi- 
ment, and  is  especially  noticeable  where  such 
sediments  are  fine  grained  with  much  clay  or 
rock  flour.  Where  much  vegetation  is  present, 
such  oxidation  is  not  readily  effected,  nor  can 
subsequent  oxidations  of  the  sediments  take 

place  easily  when  the  deposit  is  saturated  with  ground  water,  which  prevents 
the  free  access  of  the  oxygen  except  in  so  far  as  the  water  carries  it.     In  dry 


FIG.  411.  —  Photograph  of  a 
specimen  of  clay  rock  from  the 
Silurian  formation  of  Pennsyl- 
vania (Longwood  shale),  showing 
mud-cracks  formed  upon  an  an- 
cient river  flood-plain  or  playa. 
The  fissures  in  this  case  are  very 
narrow,  although  the  structure 
affects  the  rock  mass  for  a  depth 
of  many  inches.  About  one 
eighth  natural  size.  (Columbia 
University  collection;  photo  by 
B.  Hubbard.) 


FIG.  412.  —  Raindrop  im- 
pressions hi  a  layer  of 
hardened  clay  from  a 
modern  roadside  surface. 
About  one  half  natural 
size.  (B.  Hubbard,  photo.) 


492  Deposition  of  Clastic  Rock  Material 

climates,  however,  where  the  level  of  the  ground-water  sinks  low,  the  upper 
layers  of  the  deposit  are  sufficiently  dry  to  permit  the  entrance  of  the  air,  and 
here  oxidation  is  accomplished.  If  this  can  take  place  for  each  successive 
layer  added  to  the  surface  of  the  alluvial  fan  or  plain,  the  entire  deposit  will 
come  to  have  its  iron  content  oxidized  throughout.  In  the  course  of  time, 
this  hydrous  iron  oxide  will  lose  its  water,  and  the  color  will  change  from  yellow 
to  red.  In  this  manner  may  be  explained  many  of  the  extensive  red-bed  for- 
mations found  in  several  geological  series,  though  at  the  time  of  deposition 
such  beds  were  yellow  rather  than  red. 

GLACIAL  TRANSPORTATION  AND  DEPOSITION 

Transportation 

Modern  glaciers  transport  clastic  material  partly  upon  their 
.  surfaces  (superglacial),  and  partly  within  their  mass  (englacial), 
and  partly  frozen  into  their  bottoms  (subglacial).  All  the  material 
transported  by  ice,  whether  modern  or  ancient,  is  called  glacial 
drift,  while  the  large  transported  boulders  are  termed  erratics. 
The  subglacial  material  is  derived  in  part  from  the  erosion  of 
the  rock  floor  by  the  glaciers  and  in  part  from  material  which 
is  loosened  and  pried  off  on  the  margins  of  the  cirque,  and  especially 
in  the  bergschrund  (p.  366).  Englacial  drift  originates  partly  from 
the  material  carried  up  from  the  bottom  wherever  an  oblique  shear- 
ing plane  permits  faulting  in  the  ice,  the  mass  from  behind  over- 
riding that  in  front  of  it  and  carrying  its  debris  upward.  Part  of 
the  englacial  material  is,  however,  of  superglacial  origin  and  repre- 
sents the  fragments  which  have  fallen  into  the  crevasses  or  other- 
wise become  inclosed  in  the  ice  mass.  Superglacial  material  is 
derived  in  various  ways.  In  the  valley  glaciers,  the  rock  frag- 
ments falling  upon  the  ice  from  the  valley  sides,  or  sliding  down 
in  avalanches,  accumulate  upon  the  side  of  the  ice,  and  as  the 
glacier  moves  along,  such  accumulations  are  strung  out  in  lines  of 
drift  along  the  margin.  This  constitutes  the  lateral  moraine.  (See 
ante,  p.  368.)  Where  two  glacial  streams  unite,  their  adjoining 
lateral  moraines  become  confluent  and  are  continued  as  a  medial 
moraine  (p.  370). 

These  moraines  are,  however,  also  fed  from  below ;  for  it  is  known  that  in 
a  moving  glacier  there  are  diverging  currents,  by  virtue  of  which  the  lower  ice 
layers  move  outward  and  upward  along  the  sides  of  the  glacier  as  well  as  toward 
its  front  (Fig.  413  a).  Such  currents  carry  material  from  the  bottom  upward, 
and  as  this  rises  and  the  inclosing  ice  melts,  the  material  becomes  incorporated 
in  the  lateral  moraines.  Furthermore,  when  two  ice  streams  meet  they  never 


Glacial  Transportation  and  Deposition         493 

commingle  as  two  water  streams  will,  but  flow  along  side  by  side  with  more  or 
less  independence.  The  upward  currents  which  formed  part  of  the  lateral  moraine 
above  the  point  of  junction  will  continue  and  the  material  brought  up  along 
the  sides  of  the  glaciers  in  contact  with  one  another  will  thereafter  contribute 
to  the  medial  moraine  (Fig.  413  b). 


FIG.  413  a.  —  Diagram  showing  ice  currents  carrying  sub-glacial  material 
upwards  to  form  lateral  moraines.  (After  Chamberlin  and  Salisbury,  Geology; 
by  permission  of  Henry  Holt  &  Co.) 

This  change  from  subglacial  to  englacial  and  finally  to  a  superglacial  position 
of  the  material  makes  possible  the  renewal  of  moraines  on  the  surface  of  the  ice, 
when  great  floods  of  water  from  prolonged  melting  have  washed  the  surface 
clear  of  debris.  Such  renewal  of  moraines  after  the  removal  of  the  older  ones 
by  a  flood  has  actually  been  observed  in  glaciers  of  the  Alps. 

Large  ice  masses  like  the  ice-caps  of  Iceland  and  Scandinavia  and  the  con- 
tinental ice-sheet  of  Greenland  have  little  or  no  superglacial  material,  since  they 
are  not  confined  in  valleys.  It  is  true  that  near  the  margin  of  the  Greenland 
ice-cap,  rocky  peaks  or  nunataks  project  through  the  ice,  and  these  furnish  a 


FIG.  413  b.  —  Diagram  to  illustrate  ice  currents  carrying  sub-glacial  material 
upwards  to  form  a  medial  moraine.  (From  Chamberlin  and  Salisbury, 
Geology;  by  permission  of  Henry  Holt  &  Co.) 

certain  amount  of  debris  which  accumulates  on  its  surface.  Where  upward 
currents  are  developed  in  the  ice  which  is  forced  to  pass  between  two  adjoining 
nunataks,  a  semicircular  morainic  band  may  be  produced,  extending  from  one 
nunatak  to  the  other.  (See  page  386.)  But  on  the  whole,  ice-sheets  transport 
their  debris  primarily  as  subglacial  material,  and  as  englacial,  where  local 
upward  currents  or  shearing  planes  cause  the  subglacial  material  to  rise.  Such 
material  is  commonly  characterized  by  the  polishing  and  scratching  of  all  the 
coarser  fragments,  a  feature  not  characteristic  of  superglacial  accumulations 
of  rock  falls.  (See  Fig.  360  b,  p.  435.) 

Deposits  Formed  by  Modern  Glaciers 

Terminal  Moraine.  —  Modern  valley  or  mountain  glaciers  de- 
posit their  load  of  debris  chiefly  at  their  lower  ends,  where  melting 
takes  place  (Figs.  414,  415).  This  constitutes  the  terminal  moraine, 
which  is  composed  of  superglacial,  englacial,  and  subglacial  material 
more  or  less  intimately  commingled  and  showing  no  regularity 


494 


Deposition  of  Clastic  Rock  Material 


of  structure.     Many  ice  blocks  are  buried  in  such  a   terminal 
moraine,  which,  on  subsequent  melting,  will  cause  a  caving-in  of 


FIG.  414.  —  Terminal  moraine  topography  at  foot  of  Kotsina  Glacier,  Alaska. 
(Photo  by  Schraeder,  U.  S.  G.  S.     Courtesy  D.  W.  Johnson.) 


FIG.  415.  — Terminal  moraine  of  a  former  glacier,  Oregon. 

this  covering  of  debris,  and  the  production  of  a  cup  or  kettle-shaped 
hollow,  a  feature  to  which  the  name  kettle-hole  is  applied  (Fig.  416). 


Glacial  Transportation  and  Deposition         495 

These  may  be  dry  or  contain  water  forming  a  glacial  pond  (Fig. 
417).  Such  terminal  moraines  are  best  developed  where  the  ice 
front  rests  for  a  considerable  period  of  time  at  the  same  point, 
which  implies  that  the  rate  of  melting  at  the  front  and  the  rate  of 


A  B 

FIG.  416.  —  Diagrams  to  illustrate  the  mode  of  formation  of  kettle-holes  in 
glacial  deposits.  A ,  a  portion  of  a  glacial  sand-plain  burying  a  boulder  of  ice ; 
B,  the  same  after  the  ice-boulder  has  melted,  and  the  sand  caved  in  to  form 
the  nearly  circular  kettle-hole. 

ice  advance  are  equal  and  balance  each  other.  When  there  is  no 
such  equality,  the  material  left  on  melting  will  be  scattered  over 
a  belt  of  greater  or  less  width  and  form  no  pronounced  terminal 
moraine.  If  the  stream  issuing  from  the  front  of  the  glacier  is 
very  strong,  a  large  part  of  the  glacially  transported  material  may 


FIG.  417.  —  A  kettle  pond  in  a  glacial  landscape. 

be  carried  away  by  it,  and  the  terminal  moraine  will  suffer  accord- 
ingly. When  well  formed,  such  a  moraine  may  reach  a  height  of 
more  than  two  hundred  feet,  but  heights  of  100  to  200  feet  are 
more  frequent. 

Submarginal  Moraines.  —  In  some  valley  glaciers  the  material 
carried  toward  the  sides  of  the  glacier  by  the  diverging  ice  currents 


496  Deposition  of  Clastic  Rock  Material 

may  accumulate  there  as  a  submarginal  moraine,  and  become  ex- 
posed on  the  melting  away  of  the  glacier.  Such  moraines  accumu- 
late when  the  supply  of  bottom  debris  is  large  as  compared  with  the 
carrying  power  of  the  glacier.  Submarginal  moraines  are  dis- 
tinguishable from  lateral  moraines,  which  may  be  let  down  upon 
the  valley  sides  on  the  melting  of  the  ice,  by  the  fact  that  the  sub- 
marginal  accumulations  are  very  compact  from  the  pressure  of  the 
ice  and  that  the  stones  and  boulders  in  them  are  scratched  and 
polished,  features  not  found  in  the  lateral  superglacial  moraines. 

Ground  Moraines.  —  When  the  supply  of  drift  in  the  bottom  of 
the  ice  is  larger  than  can  be  moved  along  by  the  glacier,  part  of  it 
will  remain  upon  the  bottom  as  ground  moraine  and  will  be  over- 
ridden by  the  ice  as  it  moves  onward.  Such  material  will  be 
thoroughly  compacted  and  will  consist  of  an  intimate  and  irregular 
mixture  of  rock-flour,  sand,  and  striated  pebbles  and  boulders,  a 
mixture  to  which  the  name  till  is  applied.  In  modern  glaciers, 
this  accumulation  takes  place  chiefly  near  the  front,  where  the 
ice  is  thinner,  or  behind  an  obstruction  over  which  the  ice  moves 
but  behind  which  it  leaves  part  of  its  debris. 

Deposits  Formed  by  Former  Ice  Sheets 

From  the  wide  extent  of  surface  deposits  which  have  all  the 
characteristics  of  glacially  transported  material,  as  well  as  from 
scratched  and  polished  rock  surfaces  (Fig.  359,  p.  434)  and 
the  effects  of  erosion  upon  the  landscape  such  as  are  known  to  be 
produced  by  moving  ice,  it  has  become  apparent  that  a  considerable 
portion  of  northern  North  America  and  western  Europe  was 
covered  by  continental  ice-sheets  during  a  period  preceding  that 
in  which  we  live,  and  from  the  association  of  these  deposits 
with  the  remains  of  early  man  in  Europe,  we  recognize  that  this  ice- 
covering  still  existed  after  man  had  made  his  appearance  upon  the 
earth,  though  he  had  not  yet  spread  widely  over  its  surface.  The 
deposits  formed  by  these  ice-sheets'  consist  mainly  of  ground 
moraines  and  terminal  moraines  and  the  various  modifications  of 
these,  together  with  the  deposits  formed  by  the  streams  from  the 
melting  ice,  both  upon  the  land  and  in  standing  bodies  of  water, 
either  permanent  or  temporary. 

Ground  Moraines ;  Drumlins.  —  Vast  areas  of  northern  North 
America  and  western  Europe  are  covered  by  the  ground  moraine 


Glacial  Transportation  and  Deposition         497 


of  the  ice  of  the  last  glacial  period.  This  is  a  till  or  boulder  clay 
which  consists  of  a  heterogeneous  mixture  of  fine  and  coarse 
material,  with  polished  and  striated  boulders  and  without  strati- 


FIG.  418.  — Typical  section  of  glacial  till  exposed  by  cutting  a  road  through 
the  southern  end  of  a  drumlin,  Whitewater  Quadrangle,  Wis.  Note  the  unstrati- 
fied  and  unassorted  character  of  the  material.  The  appearance  of  the  pebbles 
in  this  till  is  shown  in  Fig.  419.  (Photo  by  Alden ;  from  U.  S.  G.  S.) 

fication  or  assortment.  It  is  often  so  thoroughly  compacted  that 
it  requires  almost  as  much  labor  for  its  removal  as  does  a  mass  of 
solid  rock,  on  which  account  it  is  commonly  known  as  hardpan 
(Figs.  418,  419).  This  deposit  of  till  often  fills  in  irregularities  in 


FIG.  419.  —  Glaciated  pebbles  and  boulders  from  drift  of  North  America,  show- 
ing characteristic  form  and  scratches.     (Photo  by  Alden ;    from  U.  S.  G.  S.) 


498 


Deposition  of  Clastic  Rock  Material 


the  rock  topography  beneath  it,  and  so  produces  a  more  or  less 
monotonous  surface.  At  other  times  it  rises  in  regular  rounded 
or  elliptical  hills  of  elongate  form,  with  their  major  axis  in  the 


FIG.  420  a.  —  Drumlin  near  Groton,  Mass.     (After  Frye.) 

direction  of  the  ice  movement.  Such  hills  are  known  as  drumlins, 
and  they  form  a  characteristic  feature  of  the  topography  of  eastern 
Massachusetts,  parts  of  New  York,  Wisconsin,  and  other  areas 
in  the  glaciated  regions  (Figs.  420  a,  b). 

The  thickness  of  the  till  varies  greatly,  rising  to  several  hundred 
feet  in  some  drumlins,  and  falling  to  a  few  inches  elsewhere.  The 
rock  surface  below  the  till  is  generally  smooth,  showing  much 
erosion  and  polishing  by  the  ice,  and  generally  exhibiting  a  series  of 

parallel  scratches  which 

indicate  the  direction  of 
the  ice  movement  (Fig. 
359).  In  some  cases,- 
however,  deep  grooves 
and  flutings  were  formed, 
where  large  boulders 
were  moved  along  upon 
soft  rocks  into  which 
they  gouged  deeply. 
Such  glacial  groovings 
have  been  found  wonder- 
fully developed  on  the 
limestone  surfaces  of 


FIG.  420  &.  —  Section  of  part  of  a  drumlin, 
near  Boston,  Mass.,  showing  the  hetero- 
geneous mixture  of  fine  and  coarse  material 
including  boulders.  (Photo  by  the  author.) 
(See  also  Figs.  447,  p.  531,  and  726.) 


Kelley's  Island  in  Lake 
Erie,  and  in  many  other 
regions  (Fig.  421). 

When  the  ice  first  ac- 
cumulated, many  parts  of  the  country  were  covered  by  a  mantle 
of  residual  soil  due  to  the  weathering  of  the  rock.  This  material, 
which  included  much  clay,  was  the  first  to  be  carried  away,  and 


Glacial  Transportation  and  Deposition         499 

where  it  was  deposited  the  resulting  till  consists  partly  of  clay. 
Subsequent  erosion  by  the  moving  ice,  however,  produced  only 
the  finest  rock-flour,  or  mechanically  ground-up  undecomposed 


FIG.  421.  —  Glacial  fluting  on  limestone,  Kelley's  Island,  Lake  Erie. 

rock,  besides  the  larger  fragments,  and  in  consequence  such  ma- 
terial predominates  over  the  clay,  which  may  be  largely  or  wholly 
wanting. 

Terminal  Moraines.  —  Whenever  the  ice  halted  for  a  period  of 
time  along  a  given  line,  a  terminal  moraine  was  built  up  from  the 
subglacial  and  englacial  material,  the  size  of  the  moraine  de- 


FIG.  422.  —  Bird's  eye  view  of  about  2  square  miles  of  terminal  moraine. 
Lakes  shown  by  horizontal  shading ;  swamps  are  dotted.  (Drawn  by  Fred  K. 
Morris.) 

pending  upon  the  length  of  time  during  which  the  ice  front  was 
stationary.  Such  moraines  exhibit  the  characteristic  kettle  topog- 
raphy (Fig.  422)  due  to  the  inclosure  of  ice  blocks  which  subse- 
quently melted,  while  in  many  cases  the  removal  of  the  finer 
surface  material  by  the  waters  from  the  melting  ice  has  left 
them  covered  over  with  great  boulders.  Whenever  the  sea  or 


500  Deposition  of  Clastic  Rock  Material 


other  agent  has  cut  into  such  a  moraine,  its  structure  is  seen  to  con- 
sist of  irregular  accumulations  of  sand  and  finer  material  with 
many  boulders,  often  of  large  size  and  of  heterogeneous  char- 


FIG.    423.  — Crest  of  northern  frontal  moraine,   looking  northwest.     Dog- 
town  Common.     (After  Shaler,  U.  S.  G.  S.) 

acter.     The  so-called  "  backbones  "   of  Long  Island  and   Cape 

Cod  are  formed   by   the   terminal   moraines,  and   the   Narrows 

in  New  York  harbor  represent  a  cleft  in  the  great  terminal  mo- 
raine, the  cut  ends 
of  which  can  be 
seen  on  both  sides, 
modified  of  course 
by  man's  activity. 
This  moraine  and 
many  smaller  ones 
to  the  north  of  it 
can  be  traced  with 
more  or  less  con- 
tinuity for  long  dis- 
tances across  the 
continent,  being 
FIG.  424. — Boulder  moraine,  Dogtown  Common,  recognizable  by 

Cape  Ann.    These  bpulders  are  chiefly  granite,  and     tneir      topography 

they  are  piled  up  in  great  heaps  with  little  or  no  fine 

material  between.     (Photo  by  the  author.)  and   internal    con- 

stitution. 
Boulder  Trains  and  Moraines.  —  In  some  cases  the  moraines 

appear  to  consist  almost  wholly  of  large  and  small  boulders,  forming 


Glacial  Transportation  and  Deposition         501 


a  topography  of  striking  wildness  and  complexity.     Such  a  moraine 
covers  a  part  of  the  surface  of  Cape  Ann  in  eastern  Massachusetts, 


FIG.  425.  —  Section  of  frontal  moraine  on  side  of  Warner  Street,  Gloucester, 
Mass.     (After  Shaler,  U.  S.  G.  S.) 

its  most  picturesque  portion  near  Gloucester  being  known  as  "  Dog- 
town  Common  "  (Figs.  423  to  425).  This  moraine  is  believed  to  be 
the  result  of  a  disturbance,  by  readvance  of  the  ice,  which  by  shaking 
up  an  older  moraine 
caused  the  finer  material 
to  settle  down  between 
the  coarse  blocks,  which 
then  alone  appeared  upon 
the  surface.  Part  of  the 
fine  material  may  also 
have  been  washed  away 
by  glacial  waters.  Some- 
times trains  or  long  lines 
of  boulders  alone  mark 
the  moraine,  or  they  may 
form  a  line  in  front  of  some 
projecting  rock  mass 
from  which  they  were 
derived.  Some  of  these 
glacial  boulders  are  of  as- 
tonishing size  (Fig.  426). 

Interlobate  Moraines.  —  As  the  ice  which  covered  much  of  the 
northern  lands  consisted  of  several  distinct  lobes  which  abutted 


FIG.  426.  —  House  Rock,  a  large  glacial 
boulder  or  erratic  near  Hingham,  Mass.  Its 
size  is  indicated  by  a  comparison  with  men 
standing  near  it.  (Photo  by -the  author.) 


502 


Deposition  of  Clastic  Rock  Material 


one  against  the  other,  moraines  were  formed  between  the  abutting 
lobes  by  the  drift  brought  to  the  margin  from  each  lobe.  Such 
moraines  therefore  often  extend  more  or  less  at  right  angles 
to  the  main  terminal  moraine,  and  their  opposite  sides  may  be  com- 
posed of  very  different  materials.  The  great  north-south  moraine 
which  forms  the  hills  of  Plymouth  and  extends  southward  to  Woods 
Hole  in  Massachusetts  is  such  an  interlobate  moraine. 

Modified  Deposits  of  the  Great  Ice  Age 

The  waters  resulting  from  the  melting  of  the  ice-sheet,  on  issuing 
at  its  front,  formed  many  special  types  of  deposits,  recognizable 
by  their  form  and  structure.  These,  as  a  rule,  are  more  or  less 
stratified,  often  pronouncedly  so,  and  on  this  account,  the  modified 
drift  is  also  spoken  of  as  stratified  drift  in  contradistinction  to  the 
unstratified  drift  or  till.  The  most  important  of  such  deposits  are 
the  kames,  apron-plains,  sand-plains  or  glacial  deltas,  and  the 
eskers. 

Kames.  —  These  are  more  or  less  conical  mounds  of  sand  depos- 
ited at  the  ice-front  by  temporary  glacial  streams.  They  are  not 
uncommon  in  the  moraine  belt,  of  which  they  may  constitute  a  part. 

The  Apron-plain.  —  This  is  formed  of  the  outwash  along  the 
ice  front,  and  generally  lies  in  front  of  the  terminal  moraine. 


FIG.  427.  —  Diagrams  illustrating  the  mode  of  formation  of  the  frontal 
apron-plain  and  its  relation  to  the  terminal  moraine.  (Adapted  from  J.  B. 
Woodworth.)  The  ice-front  partly  covers  the  terminal  moraine  which  is  sub- 
marginal.  In  front  of  the  ice  the  stratified  sands  washed  outward  by  the 
glacial  waters  build  a  gently  sloping  apron-plain.  When  the  ice  melts  away,  a 
depression  or  "fosse"  appears  between  the  moraine  and  the  apron-plain,  the 
upper  end  of  which,  formerly  banked  against  the  ice,  now  slumps,  and  forms  a 
steep  back-  or  ice-contact  slope. 


Glacial  Transportation  and  Deposition          503 

Near  the  moraine  border  there  is  generally  a  depression  or  fosse 
which  may  be  occupied  by  a  chain  of  lakes,  and  from  it  the  apron- 
or  outwash  plain  rises,  often  with  steep  slopes,  where  the  sand 
rested  against  the  face  of  an  ice  mass  which  occupied  the  place 
now  marked  by  the  fosse  (Fig.  427).  From  the  summit  of  this 
slope  the  plain  descends  with  a  very  gentle  inclination  and  gener- 
ally a  nearly  smooth  surface  which  may  dip  into  the  sea,  as  in  the 
case  of  the  outwash  plain  which  forms  the  southern  half  or  more  of 


FIG.  428  a.  —  Section  of  a  glacial  delta  or  sand-plain  showing  fine-grained 
fore-set  beds  dipping  to  the  left,  covered  by  coarser  top-sets.  College  Hall 
Hill,  Wellesley,  Mass.  (Photo  by  W.  P.  Haynes ;  courtesy  of  Prof.  E.  Fisher.) 

Long  Island  and  that  which  forms  the  main  southern  part  of  Cape 
Cod.  Commonly  a  series  of  transverse  channels,  the  paths  of  the 
main  streams  from  the  ice,  transects  the  outwash  plain,  and  these 
channels  may  be  dry  or  rilled  with  water  (as  on  the  Nan  tucket 
plain).  Kettle-holes  may  also  occur  on  this  plain,  and  in  regions 
of  high  ground-water  level,  as  near  the  coast,  these  may  be  con- 
verted into  ponds.  Boulders  are  rare  on  such  a  plain,  the  material 
being  mostly  sand,  generally  well  stratified. 

The  Sand-plain  or  Glacial  Delta.  —  Where  a  body  of  water  was 
held  up  by  the  ice  front  in  a  valley  which  sloped  toward  it,  a  sta- 
tionary lake  was  produced,  the  level  of  which  depended  on  the 


504          Deposition  of  Clastic  Rock  Material 


FIG.  428  b.  —  Section  of  a  glacial  delta  or  sand-plain,  showing  the  level  top, 
the  steeply  dipping  fore-set  beds,  and  the  horizontal  top-set  beds.  Brockton, 
Mass.  (Photo  by  the  author.) 


FIG.  429.  —  Ice  contact  slope  of  a  glacial  sand-plain  or  delta.    Note  the  steep 
character  of  the  slope.     (Photo  by  the  author.) 


Glacial  Transportation  and  Deposition         505 

elevation  of  the  lowest  exposed  point  in  the  rim  of  the  valley,  as  does 
the  level  of  the  Marjelen  Lake  by  the  side  of  the  Aletsch  glacier 
in  Switzerland,  on  the  level  of  the  divide  which  separates  its 
valley  from  another  on  the  east.  (See  p.  362.)  Into  such  a 
body  of  water  deltas  may  be  built  by  the  sands  washed  down  from 
the  ice,  and  such  deltas  will  have  all  the  characters  of  normal  stream 


7/y 

FIG:  430  a.  —  Diagram  illustrating  a  successive  series  of  sand-plains  or 
glacial  deltas  built  into  a  temporary  lake  held  up  by  an  ice-dam  in  a  northward 
sloping  valley.  The  southern  plains  were  built  when  the  lake  stood  at  its 
highest  level,  this  being  indicated  by  the  edge  between  the  surface  and  frontal 
slope.  As  the  ice  front  melted  backward  lower  outlets  were  uncovered  and  the 
level  of  the  lake  sank.  Then  the  lower  deltas  on  the  north  were  built. 

deltas  built  into  permanent  lakes,  including  lobate  front  and  a 
series  of  sloping  fore-set  and  nearly  horizontal  top-set  beds  (Figs. 
4280,  b).  As  ice  boulders  may  be  buried  in  such  a  delta,  kettle- 
holes  may  develop  subsequently  by  their  melting,  and  these  are  not 
uncommon  in  the  larger  deltas  of  this  type.  The  most  character- 
istic feature  of  these  deltas,  however,  is  due  to  the  fact  that  on  the 


ICL 


A  B 

FIG.  430  b.  —  Diagrams  showing  the  origin  of  eskers.  A,  A  tunnel  under  the 
ice,  nearly  filled  with  sand  and  gravel  by  the  subglacial  stream  which  built  up 
its  bed  to  the  level  of  the  outlet,  or  the  level  of  the  lake  held  in  front  of  the  ice. 
B,  The  same  deposit  after  the  melting  of  the  supporting  ice-walls.  Slumping 
has  produced  the  characteristic  steep  side-slopes. 

melting  of  the  retaining  ice  wall  of  the  lake,  this  will  be  drained,  and 
the  delta  will  then  form  a  plateau-like  elevation  in  the  midst  of  the 
valley.  The  side  of  the  delta  which  rested  against  the  ice  will,  by 
slumping,  become  a  steep  slope  —  often  as  steep  as  30  degrees  — 
and  one  easily  distinguished  by  its  angle  and  general  outline  from 
the  frontal  slopes  of  such  deltas  (Fig.  429). 

When  successively  lower  passes  are  opened  in  the  valley  side  by 
the  melting  back  of  the  retaining  ice  front,  the  level  of  the  lake 


506  Deposition  of  Clastic  Rock  Material 


FIG.  431  a. — An  esker,  near  East  Weymouth,  Ma?s.     Seen  from  the  side. 
(Photo  by  the  author.) 


FiG."43i  b.  — Top  of  the  esker  shown  in  Fig.  431  a,  East  Weymouth.     (Photo 

by  the  author.) 


Glacial  Transportation  and  Deposition         507 


will  sink  in  accordance  therewith,  and  a  succession  of  plains  or  deltas 
at  lower  levels  may  be  built  up  (Fig.  430  a) .  There  are  many  such 
plains  in  the  valleys  of  Massachusetts  which  show  the  progressive 
lowering  of  the  old  ice-dammed  lakes,  the  shores  of  a  number  of 
which  have  been  traced  and  the  successive  outlets  located,  in  con- 
formity with  the  levels  indicated  by  the  heights  of  the  abandoned 
deltas. 

Eskers.  —  Streams  flowing  in  ice  gorges  near  the  front  of  the 
great  ice  sheet  or  in  tunnels  beneath  it,  built  up  their  beds  by  de- 
positing sand  and  gravel  r 

on  their  floors,  until  a 
height  was  reached  which 
corresponded  to  that  of 
the  standing  water  level 
in  front  of  the  ice  or  the 
elevation  of  the  rocky  col 
across  which  the  stream 
had  to  discharge.  Dur- 
ing their  formation  these 
deposits  rested  against 
the  side  of  the  ice  gorge 
or  tunnel,  but  on  the 
melting  of  the  ice  this 
support  was  removed, 


FIG.  431  c. — A  small  esker,  winter  scene 
among  the  Waverly  Oaks,  Waverly,  Mass. 
(Photo  by  the  author.) 


was 

and  the  sides  of  the  sand 
and  gravel  mass  began  to 
slump  and  come  to  rest 
at  the  steepest  angle  con- 
sistent with  the  nature  of  the  material  (Fig.  430  b).  In  this  manner, 
more  or  less  winding  ridges  of  triangular  section  were  formed,  often 
extending  across  country  for  many  miles  like  a  huge  railroad  em- 
bankment, but  of  somewhat  variable  height  and  generally  winding 
course,  this  course  being  that  of  the  stream  which  made  the  de- 
posit (figs.  431  a-c).  Such  ridges  are  known  as  esker s  or  osars,  and 
they  abound  in  New  England,  in  Scandinavia,  and  in  other  regions 
formerly  covered  by  the  ice  and  of  a  topography  favorable  to  their 
formation.  They  often  constitute  striking  features  of  the  land- 
scape. Not  all  eskers  were  formed  in  this  manner,  some  probably 
representing  accumulations  of  sand  and  gravel  in  gorges  cut  into 
the  ice,  or  in  crevasses  open  to  the  sky. 


508  Deposition  of  Clastic  Rock  Material 

Ancient  Glacial  Deposits 

In  very  ancient  geological  formations  in  India,  South  Africa,  Aus- 
tralia, and  South  America  great  beds  of  rock  are  known,  which  have 
all  the  characteristics  of  a  glacial  till,  including  the  polished  and 
striated  boulders  and  bed-rock  (Fig.  3600,  p.  435).  These  rocks, 
known  as  tillites  or  consolidated  tills,  are  interpreted  as  the  deposits 
of  ice-sheets  which  covered  these  regions  in  former  geological  periods. 
Similar,  even  more  ancient,  glacial  deposits  have  been  reported  from 
Canada  and  elsewhere,  and  others  from  northern  Norway.  They 
will  be  considered  more  fully  in  the  section  on  historical  geology 
under  their  respective  periods. 


CHAPTER  XVII 

TRANSPORTATION,      SORTING,     AND     DEPOSITION     OF 
CLASTIC   MATERIAL  IN   THE   SEA 

THE  GEOGRAPHICAL  SUBDIVISIONS  OF  THE  SEA 

WHEN  we  speak  of  the  sea  we  refer  to  the  entire  extent  of  the 
connected  salt  waters  of  the  earth,  those  that  lie  between  the  con- 
tinents and  those  that  lie  within  them.  The  completely  enclosed 
salt  water  lakes  such  as  Great  Salt  Lake,  the  Caspian  Sea  (lake), 
and  the  Dead  Sea  (lake)  are  not  included  here,  though  it  is  a  gen- 
eral European  custom  to  speak  of  such  water  bodies  as  seas  (Ger- 
man, Seen).  The  term  seas  is  most  commonly  applied  to  the  more 
or  less,  but  never  completely,  enclosed  portions  of  the  sea,  such  as 
the  Mediterranean  Sea,  the  Black  Sea,  the  Yellow  Sea.  Com- 
pletely enclosed  bodies  of  water  are  properly  called  lakes,  whether 
they  are  fresh  or  salt,  though  it  will  probably  be  difficult  to  correct 
such  popular  misnomers  as  "  The  Dead  Sea,"  "  The  Caspian  Sea," 
etc.,  many  of  which  received  this  appellation  when  it  was  believed 
that  they  were  a  part  of  the'  great  salt  sea. 

Other  loosely  used  geographic  terms  are  gulf  and  bay,  which  are 
indiscriminately  applied,  sometimes  to  open  and  sometimes  to 
partly  enclosed  marginal  bodies  of  water;  sometimes  to  deep  bodies 
of  this  type,  sometimes  to  shallow.  It  is  therefore  important  that 
certain  more  precise  terms  should  be  used,  terms  which  designate 
the  various  types  of  water  bodies,  divided  not  only  according  to 
form  and  location,  but  also  according  to  depth  and  relation  to  other 
water  bodies. 

The  Oceans  or  Intercontinental  Seas 

The  Three  Continental  Masses.  —  From  our  present  point  of 
view,  we  may  consider  that  there  are  only  three  continental  masses, 
to  one  or  the  other  of  which  all  the  so-called  continents  of  the 
geographer  belong.  These  continental  masses  are  the  three  great 


510       Deposition  of  Clastic  Material  in  the  Sea 

land  blocks  of  the  earth's  crust,  and  although  several  of  them  seem 
to  be  nearly  or  quite  separated  into  distinct  continents,  they  are  in 
reality  units,  and  in  former  periods  of  time  they  were  in  some  cases 
more  intimately  united  than  they  are  to-day.  Named  in  the  order 
of  their  magnitude,  these  three  continental  masses  are:  i.  The 
Old  World  Mass,  comprising  the  geographical  continents  of  Europe, 
Asia,  Africa,  and  Australia;  2..  The  New  World  Mass,  comprising 
the  geographical  continents  of  North  and  South  America;  and 
3.  The  Antarctic  Mass,  forming  a  single  Antarctic  continent. 

The  Four  Oceans.  —  Between  these  continental  masses  lie  the 
oceans,  which  are  thus  truly  intercontinental  in  position,  and  repre- 
sent the  oceanic  blocks  of  the  earth's  crust  which  probably,  because 
of  their  greater  density,  have  a  sunken  position  with  reference  to 
the  continental  blocks.  In  the  opinion  of  many  students  of  the 
earth,  these  differences  of  relative  position  have  been  constant 
since  the  earliest  recorded  geological  time,  though  there  are  others 
who  hold  that  at  one  period  or  another  some  of  the  oceanic  blocks 
were  also  elevated  to  such  an  extent  that  their  surfaces  appeared 
as  land  above  the  sea-level.  Named  in  the  order  of  their  magni- 
tude, the  four  oceans  of  the  earth  are:  —  i.  The  Pacific  Ocean, 
lying  between  the  Old  World  block  on  the  west  and  the  New  World 
block  on  the  east,  the  convergence  of  which  bounds  this  ocean  on 
the  north,  while  the  southern  boundary  is  formed  by  the  Antarctic 
continent;  2.  The  Atlantic  Ocean,  lying  between  the  same  conti- 
nental masses  except  that  the  Old  World  block  is  on  the  east  and 
the  New  World  block  on  the  west,  while  the  Antarctic  block  also 
bounds  it  on  the  south,  and  the  convergence  of  the  other  two  blocks 
on  the  north ;  3.  The  Indian  Ocean,  lying  between  the  Old  World 
block  on  the  north,  and  the  Antarctic  block  on  the  south,  while  on 
the  east  and  the  west  it  becomes  more  or  less  confluent  with  the 
Pacific  and  Atlantic  oceans,  respectively;  4.  The  Arctic  Ocean, 
the  smallest  of  all,  which  lies  between  the  northern  ends  of  the  Olo^ 
and  New  World  blocks,  and  is  more  or  less  completely  bounded 
by  them  (Fig.  432). 

The  Continental  Shelf.  —  At  present  the  oceans  are  overfull 
of  water,  and  so  they  spread  out  over  the  margins  of  the  continental 
blocks,  in  some  places  for  a  distance  of  only  a  few  miles,  in  others 
for  a  hundred  miles  or  more.  This  margin  has  in  general  the  form  of 
a  shelf  or  gently  seaward  sloping  platform,  partly  due  to  erosion  of 
the  land  margin  and  partly  to  deposition.  From  this  shelf  arise  the 


Geographical  Subdivisions  of  the  Sea  511 

continental  islands  which  are  either  residual  masses  left  by  the 
erosion  of  the  edge  of  the  land,  or  have  been  built  up  on  the  shelf, 
by  clastic  sediments  from  the  land,  by  organisms  (coral  reefs,  etc.),' 


FIG.  432.  —  Map  of  the  Arctic.  Ocean  and  a  part  of  the  North  Atlantic  Ocean 

showing  the  depths.     The  continental  shelf  (0-200  meters)  is  left  in  white  like 

continents.     Note  its  great  width  off  the  Eur-Asian  coast  (long.  40°  K. 

«i   i      ,        Als°  n°te  the  many  epicontinental  seas  or  shallow  indentations 

the  land.    The  Baltic  Sea  (between  long.  o°  and  40°  E.  is  an  epeiric  sea.    The 

bathyal  zone  is  shown  by  the  lighter  shading  (horizontal  lines,  200  to   1000 

s)  and  the  deeper  bathyal  zone  by  the  darker  shading  (cross-lines  1000- 

>  meters).     These  two  form  the  continental  slope.     The  abyssal  district 

is  represented  in  solid  black.     Note  the  double  character  of  the  Arctic  Ocean 

and  the  submerged  ridges  which  divide  it  from  the  North  Atlantic.     These 

are  the  Wyville-Thompson  ridge  between  Scotland  and  the  Faroe  Islands,  the 

Faroe-Iceland  ridge  between  these  islands,  and  the  Denmark  straits  ridge  be- 

t*een  Iceland  and  Greenland.      (After  Nansen;  from  Grabau's  Principles  of 


512       Deposition  of  Clastic  Material  in  the  Sea 

or  by  volcanic  eruptions.  The  depth  of  water  over  this  shelf 
ranges  from  zero  to  about  100  fathoms  (600  feet),  at  which  point, 
approximately,  the  main  margin  of  the  continental  mass  is  located. 
This  margin  is  also  called  the  edge  of  the  continental  shelf  (Fig. 


FIG.  433.  —  Map  of  the  peninsula  of  Florida,  showing  the  bottom  contours 
of  the  surrounding  sea.  The  line  of  keys  and  old  reefs  is  indicated  by  crosses. 
(After  Vaughan.)  The  loo-f  athom  contour  line  marks  the  edge  of  the  continental 
shelf.  Note  the  width  of  this  shelf  on  the  west,  and  its  narrowness  on  the  south- 
east where  it  is  swept  by  the  Gulf  Stream.  Note  that  on  the  west  the  descent 
from  100  to  1000  fathoms  takes  place  in  a  shorter  distance  than  the  descent  from 
20  to  100  fathoms.  The  Keys  are  continental  islands  of  organic  origin  built 
upon  the  shelf.  The  part  of  the  sea-bottom  bounded  by  the  looo-fa thorn  line 
represents  the  abyssal  district.  (Compare  Fig.  438,  p.  519.) 

433).  It  is  upon  this  shelf  that  the  epicontinental  seas  are  located, 
while  deeper  seas  transect  it.  (See  Fig.  434.)  The  continental 
shelf  is  also  the  chief  region  of  clastic  deposition  in  the  open  sea, 
though  of  no  greater  importance  than  some  of  the  seas  next  to  be 
described.  (See  Fig.  438,  page  519.) 


Geographical  Subdivisions  of  the  Sea          513 


The  Epicontinental  and  Intracontinental  Seas 

Upon  the  edge  of  the  land,  that  is,  upon  the  continental  shelf, 
there  are  a  number  of  distinct  water  bodies  more  or  less  outlined 
by  land  extensions, 
submerged  ridges,  or 
islands  rising  from 
these.  These  are 
called  epicontinental 
seas  (epi,  upon)  be- 
cause they  lie  upon 
the  continent.  Some 
of  these  extend  far 
into  the  land  masses 
(epeiric  seas),  but  are 
still  shallow  films  of 
sea-water  resting 
upon  the  land.  Ex- 
amples of  these  are 
seen  in  the  Baltic 
Sea,  and  in  Hudson 
Bay.  Again,  the 
edge  of  the  conti- 
nental shelf  may  be 
deeply  intrenched, 
excavated,  or  hol- 
lowed by  down- 


FIG.  434.  —  Map  of  the  Gulf  of  California,  a  typi- 
cal funnel-sea  of  the  narrow  type,  transecting  the 


sinkings,  and  such  ex-     continental  shelf,  which  is  represented  by  the  hori- 
cavations    mav    also    zontal  lining  (o~6o°  ft)'     Note  the  regular  and 


extend    far    into    the 

land.    These  are  the 

•    .  ..        .  j 

true    intracontinental 


progressive  deepening  of  this  funnel-sea  and  corn- 
pare  with  the  Red  Sea,  a  typical  mediterranean 
sea  (Fig-  43  7)  •  On  the  westeJ"n  border  of  Lower 
California  is  a  partly  enclosed  epicontinental  sea, 
the  Bay  of  SaPn  Sebastian  vizcamo.  The  con^ 
seas,  because  they  lie  tinental  slope  is  shown  by  diagonal  lines  (600  to 
within  the  continen-  6oo°  ^-  or  I0°  to  I00°  fathoms),  though  the  next 

ZOIie  might  als°   be    included    here  (°°OO  to    IO,OOO 

ft  }     Below  that  is  the  deep  sea  (io>ooo  feet  and 
lower).    (See  section,  Fig.  438.) 


tnl     msec         TV,o, 

iney  are 

illustrated      by     the 
Gulf    of    California 

(Fig.  434),  and  by  the  Red  Sea  (Fig.  437).    The  principal  types 
may  be  briefly  noted. 


514      Deposition  of  Clastic  Material  in  the  Sea 


Intracontinental  Seas 


ex- 


Funnel  Seas.  —  Where  the  continental  margin  is  deeply 
cavated  or  incised,  so  as  to  form  a  trough  which  extends  into  the 
land  with  regularly  narrowing  sides  and  a  regularly  rising  bottom, 
a  funnel  sea  is  produced,  so  called  because  its  form  may  be  compared 
with  that  of  one  half  of  a  longitudinally  divided  funnel.  The  Gulf 
of  California  is  a  typical  example,  for  not  only  do  its  sides  converge 
toward  the  head,  but  its  floor  rises  progressively  (Fig.  434).  If  the 
sea-level  should  sink  or  the  land  block  rise,  this  funnel  would  be 
gradually  emptied  by  the  withdrawal  of  the  water  and  no  residual 


o-iooo  ft 


*IOOO-5OOO  ft 


sooo-io.ooofc. 


Over  lo.oooft 


FIG.  435.  —  Broad  or  Biscayan  type  of  funnel-seas,  a,  Bay  of  Biscay  be- 
tween France  and  Spain,  a  closed-head  funnel-sea ;  b,  Gulf  of  Cadiz,  an  open- 
head  funnel-sea  of  this  type,  receiving  the  waters  from  the  Mediterranean 
through  the  Straits  of  Gibraltar. 

lake  would  remain.  The  California  funnel  sea  represents  the  nar- 
row type ;  a  broader  type  is  represented  by  the  Bay  of  Biscay  on 
the  west  coast  of  Europe,  which  differs  from  the  California  type 
only  in  the  more  rapid  convergence  of  its  sides  and  corresponding 
rise  of  its  bottom  (Fig.  435  a).  A  modification  of  this  type  is  seen 
in  the  Gulf  of  Cadiz,  which  receives  the  outlet  of  the  Mediterranean 
Sea  (Fig.  435  b).  This  may  be  referred  to  as  having  an  open  head, 
as  compared  with  the  closed-head  type  shown  by  the  Bay  of  Biscay. 
It  is  evident  that  not  only  the  character  of  the  water,  its  tempera- 
ture, currents,  etc.  differ  in  the  two  types,  but  also  the  quantity 
and  character  of  the  sediments,  the  closed-head  type  receiving 
the  drainage  of  the  land  -direct,  together  with  the  sediments 
brought  in,  while  the  open  type  receives  only  the  finer  sediments 


Geographical  Subdivisions  of  the  Sea  515 

which  escape  from  the  adjoining  water  body,  besides  the  sediments 
brought  in  by  lateral  streams. 

An  example  of  the  narrow  type  of  deep  funnel  sea  with  an  open 
head  is  seen  in  the  Gulf  of  Aden  (Fig.  436)  which  receives  the  out- 
let of  the  Red  Sea,  another  mediterranean.  Finally,  there  are 
shallow  or  epicontinental  funnel  seas,  the  most  conspicuous  ex- 
ample being  the  Bay  of  Fundy.  This  is  also  a  narrow  type,  and 
this  and  its  shallowness  are  responsible  for  the  great  range  of  the 
tides  for  which  this  water  body  is  famous.  (Compare  with  Estuary.) 


FIG.  436.  —  The  Gulf  of  Aden,  which  exchanges  its  waters  with  the  Red  Sea. 
A  narrow  type  of  funnel-sea  with  open  head.  The  continental  shelf  (littoral 
zone)  is  lined  horizontally.  The  next  two  zones  belong  to  the  continental 
slope  —  the  higher  (600-3000  ft.)  is  the  typical  bathyal  zone  (diagonal  lines) ; 
the  next  (3000-6000  ft.,  cross-lines)  the  deeper  bathyal  zone.  Beyond  this  (below 
6000  feet  or  1000  fathoms,  dark  shading)  is  the  abyssal  district.  (Compare  with 
section,  Fig.  438.) 

Mediterraneans.  —  When  the  continental  shelf  or  an  inland  por- 
tion of  the  continent  is  deeply  excavated,  but  in  such  a  manner 
that  the  deeper  portion  is  everywhere  surrounded  by  a  higher  rim, 
a  mediterranean  sea  is  produced.  Such  seas  may  also  be  formed  by 
the  dropping  down  of  small  blocks  of  the  continental  mass,  or  by 
very  pronounced  slow  warping  of  a  part  of  the  continental  shelf. 
A  mediterranean  sea,  while  truly  intra-continental  or  within  the 
land  block,  as  is  the  case  with  the  deep  funnel  sea,  differs  from  the 
latter  in  having  its  bottom  rise  in  all  directions,  while  its  outline 


516      Deposition  of  Clastic  Material  in  the  Sea 

may  vary  from  nearly  circular  to  elongated  oval  or  irregular.  In 
all  cases,  however,  there  is  a  shallow  outlet  across  the  rim  or  there 
may  be  several  such,  these  outlets  leading  to  the  open  ocean  (Gulf 
of  Mexico)  to  a  funnel  sea  (Roman  Mediterranean  or  Red  Sea)  or  to 
another  mediterranean  (Black  Sea).  Should  the  land  rise,  carrying 
the  mediterranean  with  it,  or  should  the  sea-level  sink,  such  a  medi- 
terranean sea  will  be  converted  into  a  lake,  either  without  outlet, 
when  the  water  remains  salt,  as  in  the  case  of  the  Caspian,  or 
drained  by  a  river  (Lake  Baikal,  Lake  Tanganyika),  in  which  case 
the  water  will  generally  become  fresh  from  excess  precipitation. 

The  type  of  the  mediterraneans  is  the  one  generally  so  called  (the  Roman 
Mediterranean),  which  lies  between  Europe,  Asia,  and  Africa.  It  is  really  a 
compound  body,  with  several  deep  centers.  The  Red  Sea  is  another  example, 
being  land-locked  like  the  Roman,  and  opening  into  a  funnel  sea  (Fig.  437).  The 
Black  Sea,  on  the  other  hand,  is  a  very  nearly  enclosed  water  body,  almost  a 
lake.  The  Gulf  of  Mexico  is  a  mediterranean  with  several  openings  across  a 
ridge  from  which  islands  arise,  and  the  Caribbean  Sea  is  a  mediterranean  with 
a  still  more  extensively  submerged  outer  rim,  as  are  Behring  Sea  and  the  Sea 
of  Okhotsk  in  the  northern  Pacific.  These  are  generally  spoken  of  as  marginal 
mediterraneans  lying  within  the  continental  shelf. 

The  importance  of  mediterraneans  lies  in  the  fact  that  they  have  their  own 
ranges  of  temperature  and  salinity,  currents,  and  other  characteristics,  which  are 
on  the  whole  distinct  from  those  of  the  neighboring  oceans.  The  differences 
are,  of  course,  more  pronounced  as  the  enclosure  becomes  more  complete,  until 
such  types  as  the  Black  Sea,  with  practically  stagnant  deeper  waters,  are 
produced. 

Most  mediterraneans  date  their  origin  from  long  past  geological 
time.  They  are  not  as  old  as  the  oceans,  but  many  of  them  have 
existed  during  successive  geological  periods. 

Epicontinental  Seas 

Epeiric  Seas.  —  The  shallow  seas  of  an  epicontinental  character 
which  lie  within  the  land  masses,  such  as  the  Baltic  Sea  and  Hudson 
Bay,  have  come  to  be  called  epeiric  seas  (e7rei/>os,  epeiros,  a  conti- 
nent). Their  depth  seldom  exceeds  50  or  100  fathoms  (300-600 
feet),  and  they  are  formed  chiefly  by  a  gentle  down-warping  of 
the  land  surface  until  it  has  passed  below  sea-level.  Such  seas 
are  always  connected  with  the  oceans  by  shallow  passages,  the 
narrowness  of  which  often  prevents  much  change  in  level  of  the 
water  body  during  tidal  fluctuation  of  the  open  ocean.  The 
salinity  of  the  water,  its  temperature,  and  its  currents  are  also  dis- 


Geographical  Subdivisions  of  the  Sea  517 

tinct  in  such  a  water  body.  In  the  past  many  such  epeiric  seas 
existed  where  now  is  the  dry  land ;  indeed,  such  seas  have  often 
been  replaced  repeatedly  by  dry  land.  They  are  of  extreme  im- 


MER  ROUGE 

d'apres  les  sondages 

du  Navire  de  guerre  autrichien 

"POL  A".  1895  -1898. 


FIG.  437.  —  Map  of  the  Red  Sea,  a  mediterranean,  showing  the  depths  ac- 
cording to  the  soundings  of  the  Austrian  warship  Pola,  1895-1898.  (From 
Suess  and  de  Margerie.)  The  continental  shelf  (littoral  zone)  is  shown  in  white 
(0-200  meters  or  roughly,  o-ioo  fathoms).  The  zones  to  a  depth  of  2000 
meters  (roughly  1000  fathoms)  represent  the  continental  slopes,  while  the  deeper 
parts  (in  black)  are  the  abyssal  depths.  Note  that  the  Gulf  of  Suez  is  an  epi- 
continental  water-body,  resting  entirely  upon  the  continental  shelf,  while  the 
Gulf  of  Akaba  is  a  subsidiary  mediterranean. 


518       Deposition  of  Clastic  Material  in  the  Sea 

portance  in  the  history  of  older  sediments.  Marginal  seas  of  this 
type  situated  on  the  continental  shelf  are  the  North  Sea,  Irish  Sea, 
Tasmanian  Sea,  and  others. 

Geosynclines.  —  Along  the  base  of  high  or  mountainous  land 
masses,  there  generally  extends  a  belt  of  subsidence  in  which  the 
bulk  of  the  sediment  brought  from  the  mountains  is  deposited. 
This  is  the  geosyncline.  The  rate  of  subsidence  is  usually  con- 
stant, so  that  great  thicknesses  of  material  of  uniform  character 
may  accumulate.  Sometimes  this  material  accumulates  wholly 
above  sea-level,  as  i,n  the  case  of  the  north  Indian  geosyncline  at  the 
southern  foot  of  the  Himalayas  already  referred  to  (page  468).  In 
other  cases  the  sea  has  access  to  this  trough,  when  great  thicknesses 
of  limestones  or  of  other  marine  deposits  may  form.  Not  infre- 
quently there  is  an  alternation  of  continental  and  marine  deposits, 
showing  a  variable  rate  of  subsidence  or  a  variable  volume  of  sedi- 
ment. In  general  the  rate  of  subsidence  and  that  of  supply  of 
material  is  evenly  balanced,  so  that  for  a  long  time  similar  deposits 
may  be  formed  at  essentially  the  same  elevation  above,  or  depres- 
sion beneath  the  sea-level.  Such  geosynclines  of  the  past  will  be 
referred  to  at  greater  length  under  their  proper  periods  in  the  section 
on  historical  geology. 

BATHYMETRIC  DISTRICTS  AND  ZONES 

The  bathymetric  districts  and  zones  of  the  sea  are  those  deter- 
mined by  the  depth  of  water.  As  we  have  seen,  this  depth  over  the 
continental  shelf  does  not  exceed,  as  a  rule,  100  fathoms  (600  feet). 
This  district  and  the  shallow  epeiric  sea  is  illumined  throughout 
by  sunlight  and  agitated  by  waves,  and  constitutes  the  littoral 
district  of  the  sea,  though  this  term  is  restricted  by  some  to  the  shore 
zone.  This  district  naturally  falls  into  two  zones,  (a)  that  of  the 
shore  between  high  and  low  tide  (shore  zone)  and  (b)  that  perma- 
nently submerged  even  at  low  water  (neritic  zone)  (Fig.  438). 
Beyond  the  edge  of  the  continental  shelf  the  ocean  floor  slopes 
more  rapidly  to  depths  of  about  1000  fathoms  (6000  feet),  and  this 
constitutes  the  bathyal  district,  though  some  authorities  limit  this 
zone  to  depths  of  500  fathoms.  Beyond  this  lies  the  abyssal 
district,  which  comprises  the  larger  area,  between  1000  and  3000 
fathoms  (2400  and  5500  meters)  and  the  smaller  oceanic  depressions 
or  deeps,  which  descend  to  greater  depths  (30,000  feet  or  more). 


Waves  and  Currents  of  the  Sea 


519 


The  characteristic  deposits  of  these  several  districts  and  zones  will 
be  considered  in  a  later  section  of  this  chapter. 


Littoral       District- 


Strdnd     ShoreZone 


Df  strict 


(  A  tyjso-  Pelagic  District 
Abyssal  District 


FIG.  438.  —  Diagrammatic  cross-section  of  a1  part  of  the  sea  to  show  the 
several  life-districts  and  the  bathymetric  zones.  The  edge  of  the  continental 
shelf  is  marked  by  the  2OO-meter  or  ico-fathom  line  (approximately).  That 
part  of  the  sea  which  lies  above  the  continental  shelf  and  extends  to  "high- 
water  mark  is  called  the  Littoral  District,  this  being  divided  into  the  shore-zone 
between  high  and  low  water  and  the  neritic  zone  beyond  that.  From  the  edge 
of  the  continental  shelf  the  "continental  slope"  descends  to  the  deep  sea  (200 
to  2400  meters,  100  to  1000  fathoms,  roughly).  The  upper  part  of  this,  to  a 
depth  of  500  fathoms,  is  called  the  bathyal  zone  proper,  the  lower  part  the  deeper 
bathyal  zone.  Above  the  floor  of  the  deep  sea  lies  the  abyssal  district  of  the  ocean. 
Long,  narrow  depressions  of  the  ocean  floor  parallel  to  the  continents  form  the 
fore-deeps.  The  pelagic  district  is  the  upper  part  of  the  ocean  to  the  depth  to 
which  sunlight  penetrates  —  approximately  100  fathoms. 


WAVES  AND  CURRENTS  or  THE  SEA 

Waves.  —  There  are  many  kinds  of  water  waves,  produced  in 
a  variety  of  ways ;  of  these  the  waves  produced  by  winds  are  the 
most  important,  because  most  common,  though  those  produced  by 
earthquakes  and  explosions,  the  so-called  tsunamis,  are  often  more 
destructive.  Some  of  these  will  be  noted  in  connection  with  our 
studies  of  earthquakes.  The  great  waves  known  as  the  tides, 
two  of  which  sweep  around  the  earth  in  somewhat  more  than 
twenty-four  hours,  are  of  great  importance  as  producers  of  oceanic 
currents,  while  the  periodic  rise  and  fall  of  the  waters  becomes 
significant  along  the  shores,  influencing  not  only  the  forces  which 
modify  such  shores,  but  the  life  of  the  shallow  sea  as  well. 

The  waves  of  the  open  ocean  are  due  to  a  rotary  movement  of 
the  particles  of  water  (Fig.  439).  Originally  these  are  set  in  motion 
by  the  wind,  but  they  are  propagated  beyond  the  zone  of  wind 
activity,  so  that  even  on  calm  days  great  waves  or  swells  will  be 
found  in  the  ocean,  far  from  their  source  of  origin.  As  a  result  of 


520      Deposition  of  Clastic  Material  in  the  Sea 

the  rotary  motion  of  the  water  particles  the  mass  of  the  water 
rises  and  falls  rhythmically,  forming  alternately  the  crest  and  the 
trough  of  the  wave.  The  water  particles  move  forward  on  the 
crest  of  the  wave,  downward  on  the  back,  backward  in  the  trough, 


FIG.  439.  —  Diagram  illustrating  the  formation  of  waves  in  the  open  ocean 
by  a*rotary  motion  of  the  water  particles.  As  the  particles  revolve  in  the 
direction  indicated  by  the  small  arrows,  the  wave  form  advances  as  shown  by 
the  heavy  arrows.  (From  Grabau,  Principles  of  Stratigraphy.} 

and  upward  on  the  front  of  the  wave.  In  water  of  limited  depth, 
however,  the  path  of  the  moving  particles  is  an  ellipse,  with  the 
major  axis  horizontal. 

The  height  of  the  wave  is  measured  by  the  vertical  distance  between  the 
trough  and  the  crest,  and  corresponds  to  the  diameter  of  the  orbit  in  which 
the  water  particles  move.  In  the  open  ocean,  wave  heights  from  5  to  15  feet 
are  most  common,  but  20  or  40  feet  is  not  an  unusual  height  in  storms,  while 
heights  of  45  to  50  feet  have  been  recorded.  The  length  of  the  wave  is  the 
horizontal  distance  from  crest  to  crest.  During  stormy  weather,  in  the  open 
ocean,  the  wave  length  may  be  from  200  to  500  feet  or  over.  When  the  wave 
reaches  a  length  of  a  thousand  feet  or  more,  it  is  called  a  swell  or  ground  swell 
and  such  waves  have  been  found  in  exceptional  cases  to  have  a  length  of  more 
than  2700  feet  from  crest  to  crest.  The  height  and  length  of  the  wave  depend 
upon  the  strength  and  continuity  of  the  wind,  and  to  some  extent  on  other 
factors.  The  height  of  the  wave  decreases  downward  at  a  rate  proportional 
to  the  wave  length.  If  the  diameter  of  the  orbit  of  motion  of  the  water  particles 
is  20  feet,  giving  surface  waves  20  feet  high,  and  if  the  wave  length  is  400  feet, 
the  diameter  of  the  orbit  of  the  water  particles,  at  a  depth  of  400  feet,  is  T4^  of 
an  inch. 

Because  of  the  regular  rise  and  fall  of  the  water  in  wave  formation,  the  crest 
of  the  wave  advances,  although  the  water  itself  only  rises  and  falls.  The 
rapidity  with  which  the  wave  crest  travels  is  called  the  velocity  of  the  wave, 
and  this  varies  from  15  miles  or  less  to  35  miles  or  more  an  hour  and  may  be 
as  high  as  60  miles.  For  ocean  waves  of  large  size,  the  wave  velocity  is  apt 
to  be  six  or  seven  times  as  great  as  the  orbital  velocity  of  the  water  particles. 
For  example,  a  wave  400  feet  long  and  15  feet  high  will  have  a  velocity  of  about 


Waves  and  Currents  of  the  Sea 


45  feet  per  second  (about  30  miles  an  hour),  while  the  surface  water  particles 
will  move  round  in  their  orbits  at  a  speed  of  only  5^  feet  per  second.  The 
time  taken  by  a  crest  to  travel  a  wave  length  is  called  the  period  of  the  wave, 
and  this,  in  storm  waves,  varies  from  6  to  10  seconds  or  over.  It  corresponds 
to  the  time  required  for  a  particle  to  move  around  its  orbit. 

When  great  waves,  by  propagation,  reach  shallow  water  the 
free  orbital  movement  of  the  water  particles  is  interfered  with, 
partly,  as  some  hold,  by  friction  upon  the  bottom,  which  retards 
the  backward  motion,  but  more  especially  because  of  the  insuffi- 
cient quantity  of  water  to  keep  up  the  normal  rotary  motion. 
The  wave  becomes  higher  and  shorter,  its  front  steepens,  while 
the  crest  arches  forward  (Fig.  440,  D,  £),  and  being  unsupported 


FIG.  440.  — Diagram  showing  the  development  of  the  breaking  wave  (A-G), 
and  the  "swash"  after  breaking  (//).  (After  Davis.)  In  general  the  wave 
breaks  when  the  depth  of  water  reckoned  from  the  undisturbed  sea-level  is 
equal  to  the  height  of  the  crest  above  the  trough. 

by  sufficient  water  in  front  of  it,  dashes  downward  with  a  roar, 
producing  a  breaking  or  combing  wave  (Fig.  440,  F,  G,  Fig.  441). 
After  the  wave  has  broken,  the  water  rushes  up  on  the  beach, 
forming  the  "  swash,"  and  returns  seaward,  carrying  the  sand  and 
pebbles  with  it,  the  impact  of  the  latter  against  one  another  often 
producing  a  loud  rattling  noise.  In  this  manner  rock  fragments 
are  rapidly  worn  into  round  pebbles  (see  p.  430).* 

Waves  of  Translation.  —  The  waves  so  far  described  are  the  ordinary  ones, 
and  are  called  waves  of  oscillation.  There  is,  however,  another  kind,  illustrated 
by  the  movement  produced  when  still  water  is  pushed  into  a  mound  by  the 
shoving  action  of  a  boat.  In  that  case  a  single  prominent  wave  rolls  forward 
over  the  surface  of  the  water,  there  being  no  trough  as  in  the  oscillation  wave. 
Such  waves  are  called  waves  of  translation,  because  the  water  particles  rise  and 
move  forward  to  a  new  position  as  the  wave  form  advances.  When  large  waves 

1  For  further  details  see  A.  W.  Grabau,  Principles  of  Stratigraphy,  A.  G.  Seller  and 
Co.,  Chapter  V,  and  D.  W.  Johnson,  Shore  Processes  and  Shoreline  Development,  John 
Wiley  and  Sons,  1919,  Chapters  I  and  II. 


522       Deposition  of  Clastic  Material  in  the  Sea 

of  oscillation  break  on  a  gently  sloping  shore  far  from  land  they  give  rise  to  a 
series  of  waves  of  translation  which  will  advance  up  the  beach,  carrying  the 
wrack  of  the  sea  landwards. 


FIG.  441.  —  Combing  wave,, showing  water  completing  orbital  movement 
although  insufficient  in  quantity  to  fill  the  wave  form.  Note  the  steep  wave 
front  in  the  foreground,  and  the  decreasing  steepness  away  from  the  comber 
in  the  distance.  (Compare  with  Fig.  440.)  (After  D.  W.  Johnson,  Shore 
Processes,  etc.  John  Wiley  and  Sons.) 

Wave  Currents.  —  In  shallow  water,  where  the  wave  front  is 
steepest,  it  will  rush  forward  with  a  short,  quick  movement,  while 
the  return,  or  backward,  motion  is  slower  and  of  longer  duration. 
The  forward  movement  may  be  strong  enough  to  carry  large 
pebbles  and  cobblestones  which  the  backward  current  is  unable 
to  move.  Hence  this  coarser  material  will  be  carried  landwards, 
while  the  finer  material  moves  seawards.  Coarse  material  dropped 
at  a  distance  of  7  to  10  miles  from  land,  where  the  water  was  from 
10  to  20  fathoms  deep,  has  been  thrown  on  shore  by  storm  waves, 
and  even  pig  lead,  from  a  vessel  wrecked  more  than  a  mile  from 
shore,  has  been  cast  upon  the  beach.  Stones  with  large  seaweeds 
or  kelp  attached  to  them  are  often  cast  ashore  from  the  deeper 
water  in  which  these  seaweeds  grow.  Here  a  certain  buoyancy 
is  given  to  the  stone  by  the  seaweed.  On  steep  coasts,  however, 
the  backward  current  or  undertow  may  be  very  strong. 

Beach  Drift.  —  This  term  has  been  applied  by  D.  W.  Johnson 
to  the  transportation  of  material  on  the  beach-slope,  and  parallel 


Waves  and  Currents  of  the  Sea  523 

to  the  shore,  by  current  action  due  to  waves  which  break  obliquely 
upon  the  shore.  In  this  case  the  swash  advances  obliquely  up 
the  slope  of  the  beach,  but  the  back-wash,  moving  under  the  control 
of  gravity,  tends  to  return  directly  down  the  steepest  slope.  Thus 
such  material  will  zigzag  along  the  shore,  but  the  path  of  the 
particles  will  in  reality  be  a  series  of  parabolic  curves,  as  shown 
in  Fig.  442.  These  paths 
are  larger  for  the  finer  than 
for  the  coarser  material,  the 
finer  particle  therefore  trav- 
eling faster  along  the  shore. 
The  direction  of  the  drift  FlG.  ^  _  Diagram  to  "niustrate  the 
Will  depend  on  the  direction  "zigzagging"  of  the  particles  in  beach 
of  the  prevailing  winds,  on  drift  under  the  influence  of  oblique  waves. 
the  position  Of  the  greatest  The  direction  of  movement  is  indicated 

by  the  arrows.     The  paths  are  parabolic 
stretch  of  open  water,  since      curves.      (After  Johnson.) 

this  largely  determines  the 

direction  from  which  the  larger  waves  will  come,  and  on  other 
factors.  In  some  cases  drift  movement  is  in  opposite  directions 
from  a  given  point.  A  similar  movement  of  particles  goes  on  in 
the  off-shore  deeper  water  region,  where  the  material  is  moved  to 
and  fro  on  the  bottom  by  the  waves  which  advance  parallel 
to  the  shore,  but  moves  in  a  series  of  parabolic  curves,  if-  the 
wave  advance  is  oblique.  This  is  called  long-shore  drifting. 

Undertow  and  Long-shore  Currents.  —  When,  by  the  breaking 
of  waves  off  shore,  a  succession  of  forward-moving  water  waves 
(waves  of  translation)  is  formed,  the  water  will  be  piled  up  on  the 
shore,  above  the  normal  level  of  the  sea,  and  has  to  escape  either 
seaward  along  the  bottom  or  parallel  to  the  shore.  In  the  first 
case  it  forms  the  undertow,  .in  the  second  the  long- shore  current. 
The  undertow  is  especially  marked  in  regions  of  steep  off-shore 
bottom,  and  deep  water  close  to  shore,  and  further,  where  broad 
zones  of  waves  strike  the  shore  at  right  angles.  The  long-shore 
current  is  best  developed  where  the  waves  strike  the  shores 
obliquely,  and  where  the  bottom  shallows  gradually. 

Tidal  Currents.  —  Under  the  influence  of  the  attraction  of  the 
moon  and,  to  a  lesser  degree,  that  of  the  sun,  the  water  of  the  sea 
rises  into  two  elevations  on  opposite  sides  of  the  earth,  and  these 
would  sweep  around  the  earth  as  two  great  continuous  tidal-waves, 
were  it  not  for  the  presence  of  the  continents.  Twice  each  month, 


524      Deposition  of  Clastic  Material  in  the  Sea 

at  new  moon  and  at  full  moon,  the  tides  are  exceptionally  high, 
owing  to  the  relative  position  of  the  sun  and  moon  at  such  times, 
when  they  exert  a  combined  influence  of  the  same  character  upon 
the  waters.  Such  tides  are  called  Spring  Tides.  Twice  a  month 
also,  at  the  period  of  the  first  quarter  and  last  quarter  of  the  moon, 
the  interval  between  high  and  low  water  is  at  its  lowest,  since  at 
such  times  the  moon  and  sun  act  in  contrary  direction  upon  the 
waters,  each  tending  to  neutralize  the  force  of  attraction  of  the 
other.  This  constitutes  the  Neap  Tides. 


FIG.  443.  —  Reversible  Falls,  St.  John,  N.  B.     High  tide, 
westward  into  the  harbor. 


Water  pouring 


At  a  given  point,  the  crest  of  the  tidal  wave  arrives  at  intervals  of  1 2  hours 
and  26  minutes.  This  produces  the  flood-tide.  Six  hours  and  thirteen  minutes 
later  the  trough  of  the  tidal  wave  replaces  the  crest,  and  low  water  or  ebb-tide 
results.  Because  of  the  interference  of  the  lands,  the  tidal  wave  is  retarded 
and  the  time  of  high  or  low  tide  may  differ  widely  from  that  of  neighboring 
regions,  though  the  interval  remains  the  same.  It  may  even  happen  that  a 
low  tide  from  one  direction  meets  a  high  tide  from  another,  as  at  Hellgate, 
where  the  high  tide,  entering  from  New  York  Harbor,  arrives  simultaneously 
with  a  low  tide  approaching  through  Long  Island  Sound,  while  six  hours  later 
the  reverse  is  true.  The  strong  tidal  disturbances  of  the  Straits  of  Dover, 
experienced  by  all  who  have  made  the  crossing,  are  due  to  the  interferences 
of  tides  arriving,  the  one  from  the  North  Sea,  the  other  from  the  Atlantic 


Waves  and  Currents  of  the  Sea 


525 


through  the  Channel.  The  famous  "  Scylla  "  and  "  Charybdis  "  in  the  Straits 
of  Messina  are  whirlpools,  produced  by  the  meeting  of  tides  from  opposite 
directions,  and  a  similar  phenomenon  is  noted  on  the  coast  of  Massachusetts  at 
Woods  Hole. 

Where  the  tides  pass  through  narrow  channels,  powerful  and 
dangerous  currents  are  generated,  and  these  may  affect  the  bottom, 
even  at  considerable  depth,  by  sweeping  away  all  loose  material 
and  depositing  it  elsewhere.  When  the  tides  are  compressed  in 


FIG.   444.  —  Reversible  Falls,  St.  John,  N.  B.    Low   tide.     Water   pouring 
from  the  harbor,  flowing  eastward. 

narrow  inlets,  such  as  the  Bay  of  Fundy,  they  may  pile  up  near 
the  head  of  the  bay  to  a  height  of  30  or  40  feet  and  in  exceptional 
cases  to  a  height  of  70  feet.  The  rapidly  rising  tide  of  this  bay 
pours  into  St.  John  Harbor  through  a  narrow  inlet,  the  surface  of 
this  basin  rising  more  slowly  than  that  of  the  ocean  without, 
on  account  of  the  narrowness  of  the  passage.  Conversely,  the 
tide  in  the  Bay  of  Fundy  falls  more  rapidly  than  the  water  can 
pour  out  from  St.  John  Harbor.  Hence  a  reversible  fall  is  pro- 
duced, facing  inward  when  the  ocean  is  highest  and  outward 
when  it  is  lowest.  The  maximum  difference  of  elevation  of  the 
water  surfaces  is  nearly  10  feet  (Figs.  443,  444).  Related  phe- 


526      Deposition  of  Clastic  Material  in  the  Sea 


nomena  exist  upon  the  coast  of  Massachusetts.  The  influence  of 
the  tidal  currents  is  very  marked  in  estuaries  and  will  be  referred 
to  later. 

Planetary  Currents.  —  These  represent  the  great  oceanic  circu- 
lation set  in  motion  primarily  by  the  planetary  winds  and  modified 

by  the  rotation  of  the  earth,  the  con- 
figuration of  the  lands,  and  by  other 
factors.  In  an  ideal  ocean  of  sym- 
metrical form,  reaching  from  pole  to 
pole,  and  covering  a  meridional  dis- 
tance of  90°,  the  circulation  would  be 
essentially  like  that  shown  in  Fig.  445. 


FIG.  445.  —  Diagram  show- 
ing the  circulation  in  an  ideal 
ocean  extending  from  pole  to 
pole  and  covering  one  fourth 
the  circumference  of  the  earth 
(90°).  (After  Krummel.) 


On  both  sides  of  the  equator,  at  10°  north 
and  south  latitudes,  the  equatorial  stream 
would  flow  westward  under  the  influence  of 
the  easterly  winds.  On  approaching  the 
western  shore  it  would  bend  northward  and 
southward,  respectively,  crossing  eastward 
again  in  latitudes  50°  north  and  south,  and 
returning  to  the  equatorial  region  on  the 
eastern  side.  This  constitutes  the  principal 
circulation.  Arctic  and  antarctic  currents 
would  also  exist,  flowing  as  indicated  in  .the 
diagram.  Between  the  two  westward-flowing 
equatorial  currents  is  an  eastward-flowing 

equatorial  counter-current,  which  is,  however,  much  weaker.  The  equatorial 
currents  are  warm  and  they  impart  their  heat  to  the  westerly  shores  where 
they  turn,  respectively,  north  and  south,  while  the  return  currents  are  cooled 
in  their  northerly  and  southerly  passages. 

The  outline  of  the  continents  is  responsible  for  marked  deflections 
of  the  currents,  while  the  position  of  the  equatorial  currents  is  also 
modified  by  the  shifting  of  the  heat  equator.  The  south-equatorial 
current  crosses  the  Atlantic  with  an  average  velocity  in  June, 
July,  and  August  of  from  1.6  to  1.8  km.  per  hour.  Striking  the 
projecting  point  of  South  America,  at  Cape  St.  Roque,  it  divides, 
one  arm  passing  southward  to  become  the  Brazil  current,  while 
the  other  (northern)  unites  with  the  north-equatorial  current  to 
produce  the  Guiana  current,  which  later  becomes  the  Gulf  Stream. 
This  makes  the  circuit  of  the  Gulf  of  Mexico  and  escapes  by  the 
narrow  passage  between  Cuba  and  Florida  (Fig.  225,  p.  301).  It 
turns  northward  between  Florida  and  the  Bahama  banks,  crossing 


Sources  of  Clastic  Sediments  in  the  Sea          527 

the  Atlantic  approximately  in  latitude  40°  N.  Its  further  course 
may  be  noted  by  reference  to  a  map  of  these  currents  in  any  good 
atlas.  At  the  Florida  Straits,  the  average  velocity  is  5.55  km.  per 
hour,  but  rises  to  9  km.  per  hour  in  the  warmer  season.  The 
velocity  rapidly  decreases  northward,  and  where  the  Gulf  Stream 
crosses  the  Atlantic,  it  averages  not  much  over  i  km.  per  hour.  In 
the  center  of  the  great  North  Atlantic  eddy,  the  ocean  is  relatively 
quiet,  and  is  filled  with  the  floating  sea-weed  Sargassum.  This 
region  has,  therefore,  become  known  as  the  Sargasso  Sea.1 

It  should  be  noted,  however,  that  these  great  oceanic  currents  seldom  come 
near  the  coast,  or  if  they  do,  tKeir  effect  is  overcome  by  other  local  currents. 
This  is  true  even  of  the  Gulf  Stream  in  the  Florida  Straits,  which  is  separated 
from  the  land  by  another  and  slower  current  moving  in  the  opposite  direction. 

Other  Currents.  —  Besides  those  described  there  are  other 
currents  in  the  sea  and  in  great  lakes,  most  notable  among  which 
are  those  produced  by  winds  which  blow  steadily  in  a  given  direc- 
tion. These  currents  are  often  of  great  importance,  especially 
in  enclosed  or  nearly  enclosed  water  bodies.  Other  currents  are 
due  to  difference  in  salinity  of  the  waters,  to  convection,  to  rivers 
entering  the  sea  or  lakes,  and  to  other  causes.2 


SOURCES  OF  CLASTIC  SEDIMENTS  DEPOSITED  IN  THE  SEA 

The  clastic  material  which  finds  its  final  resting  place  in  the  sea 
is  derived  from  a  number  of  sources,  and  brought  to  it  in  a  variety 
of  ways.  The  principal  of  these  are  as  follows : 

Land-derived  or  Terrigenous  Material 

Clastic  material  derived  from  the  land  is  called  terrigenous 
(earth-born) ;  and  this  may  be  the  product  of  weathering  or  of 
mechanical  erosion  as  already  outlined  in  an  earlier  chapter  (p.  424). 
This  material  is  supplied  to  the  sea:  (a)  by  rivers,  glaciers, 
icebergs,  etc.  which  bring  it  from  the  land,  (b)  by  the  wind,  often 
from  great  distances,  (c)  by  erosion  of  the  seashore  by  the  waves, 
and  (d)  by  the  scouring  action  of  the  oceanic  currents. 

1  For  further  details,  and  for  description  of  currents  in  the  other  oceans,  see  the 
author's  Principles  of  Stratigraphy,  pp.  231-244. 

2  For  a  discussion  of  these  see  Johnson,  D.  W.,  Shore  Processes,  etc.,  chapter  iii. 


528      Deposition  of  Clastic  Material  in  the  Sea 

Clastic  Material  Derived  from  Organic  or  Chemical  Deposits  in  the  Sea 

Under  this  heading  belongs  the  clastic  material  which  is  torn  from 
structures  built  in  the  sea  from  material  formerly  in  solution; 
namely,  coral  reefs,  shell  deposits,  and  the  like,  as  well  as  from 
chemical  deposits  in  the  sea.  Of  these  the  coral  reefs  are  by  far 
the  most  important  sources  of  clastic  material,  this  being  of  course 
wholly  composed  of  carbonate  of  lime  with  some  magnesia.  The 
destructive  work  is  performed  by  the  waves  and  by  animals  which 
feed  upon  the  reef-building  organisms  and  crush  their  calcareous 
structures  (see  ante,  p.  437). 

Clastic  Material  of  Subcrustal  Origin 

Explosive  eruptions  of  submarine  volcanoes  furnish  clastic 
material  to  the  waves  and  currents,  this  being  of  ten  of  considerable 
amount.  Lava  streams  poured  out  upon  the  sea  bottom  will,  if 
they  come  within  the  reach  of  the  waves,  be  subject  to  their  attack 
and  so  furnish  clastic  material  for  other  sediments. 

Material  of  Meteoric  Origin 

Meteoric  dust  and  stones  from  spaces  outside  of  our  solar  system 
frequently  reach  the  earth,  and  probably  fall  into  the  sea  in  no  in- 
considerable quantities.  These  are  incorporated  in  all  kinds  of 
marine  sediments. 

TRANSPORTATION  AND  SORTING  OF  CLASTIC  MATERIAL 
IN  THE  SEA 

The  material  furnished  to  the  waves  and  currents  of  the  sea  by 
the  various  agencies  above  enumerated  may  be  transported  by  them 
over  wide  areas  before  it  finally  comes  to  rest.  The  work  of  the 
waves  consists  mainly  in  stirring  up  the  material  so  as  to  place  it 
in  the  best  position,  that  of  suspension,  for  the  currents  to  trans- 
port it.  In  general,  such  transportation  is  most  effective  in  the 
shore-zone  of  the  littoral  district  and  in  the  shallow  part  of  the  sub- 
merged zone,  but  it  is  also  effective  in  the  deeper  part  of  that  zone. 
On  the  shore  the  chief  currents  are  the  long-shore  current,  which 
carries  the  material  more  or  less  parallel  to  the  coast,  and  the  under- 
tow, which  is  the  return  current  on  the  bottom,  inaugurated  upon 
the  breaking  of  a  wave  against  the  shore.  This  current  tends  to 


Transportation  and  Sorting  529 

carry  the  material  farther  out  to  sea.  There  are,  however,  also 
other  bottom  currents  in  the  deeper  parts  of  the  submerged  zone 
which  may  do  no  inconsiderable  amount  of  transportation.  Chief 
among  these  are  probably  the  tidal  currents,  though  others,  set  in 
motion  by  the  normal  circulation  of  the  ocean  (the  great  oceanic 
or  planetary  currents),  are  also  active. 

Both  waves  and  currents  perform  a  certain  amount  of  sorting 
of  the  material  supplied ;  the  finer-grained  the  material  the  longer 
will  it  be  kept  in  suspension  by  the  waves  and  the  farther  away 
will  it  be  carried  by  the  currents.  Sorting  of  clastic  material  is 
best  accomplished  when  there  is  vigorous  wave  action  accompanied 
by  strong  undertow,  and  when  the  volume  of  sediment  supplied  is 
moderate.  On  the  other  hand,  if  large  quantities  of  sediment  are 
furnished  to  the  waves,  only  a  moderate  amount  of  sorting  is  ef- 
fected. Where  strong  currents  support  material  in  suspension  for  a 
long  period  of  time,  picking  it  up  again  repeatedly  after  dropping  it, 
a  considerable  amount  of  sorting  may  be  produced.  Such  sorting 
will  be  according  to  both  size  of  grain  and  character  of  material, 
and  as  a  result  pure  quartz  sands  will  be  found  in  some  localities, 
sands  largely  composed  of  -heavy  minerals,  such  as  garnet  and 
magnetite,  in  others,  and  muds  in  still  other  localities.  By  mutual 
attrition  of  the  grains,  a  considerable  amount  of  rounding  is  pro- 
duced, but  it  must  be  recognized  that  such  rounding  is  seldom  or 
never  as  perfect,  nor  the  sorting  according  to  size  of  grain  as  com- 
plete, as  is  the  case  with  wind-transported  material.  As  a  result, 
the  sands  of  the  sea-beach  generally  consist  of  commingled  coarser 
and  finer  grains,  most  of  them  subangular,  —  this  angularity,  and 
the  presence  of  water  held  by  capillary  attraction  between  the 
grains,  serving  to  bind  them  into  a  compact  mass.  This,  in  some 
sections,  results  in  the  production  of  so  firm  a  floor  at  low  tide  that 
it  forms  a  favorite  region  for  the  trials  of  speed  of  high-power 
racing  machines.  (See  Fig.  357,  p.  431.) 

Fine  sediments  may  be  carried  out  to  sea  for  hundreds  of  miles, 
and  this  accounts  for  the  fact  that  the  entire  surface  of  the  conti- 
nental shelf  is  covered  with  sand,  while  the  mud-line  begins,  as  a  rule, 
at  the  edge  of  the  shelf  and  continues  over  the  bathyal  district. 
Terrigenous  muds  are  probably  seldom  carried  to  the  deeper  sea, 
where  volcanic  or  meteoric  matter  forms  the  chief  clastic  sediment, 
together  with  the  inorganic  matter  derived  from  the  structures  of 
organisms. 


530      Deposition  of  Clastic  Material  in  the  Sea 

In  such  regions,  however,  rafted  material  may  form  important  deposits. 
We  have  already  referred  to  the  ice-rafted  material  which  forms  some  subma- 
rine banks  and  which  has  been  brought  there  by  floating  icebergs.  Materials 
attached  to  or  held  by  the  roots  of  floating  trees  and  other  vegetation  from  the 
land  may  also  be  distributed  widely.  Thus  leaves  of  land  plants  are  frequently 
dredged  in  deep  water,  and  tree  trunks  from  the  tropical  rivers  of  America  have 
been  carried  by  the  Gulf  Stream  to  the  Arctic  regions  where  they  have  been  cast 
ashore  on  far-distant  northern  lands  and  islands. 


TYPES  OF  CLASTIC  DEPOSITS  IN  THE  SEA 

Deposits  in  the  Shore  Zone 

The  Beach.  —  The  sea  beaches  of  the  modern  oceans  consist 
very  largely  of  sand.     Even  where  the  sea  is  faced  by  a  rocky  cliff 


FIG.  446.  —  Rock  fragment  beach  at  the  foot  of  a  rocky  cliff,  a  portion  of 
which  remains  in  the  foreground  as  a  marine  "stack"  or  chimney,  coast  of 
France.  The  beach  here  forms  a  bench  and  the  coast  topography  is  young. 
(From  Johnson's  Shore  Processes ;  John  Wiley  and  Sons.) 

which  is  actively  undergoing  erosion  by  the  waves  at  the  base  and  by 
weathering  at  the  top,  coarse  materials  such  as  large  blocks,  boul- 
ders, and  pebbles  are  restricted  to  a  relatively  narrow  zone,  beyond 
which  the  material  of  the  sea-bottom  is  sand  (see  Fig.  356  b,  p.  430). 
Where  cliffs  are  undermined  by  the  waves,  huge  blocks  often  cover 


Types  of  Clastic  Deposits  in  the  Sea          531 


the  beach  at  the  foot  of  the  cliff,  but  these  will  undergo  gradual 
destruction  in  time  (Fig.  446).  Sometimes,  however,  the  condi- 
tions are  favorable  for  their  preservation,  the  spaces  between  them 
being  filled  by  finer  material  and  the  whole  becoming  bound  to- 
gether into  rock.  Such  rock  formed  during  the  Jurassic  period  of 
the  earth's  history,  and  carrying  blocks  of  stone  twenty  feet  in 
length,  is  found  exposed  on  the  eastern  coast  of  Scotland,  where 
the  blocks  had  accumu- 
lated much  as  they  ac- 
cumulate in  parts  of 
that  region  to-day. 
Mingled  with  the  ma- 
terial of  the  coarse  frag- 
ments of  rock  are  heads 
of  corals  and  other  or- 
ganic remains,  the  larger 
ones  generally  water- 
worn. 

Boulders  are  also 
characteristic  of  the 
coast  where,  old  glacial 
moraines,  either  termi- 
nal or  ground  moraines, 
are  exposed  to  the  at- 
tack of  the  waves  (Fig. 
447).  At  "Woods  Hole 
and  along  the  shores  of 
the  Elizabeth  Islands 
group  on  the  south- 
eastern coast  of  Massa- 
chusetts, many  good 

sections  of  the  terminal  moraines  are  exposed,  and  huge  boulders 
are  strewn  over 'the  beach  for  some  distance  out  from  the  shore. 
At  several  places  in  Boston  Bay  drumlins  have  been  cut  into  by 
the  sea  and  great  accumulations  of  boulders  form  the  shore  zone 
(Fig.  447).  These  boulders  are  often  overgrown  with  seaweeds, 
which  sometimes  act  as  a  buoying  agent, making  possible  their 
movement  by  the  waves  even  on  the  gently  sloping  coast.  This 
buoyant  power  of  seaweeds  together  with  that  of  shore  ice  in 
winter  has  enabled  the  waves  to  arrange  the  boulders  in  close  juxta- 


FIG.  447.  —  Section  of  Grover's  Cliff,  near 
Winthrop,  Mass.,  showing  erosion  of  drumlin 
by  waves  and  the  formation  of  a  boulder  beach 
from  the  larger  stones  of  the  drumlin  material. 
These  boulders  are  partly  overgrown  with 
seaweeds  which  give  them  a  certain  buoyancy 
at  high  tide.  This  and  the  shore-ice  serves  to 
arrange  these  rocks  into  a  solidly  packed 
pavement  of  boulders.  (Photo  by  the  author.) 


532      Deposition  of  Clastic  Material  in  the  Sea 


position  so  as  to  produce  at  low  tide  the  appearance  of  a  boulder 
pavement.     On  the  gently  sloping  portion  of  the  North  Sea  coast 

of  Scotland,  such 
closely  arranged 
boulders  are  not  only 
thickly  overgrown 
with  seaweeds,  but 
also  serve  the  limpets 
and  other  shell-bear- 
ing mollusks,  and 
even  the  soft-bodied 
sea  anemones,  as  a 
place  of  attachment. 
Where  the  coast 
slopes  more  steeply, 
the  boulders  are 
moved  by  the  strong 
waves,  and  here  all 
organisms  are  ground  up  and  the  boulders  appear  barren.  If 
strong  waves  break  upon  the  shore,  they  often  effect  the  forma- 
tion of  terraces  with 
gently  sloping  upper  sur- 
faces and  steeper  sea- 
ward faces.  These  ter- 
races are  more  common 
where  the  rock  fragments 
are  relatively  small,  as 
of  the  size  of  a  man's 
fist,  and  in  that  case  the 
fragments  are  mostly 
worn  into  round,  smooth 
cobble-stones.  Such  ter- 
races of  cobble-stones  are 
shown  along  some  por- 


FIG.  448  a.  —  Cobble-stone  terrace  on  the  coast  of 
Marblehead,  Mass.  Note  the  flat  summit  and  the 
steep  frontal  slope  of  the  terrace.  (Photo  by  the 
author.) 


tions  of  the  Massachu- 
setts coast  as  at  Marble- 
head  (Fig.  4480),  and 
also  along  the  borders  of 
some  of  our  Great  Lakes. 


FIG.  448  b.  —  Barrier  beach,  or  cobble  ter- 
race, North  Sea  Coast  of  Scotland.  (Photo 
by  M.  I.  Goldman.) 


They  are  also  characteristic  of  many 


parts  of  the  Scottish  and  other  coasts  (Fig.  4486). 


Types  of  Clastic  Deposits  in  the  Sea          533 


Where  cobble  and  pebble  beaches  have  been  raised  by  recent 
earth  movements,  as  in  many  places  along  the  Scottish  and  other 
coasts,  these  beach  deposits  are  often  full  of  the  shells  of  shore  mol- 
lusks,  such  as  the  limpets,  as  well  as  the  hard  parts  of  other  marine 
organisms.  When  consolidated  they  will  form  a  fossiliferous  con- 
glomerate. Examples  of 
such  rock  are  known 
from  many  older  geo- 
logical formations. 

Sections  of  boulder 
beaches  and  terraces  are 
rarely  seen,  but  it  is  ap- 
parent that  their  struc- 
ture must  be  an  irregular 
one,  with  perhaps  the 
development  of  coarse 
cross-bedding.  Where 
rivers  descend  from  high 
mountains  near  the  coast, 
great  beds  of  "  shingle" 
or  pebble  beaches  are 
formed  along  the  coast, 
as  is  shown  along  the 
base  of  the-  Maritime 
Alps  between  Toulon  and 
Genoa,  and  especially 
near  Nice,  on  the  coast 
of  the  Mediterranean. 
A  great  part  of  the 
pebbles  is,  however, 
swept  into  the  Mediterranean,  which  in  some  sections,  as  at  Nice, 
drops  off  rapidly  to  a  depth  of  2000  feet  at  a  few  hundred  yards 
from  the  beach.  North  of  Messina  on  Sicily,  a  torrential  river 
carries  annually  vast  masses  of  granite  pebbles  into  the  sea. 

Sand  beaches,  on  the  other  hand,  are  commonly  broad  and  flat, 
sloping  gently  toward  the  water  and  under  it.  Here  and  there 
the  surface  will  be  characterized  by  ripple  marks  and  the  peculiar 
lines  which  are  left  on  flat  shores  by  each  little  wavelet,  the  extent 
of  which  they  outline.  Shells  of  mollusks  and  often  those  of  forami- 
niferans,  and  the  remains  of  crustaceans,  fish,  and  other  animals 


FIG.  449.  —  Low  tide  near  the  head  of  the 
Bay  of  Fundy.  The  rocks  and  boulders  are 
overgrown  with  seaweeds  (Fucus,  etc.),  and 
these  are  exposed  between  tides.  The  stacks 
in  the  view  are  the  result  of  marine  erosion 
at  high  tide.  (Photo  by  M.  O'Connell.) 


534      Deposition  of  Clastic  Material  in  the  Sea 

are  buried  in  the  sand,  sometimes  in  large  numbers,  sometimes  as 
scattered  individuals.  Frequently  the  currents  and  waves  collect 
the  shells  in  protected  embayments,  and  there  they  accumulate  in 
large  quantities  and  may  become  embedded  as  shell  layers  in  the 
sands.  On  the  coast  of  Florida  such  shell  layers  of  lenticular  form 
are  inclosed  in  the  beach  sand,  and  because  of  their  consolidation 
into  a  shell  rock,  or  coquina,  they  form  limestone  shelves  at  the 
water's  edge.  A  number  of  these  have  been  described  by  J .  F.  Kemp. 
One  of  the  most  remarkable  beaches  in  existence,  is  found  in  the 
Bay  of  Fundy,  where  the  rise  of  the  tide  is. unusual,  being  commonly 
from  30  to  40  feet  or  over,  as  noted  previously  (Fig.  449) .  Where 


l^^^^^^^^H^B^^^^^^^^^^^^BMBBBMBB^^BBBBBBBH 

FIG.  450-  —  The  tidal-bore  at^t.  John,  N.  B.  The  water  rushes  up  the  river 
valley,  presenting  a  steep  front  and  producing  a  roaring  sound  as  of  violent 
rapids. 

the  beach  is  gently  sloping,  as  in  the  Basin  of  Minas,  the  water 
recedes  for  miles,  laying  bare  huge  flats  of  red  mud  and  sand, 
which  were  derived  from  the  erosion  of  the  older  red  beds  of  the 
shore.  Across  these  flats,,  the  streams  and  retreating  tidal  waters 
cut  channels  of  varying  complexity,  with  rill  marks  upon  their 
borders,  while  raindrop  impressions  are  found  wherever  a  shower 
strikes  the  drying  mud.  Worms  crawl  over  the  surface  or  move 
just  beneath  it,  forming  characteristic  tracks ;  shells  of  dead 
mollusks  are  left  stranded  upon  the  red  mud  surface,  and  the  re- 
mains of  many  other  organisms  are  strewn  about.  When  the  tide 
returns,  it  sweeps  across  the  flat  surface  with  surprising  speed  and 
enters  the  narrow  river  channels  in  the  form  of  a  steep  frontal  wave 
called  the  "  bore,"  which  rushes  up  the  river  with  the  noise  and  the 
rapidity  of  a  breaking  wave  upon  the  sea-shore  (Fig.  450).  The 


Types  of  Clastic  Deposits  in  the  Sea          535 

mud  of  the  bottom  is  stirred  up,  a.nd  the  water  is  thick  with  sedi- 
ment which  later,  as  the  agitation  of  the  water  subsides,  settles 
upon  the  bottom,  covering  any  structure  not  destroyed  by  its  mad 
shoreward  rush. 

Here  we  have  a  good  example  of  the  reworking  of  red  sands  and  muds  de- 
rived by  erosion  of  older  red  beds,  which  themselves  were  a  continental  deposit- 
These  reworked  beds  will  possess  the  characters  of  a  seashore  deposit,  with  its 
peculiar  features  impressed  upon  it.  Among  the  older  rock  series  of  the  earth's 
crust,  there  are  several  such  red  stratified  formations  with  marine  features  and 
with  characters  which  appear  in  all  respects  to  conform  to  the  sediments  now 


FIG.  451.  —  Beach  cusps  on  shore  lines  of  Carmel  Bay,  Monterey  County, 
California.     Photo  by  W.  S.  Cooper.     (Courtesy  of  D.  W.  Johnson.) 

forming  in  parts  of  the  Bay  of  Fundy.  These  can  confidently  be  regarded  as 
ancient  products  of  activities  similar  to  those  visible  in  Nova  Scotia  to-day.  One 
of  the  best  known  of  these  is  the  so-called  Medina  sandstone,  which  was  formerly 
extensively  quarried  in  western  New  York,  and  has  been  used  widely  for  flagstone 
sidewalks.  On  this  rock,  not  only  solid  ripple  marks  are  seen  (often  recogniz- 
able in  the  older  sidewalks  of  Buffalo,  Rochester,  and  other  cities  where  this  rock 
is  used  for  flagstones),  but  beautifully  preserved  wave  marks,  shell-protected 
sand  ridges,  and  other  characteristic  beach  features  are  found.  Here,  too, 
are  preserved,  in  some  cases,  the  beach  cusps  similar  to  those  formed  on  modern 
beaches  and  next  to  be  described.  The  beach  here  fossilized  was  a  very  ancient 
one,  belonging  to  the  interior  or  epeiric  seas  which  covered  North  America  in 
Silurian  time. 

Beach  Cusps  and  Ripple  Marks.  —  When  the  waves  strike  the 
shore  directly,  the  front  of  the  beach  will  commonly  become  exca- 
vated into  a  series  of  scallops  or  concavities  with  sharp  ridges  or 


536       Deposition  of  Clastic  Material  in  the  Sea 

cusps  between  them  (Fig.  451).  These  beach  cusps  project  at  right 
angles  from  the  front  of  the  beach  and  form  a  series  of  sand  or 
pebble  ridges  of  triangular  outline  with  the  base  of  the  triangle  on 
the  shoreward,  and  the  apex  on  the  water  side.  They  vary 
greatly  in  their  distance  apart,  according  to  the  size  of  the  waves 
which  produce  them,  but  in  any  given  series  they  have  a  relatively 
uniform  spacing.  "  Where  the  waves  are  about  an  inch  in  height, 
the  cusps  are  from  3  to  9  inches  apart;  where  the  waves  are 
from  one  and  a  half  to  two  and  a  half  feet  high,  they  are  30  to  60 


FIG.  452.  —  Remnant  of  the  concavity  between  two  beach-cusps,  preserved 
because  of  the  consolidation  of  the  old  beach  sand  into  a  solid  rock.  This  rock 
forms  a  part  of  the  Medina  series  (Whirlpool  sandstone)  of  the  Silurian,  and  the 
position  of  the  cusps  indicates  that  the  old  Medina  shore-line  was  approxi- 
mately parallel  to  the  railroad  track  of  to-day.  Niagara  gorge.  (Photo  by 
G.  K.  Gilbert;  from  U.  S.  G.  S.) 

feet  apart,  while  large  storm  waves  build  cusps  100  feet  or  more 
apart  "  (Johnson) .  What  appear  to  be  fossil  beach  cusps  have  been 
found  in  ancient  beach-formed  sandstones,  in  one  of  which  (Medina 
sandstone  of  Silurian  age,  already  referred  to),  exposed  in  the  gorge 
of  Niagara  below  the  whirlpool,  they  are  wonderfully  well  preserved 
(Fig.  452).  Ripple  marks  are  not  common  on  open  beaches,  but  in 
protected  areas  they  are  often  well  developed  (Fig.  453).  They 
are  of  the  asymmetrical  or  current  ripple  type.  (See  further,  p.  550.) 
It  is  difficult  to  get  at  the  internal  structure  of  modern  beaches, 
but  in  a  few  cases,  where  sections  are  exposed  by  wave  cutting,  an 
irregular  but  strongly-marked  stratification  is  seen.  The  layers 


Types  of  Clastic  Deposits  in  the  Sea  537 

are  essentially  horizontal  and  in  some  cases  very  perfect,  while 
in  others  a  certain  amount  of  pinching  out  and  overlapping  of  the 
ends  of  layers  is  seen.  Cross-bedding  on  a  large  scale  is  rarely  in- 
dicated, though  that  due 
to  ripple  marks,  on  a 
scale  of  inches,  is  not 
uncommon. 

The  Bar.  --  On  a 
gently  sloping  coast  the 
large  waves  commonly 
break  at  a  distance  from 
shore,  often  several  miles 
from  it,  where  the  depth 
is  such  that  the  waters 
in  their  rotary  motion 
just  strike  the  bottom. 

The  breaking  wave 
(Fig.  441)  digs  up  the 
sand  of  the  sea-bottom 
and  hurls  it  forward,  de- 


.  453.  —  Beach  near  St.  Monans,  east 
coast  of  Scotland,  showing  current  ripple 
marks  in  protected  embayment  of  shore  with 
numerous  worn  castings  in  the  troughs.  Dr. 
Benjamin  Peach  of  the  Scottish  Geological 
Survey  on  the  right.  (M.  I.  Goldman 
photo.) 


positing  it  in  front  of  the 

line  of  'breakers,  and  by 

repeated    work    of    this 

kind   builds   up   a  sand 

ridge  or  bar  parallel  to 

the  line  of  the  breaking  wave  and  therefore  in  general  parallel  to 

the  shore.     When  this  bar  has  been  built  so  high  that  at  low  tide 

it  is  exposed,  sand  dunes  are  likely  to  be  formed  upon  it,  and  it 

becomes  a  barrier  beach  with  a  lagoon  between  it  and  the  land  as 

already  outlined  (page  330). 

When  the  emerged  bar  or  barrier  beach  is  attached  at  one  end 
to  a  projecting  headland  of  sand  or  gravel,  a  large  amount  of 
material  is  furnished  by  the  erosion  of  this  headland.  If  this 
material  is  carried  by  the  long-shore  current  parallel  to  and  along 
the  shore  of  the  barrier  beach,  it  tends  to  fill  up  the  tidal  inlets 
or  breaks  in  the  continuity  of  this  beach.  The  result  is  that 
barrier  beaches  are  always  most  continuous  in  those  portions 
which  spring  from  the  sand-supplying  headlands,  becoming  less 
so  away  from  this.  On  the  south  shore  of  Long  Island  the  great 
bar  known  as  Fire  Island  Beach  extends  unbroken  for  more  than 


538       Deposition  of  Clastic  Material  in  the  Sea 

40  miles  soumwestward,  while  the  three  main  barrier  beaches 
farther  west,  which  are  not  joined  to  headlands,  are  each  less  than 
10  miles  in  length.  Rockaway  Beach,  still  farther  west,  also 
springs  from  a  sand-supplying  headland,  and  is  rapidly  growing 
in  length,  as  shown  in  the  maps  (Fig.  457  b).  When  the  beaches 
are  shorter,  that  is,  when  the  tidal  inlets  across  the  bar  are  more 
numerous,  the  normal  condition  for  the  growth  of  salt-water 
vegetation  is  best  developed,  and  the  lagoon  is  converted  into  a 
marsh,  as  previously  described  (p.  331).  The  protected  lagoon 
behind  the  continuous  bar,  however,  favors  such  growth  to  a 
lesser  degree,  chiefly  because  there  is  less  rapid  filling  of  the  lagoon 
to  the  required  depth  for  the  growth  of  such  plants,  and  also 
because  this  portion  is  often  freshened  by  the  inpouring  of  stream- 
waters.  Hence  the  protected  lagoon  will  remain  open  for  a  longer 
time.  Conditions  of  this  type  also  exist  on  the  New  Jersey  and 
other  coasts  characterized  by  barrier  beaches. 

From  the  way  in  which  the  sands  are  piled  up  to  form  the  bar,  we  may  infer 
that  its  internal  structure  is  very  irregular,  and  that  if  a  section  were  cut  across 
it,  an  irregular  cross-bedded  structure  would  appear.  Such  a  bar  may,  under 
favorable  conditions,  harden  into  a  rock  by  cementation  of  its  sand  grains  by 
lime  or  other  substance,  as  happens  on  the  coast  of  Brazil,  where  wave-formed 
bars  form  lines  of  solid  "  stone  reefs  "  as  they  are  called.  Again,  near  the  shore 
such  a  bar  may  be  preserved  by  changes  in  sea-level,  which  leave  it  submerged 
and  allow  the  formation  of  other  sediments  over  it.  When  such  a  bar  is  changed 
to  rock,  this  rock  will  exhibit  in  its  structure  the  characteristic  oblique  bedding 
which  is  probably  not  very  different  in  general  appearance  from  the  cross-bedding 
of  wind-laid  sediments. 

As  has  been  briefly  outlined  in  a  previous  chapter  (p.  332),  the 
off-shore  bar  is  not  a  stationary  structure,  but  is  subject  to  con- 
tinued wave  attack,  for  now  that  the  ocean  bottom  has  been 
sufficiently  deepened  in  front  of  the  bar  by  the  excavation  of  the 
sand  from  which  that  bar  was  built  (see  Fig.  273,  p.  330),  the 
waves  will  break  close  to  the  shore  of  the  bar  and  will  therefore 
be  able  to  erode  it.  Such  erosion  is  more  active  of  course  where 
little  material  is  supplied  by  the  long-shore  currents,  and  there- 
fore in  those  portions  of  the  bar  which  are  farthest  away  from  the 
sand-supplying  headlands.  If  the  lagoon  has  been  transformed 
into  a  peat  marsh  (see  p.  332),  the  shore  dunes  will  advance  over 
it,  as  the  bar  is  being  eroded,  the  sand  compressing  the  peat  by 
its  weight.  The  barrier  beach  thus  migrates  shoreward  in  the 


Types  of  Clastic  Deposits  in  the  Sea          539 

course  of  time.  As  the  erosion  on  the  seaward  side  continues,  the 
peat-beds  of  the  lagoon  will  be  reached,  and  become  exposed 
upon  the  shore  at  low  tide.  Such  peat  exposures  are  seen  in  a 
number  of  places  upon  the  Atlantic  coast,  those  most  readily 
accessible  from  New  York  being  just  east  of  Fort  Hamilton  on 
the  Long  Island  coast.  Eventually,  with  continued  erosion  the 


FIG.  454.  — -  Diagram  illustrating  the  development,  migration,  and  complete 
destruction  of  an  off-shore  bar  and  the  lagoon  deposits  behind  it.  (After 
Davis.)  i.  The  bar  just  beginning  to  emerge,  the  lagoon  open,  and  the  shore 
of  the  mainland  marked  by  a  small  erosion  scarp  or  "nip";  2.  Completion 
of  the  barrier-beach  and  formation  of  salt  marsh,  with  lagoon  in  the  center; 
3.  Advance  of  the  beach  and  dunes  on  the  marsh  with  progressive  erosion 
of  the  bar  on  the  seaward  side.  The  lagoon  is  now  converted  into  a  salt  marsh 
intersected  by  a  tidal  stream  N;  4.  Complete  destruction  by  the  waves  and 
currents  of  the  original  bar  —  advance  of  the  beach  and  dunes  over  the 
marsh  which  has  now  become  exposed  upon  the  shore;  5.  Final  stage,  show- 
ing complete  destruction  of  the  lagoon  and  its  deposits,  and  the  advance  of 
the  deeper  water  to  the  original  shore,  which  is  now  being  cliffed  by  the 
waves.  The  dotted  line  shows  the  original  slope  of  the  sea-bottom. 

entire  lagoonal  deposit  is  cut  away,  and  the  sea  is  able  to  attack 
the  mainland  with  the  full  force  of  its  waves,  which  it  was  unable 
to  do  before  the  building  of  the  bar  because  of  the  shallowness 
of  the  water  near  shore.  The  various  stages  in  the  building  and 
migration  of  the  bar  and  barrier  beach  are  shown  in  the  preceding 
diagram  (Fig.  454). 

Bars  of  this  type  are  found  to  form  a  more  or  less  continuous 
series  along  the  Atlantic  coast  of  North  America  from  Long  Island 
southward  (Fig.  455,  p.  540).  They  also  occur  on  the  Massachusetts 
coast,  and  are  characteristic  of  many  other  coasts  as  well.  On 
their  seaward  side  they  exhibit  all  the  normal  features  of  the 
beach.  Because  of  the  exposed  character  of  the  beaches,  the  winter 


540       Deposition  of  Clastic  Material  in  the  Sea 


storms  often  effect  a  considerable  amount  of  cutting,  sometimes 
doing  much  damage  to  buildings  and  other  structures  (Fig.  456). 

The  bars  of  the  Baltic,  and  the 
movements  of  the  sand  dunes 
upon  them  have  been  de- 
scribed in  a  previous  chapter 

(P-  445  >  Figs-  364  a,  fy- 

The  Sand-spit. — The  usual 
on-shore  movement  of  the 
waves  is  seldom  direct,  but 
nearly  always  more  or  less 
oblique.  As  a  result,  the  sands 
(and  pebbles),  moved  up  the 
beach  by  the  swash  or  the 
breaking  wave  itself,  proceed 
in  an  oblique  direction,  but  the 
undertow  carries  this  material 
back  at  right  angles  to  the 
shore  line.  In  consequence, 
the  material  moves  in  a  zigzag 
fashion  along  the  beach,  here 
in  one  direction,  there  in 
another,  according  to  the  con- 
formation of  the  shore.  (See 
p.  523.)  If  the  shore  recedes 
abruptly  into  the  land  as  at  an 
inlet,  the  moving  sands  reach 
water  too  deep  for  their  further 
progress,  and  they  come  to  a 
temporary  rest.  By  constant 
piling  up  of  such  sands,  etc.,  a 
tongue  begins  to  project  into 
the  water,  and  this  continues 
to  grow  in  length  until  a  sand- 
spit  of  considerable  extent  is 
formed.  Where  strong  tidal 
or  wind-driven  shore  currents 
exist,  they  greatly  aid  in  the 


FIG.  455.  —  Map  of  the  coast  of 
North  Carolina,  showing  a  coast  with 
drowned  river  valleys,  modified  by  the 
subsequent  formation  of  off-shore  bars 
and  barrier  beaches,  built  at  a  great 
distance  from  shore  because  of  the 
gently  sloping  character  of  the  sub- 
coastal  plain.  CK,  Corrituck  bar  and 
lagoon;  N,  New  inlet;  H,  Cape  Hat- 
teras;  0,  Ocracoke  Inlet;  L,  Cape 
Lookout.  The  sharp  angle  at  which 
the  bars  meet  to  form  Cape  Hatteras 
has  been  explained  by  some  as  due  to 
deposition  of  debris  derived  from  both 
north  and  south,  in  a  triangle  of  quiet 
water  between  two  adjacent  circling 
currents,  while  others  have  suggested 
the  influence  of  an  initial  projecting 
shore  line  or  shoal. 


transportation  of  the  material  along  the  shore.     Sand-spits  thus 
represent  a  prolongation  of  a  beach  into  the  deeper  water  where 


Types  of  Clastic  Deposits  in  the  Sea          541 


FIG.  456.  —  Ruins  of  summer  residence  at  North  Long  Branch,  New  Jersey. 
(Photo  by  D.  W.  Johnson.) 


FIG.  457  a.  —  A  sand-spit,  Traverse  Bay,  Lake  Michigan.     (Shaler.) 


542      Deposition  of  Clastic  Material  in  the  Sea 


Charts  from  George  R.  Putnam 


FIG  457  b.  —  Westward  growth  of  the  compound  sand-spit  of  Rockaway 
Beach  between  the  years  1835  and  1908  (National  Geographic  Magazine).  In 
1889  the  curved  spit  near  meridian  73°  54'  was  the  westernmost,  although  at 
that  time  it  was  broader  and  extended  across  the  meridian.  It  has  suffered 
erosion  during  the  formation  of  the  later  spits.  The  outermost  spit  has  been 
built  since  1905.  Between  1908  and  1912  the  change  has  been  very  slight. 
The  influences  of  the  tidal  current  through  Rockaway  inlet  in  producing  the 
curvature  of  the  spits  is  well  shown. 

the  shore  has  receded.  According  to  circumstances  and  the  direc- 
tion and  interference  of  currents,  sand-spits  may  be  straight  or 
curved,  or  pass  from  one  form  to  the  other  (Figs.  457  a,  b).  Sand- 
spits  of  great  extent  are  shown  at  the  end  of  the  northward  pro- 
jecting forearm  of  Cape  Cod  (see  Fig.  714),  where  they  have 


Types  of  Clastic  Deposits  in  the  Sea          543 


combined  to  form  the 
Provincetown  headland, 
now  crowned  with  sand 
dunes,  and  in  the  simi- 
lar northward  project- 
ing strip  of  land  on 
the  New  Jersey  coast 
known  as  Sandy  Hook. 
Smaller  sand-spits  are 
built  within  the  waters 
partly  enclosed  by  both 
of  these  larger  dune- 
covered  spits.  Sand- 
spits  are  found  on  many 
other  parts  of  our  coast, 
and  in  our  Great  Lakes 
as  well  (Fig.  457  a).  FlG.  45?  Ct  _  A  Bay-bar,  with  narrow  inlet 
They  are  equally  com-  which  leaves  the  water  of  the  bay  salty, 
mon  on  foreign  coasts.  South  Shore  of  Martkas  Vineyard.  (U.  S.  G.  S.) 
When  the  sand-spit  is  built  completely  across  the  mouth  of  a  bay, 
so  that  only  a  narrow  inlet,  or  none  at  all,  remains,  it  is  called  a 

bay-bar.  If  the  bay  re- 
mains connected  with  the 
sea,  its  waters  will  differ 
little  in  salinity  from 
that  of  the  ocean;  but 
if  the  bar  is  complete 
(Fig.  457  d)j  the  waters 
of  the  cut-off  lagoon  will 
become  fresh  if  the  cli- 
mate is  pluvial,  or  saltier 
than  the  sea  if  the  cut- 
off is  in  an  arid  region. 
The  increase  in  salinity 
may  also  take  place  if 
the  inlet  is  so  shal- 
low and  narrow  that 
FIG.  457  d.  —  A  Bay-bar  completely  cut-  there  is  no  free  inter- 
tmg  off  a  bay  and  converting  it  into  a  closed  i,  r 
shore-pond.  South  Shore  of  Marthas  Vine-  ChangC  °f  Waters>  a  con- 
yard.  (U.  S.  G.  S.)  dition  characteristic  of 


544      Deposition  of  Clastic  Material  in  the  Sea 


tideless  waters  like  the  Caspian,  where  the  Kara  Bugas  Gulf, 
described  on  p.  239,  forms  a  typical  example.  In  many  such  bar- 
enclosed  bays,  salt  deposits  are  forming,  while  in  others,  situated 

in  pluvial  climates,  fresh- 
water vegetation  and 
fresh-water  animals  re- 
place the  marine  types 
which  formerly  inhabited 
the  bay.  In  this  way  may 
be  readily  explained  the 
association  of  fresh-water 
with  marine  deposits, 
which  is  sometimes  found 
in  the  older  rock  series. 
The  bay-bar  may  be 
formed  by  a  single  sand- 
spit  growing  in  one  di- 
rection (Fig.  457  d)  or  by 
two  spits  growing  from 
opposite  sides.  The  con- 
ditions necessary  for  the 
formation  of  such  bay- 
bars  are:  sufficient  sup- 
ply of  detritus  or  suffi- 


FIG.  457  e.  —  Nahant,  a  rocky  headland 
tied  to  the  shore  by  a  sand  beach  or  tombolo. 
U.  S.  G.  S. 


cient  strength  of  the  long-shore  current,  or  both,  to  overcome  the 
effects  of  the  currents  flowing  into  and  out  of  the  bay,  so  that  the 
equilibrium  is  not  established  until  the  bay  is  nearly  or  entirely  closed. 

Sand-spits  may  grow  between  islands,  eventually  tying  them 
together,  or  between  an  island  and  the  mainland,  tying  the  former 
to  the  latter.  Such  a  connecting  bar  has  been  called  a  tombolo. 
It  is  well  illustrated  by  the  bar  which  unites  Great  and  Little 
Nahant,  and  by  that  which  ties  this  group  to  the  mainland  at 
Lynn,  Mass.  (Fig.  457  e).  It  is  also  shown  in  the  beach  which 
unites  Marblehead  Neck  and  the  mainland  (Fig.  716)  and  in  the 
Nantasket  beaches  (Figs.  727-730).  These  examples  will  be  more 
fully  described  in  a  later  chapter.  A  complex  shore-form  produced 
by  sand-spits  is  shown  in  Fig.  458. 

Cape  Canaveral,  on  the  eastern  coast  of  Florida,  illustrates  a 
striking  series  of  modifications  produced  in  the  coast  line  by  beach, 
bar,  and  spit  building  by  waves  and  currents  (Fig.  459).  The 


Types  of  Clastic  Deposits  in  the  Sea          545 

original  coastline,  shown  on  the  left  of  the  figure,  was  nearly 
straight,  being  the  shore  of  a  coastal  plain.  In  front  of  this  was 
built  a  cuspate  foreland,  i.e.,  a  triangular  projecting  land-mass  of 
low  relief,  with  the  apex  of  the  triangle  pointing  seaward.  A 
body  of  water,  known  as  Indian  River,  lies  between  the  mainland 
and  the  cuspate  foreland,  which  terminates  in  what  is  now  called 
the  False  Cape.  An  irregular  winding  stream  also  bisects  this 
foreland,  extending  nearly  to  the  apex.  This  cuspate  foreland 
consists  of  a  series  of  beaches  or  bars,  built  one  in  front  of  the 


FIG.  458.  —  Tidal  lagoon  formed  by  sand-spit  at  the  mouth  of  San  Luis 
Obispo  Creek,  California.  Oxbow  in  the  foreground.  (G.  W.  Stose,  photo ; 
from  U.  S.  G.  S.) 

other  because  of  an  excessive  supply  of  sand,  both  from  the  north 
and  from  the  south,  the  relative  uniformity  in  the  amount  supplied 
from  both  directions  having  determined  the  symmetry  of  outline 
of  the  old  foreland  of  the  False  Cape.  Such  a  broad  plain,  formed 
of  a  succession  of  beach-ridges  with  shallow  depressions  or  swales 
between,  is  also  called  a  beach-plain.  The  swales  are  sometimes 
marshy,  or  may  be  occupied  by  narrow  lagoons;  for  the  most 
part  they  are  dry.  The  ridges  are  commonly  dune-covered. 

Such  beach-ridges  are  formed  by  direct  building  of  new  bars 
in  front  of  the  older  ones  by  successive  storm  waves ;  by  addition 


546      Deposition  of  Clastic  Material  in  the  Sea 


FIG.  459.  —  Map  of  Cape  Canaveral,  Florida.  (U.  S. 
Coast  Survey  chart.)  Showing  formation  of  bars, 
beach  plains,-  spits,  etc.  For  location  see  map,  Fig. 
433>  P-  512.  (Copied  from  de  Martonne.) 


of  material  along 
the  whole  front, 
because  of  diver- 
gence of  shore- 
currents,  which 
then  assume  a 
greater  cross- 
section  and  a  di- 
minished velocity, 
and  in  conse- 
quence permit  de- 
position of  trans- 
ported debris;  by 
the  extension  of 
su  c  ce  s  sively 
formed  embank- 
ments ;  or  by  the 
interaction  of  all 
three  processes. 
Again  they  may  be 
produced  by  nor- 
mal bar-building, 
one  in  front  of  the 
other,  because  an 
excess  of  debris  is 
supplied  by  the 
shore-currents, 
which  become  de- 
flected as  the  re- 
sult of  this  build- 
ing of  successive 
bars. 

A  pronounced 
migration  of  the 
site  of  bar-build- 
ing has  produced 
the  later  addi- 
tions, which  form 
the  present  Cape 
Canaveral.  This 


Types  of  Clastic  Deposits  in  the  Sea          547 


is  separated  from  the  older  cape  by  a  broad  lagoon  called  the 
Bananc  River,  while  a  similar  lagoon  lies  between  the  outer  bar 
and  the  old  cape  on  the  north.  In  front  of  the  present  cape  is 
built  a  new  series  of  sand-spits  and  bars  which  are,  for  the  most 
part,  still  submerged. 

Estuarine  Deposits.  —  Estuaries  are  indentations  in  the  sea- 
coast  where  large  rivers  enter  the  sea.  They  may  indeed  be  re- 
garded as  the  broadened  mouths  of  rivers,  and  are  often  formed  by 
the  sinking  of  the  land  which  is  traversed  by  the  lower  river  valley, 


52' 


CHANNEL 


FIG.  460  a.  —  Map  of  the  Estuary  of  the  Severn  River,  England. 

so  that  the  sea  has  access  to  this  valley.  The  lower  Hudson  River 
is  an  example  of  such  a  submerged  valley  cut  in  a  rocky  land  sur- 
face, and  because  of  that  it  has  nearly  parallel  sides.  Most  estuaries, 
however,  have  their  sides  diverging  seawards  so  that  their  form  is 
more  or  less  funnel-shaped.  The  estuaries  of  the  River  Severn 
on  the  west  coast  of  England  (Fig.  460  a),  and  of  the  La  Plata 
(Figs.  460  b,  c]  in  South  America  are  of  this  type.  The  latter  is  125 
miles  long  and  receives  the  waters  of  the  Parana  and  the  Uruguay 
rivers,  and  the  currents  of  these  rivers  come  into  periodic  conflict 
with  those  of  the  entering  tides  from  the  Atlantic.  Where  the  two 


548      Deposition  of  Clastic  Material  in  the  Sea 

currents  neutralize  each  other,  sedimentation  is  most  pronounced, 
the  maximum  being  between  10  and  20  miles  above  the  mouth  of 
the  estuary,  where  slack  water  conditions  may  continue  for  hours. 
From  the  sediment  dropped  here,  chiefly  the  mud  brought  by  the 
rivers,  submerged  banks  are  built  up  which  may  eventually  rise 
into  islands.  At  Buenos  Aires,  constant  dredging  is  necessary, 
to  keep  the  channels  open  for  navigation.  In  the  narrower 
channels  between  the  islands,  the  tidal  current,  aided  by  the 
river  currents,  will  effect  scouring  and  little  or  no  sedimentation 


Bdcwofr. 


FIG.  460  b.  —  Map  of  the  La  Plata  Estuary. 


takes  place.  The  original  length  of  the  La  Plata  estuary  was  325 
miles,  but  about  two-thirds  of  this  has  been  filled  up  by  sediments. 
In  these  sediments  fine  muds  prevail,  and  with -them  are  buried 
organic  remains  brought  by  the  rivers. 

Marine  organisms  of  floating  or  swimming  habit  may  also  enter 
with  the  tide,  and  being  killed  by  the  freshening  of  the  water, 
their  remains  become  embedded  in  the  sediments.  Thus  such 
sediments  contain  a  mixture  of  marine  and  fresh-water  organisms, 
though  on  the  whole  the  organic  remains  are  few  in  kind  as  well  as 
number. 

The  floor  of  the  Hudson,  too,  has  been  built  up  in  places  by  mud 
deposits  to  a  considerable  extent.  In  these  muds  have  been  found 


Types  of  Clastic  Deposits  in  the  Sea          549 

the  remains  of  marine  animals  of  a  dwarfed  character,  this  dwarfing 
being  due,  according  to  Shimer,  to  the  freshening  of  the  water. 

The  waters  of  the  estuary  of  the  Severn,  southwestern  England,  at  high  tide 
are  thick  and  opaque  with  tawny-colored  sediments,  while  at  ebbing  tide  a 
broad  expanse  of  shining  mud-flat  is  revealed  along  the  coast.  Owing  to  the 
heavy  load  of  silt  in  the  water,  the  boundary  line  between  the  water  and  the 
mud-flat  is  not  easily  distinguishable,  especially  from  a  distance.  Part  of  the 
mud  is  brought  down  by  the  river,  part  of  it  is  derived  by  wave  and  current 
erosion  along  the  banks  and  is  carried  into  the  estuary  by  the  tide.  With  the 
muds  and  fine  sands  many  floating  marine  organisms,  especially  Foraminifera, 


FIG.  460  c.  —  View  near  the   mouth  of  the   Estuary   Rio   de  La    Plata    (at 
Montevideo).     A  mud  bank  is  shown  in  the  foreground. 

and  the  fragments  of  larger  organisms  are  brought  into  the  estuary  and  buried 
in  the  muds,  while  farther  up  the  remains  of  fresh-water  sponges  and  of  diatoms 
are  found.  Much  vegetable  matter  also  occurs  in  the  older  muds  of  the  estuary. 
A  special  case  of  river-borne  muds  is  seen  in  the  Bay  of  Danzig,  on  the  Baltic 
Sea,  where  the  river  Vistula  brings  black  carbonaceous  muds  from  the  Russian 
plains  (the  tschernosem,  see  p.  459).  This  bay  is  of  low  salinity  and  so  corre- 
sponds in  character  to  the  water  of  an  estuary.  Extensive  deposits  of  deep  black 
mud,  inclosing  dwarfed  marine  organisms,  are  formed  upon  the  floor  of  this  bay, 
and  they  illustrate  one  way  in  which  black  shales  with  dwarfed  marine  organisms 
may  originate. 

Deposits  in  the  Of -Shore  Shallow  Waters 

The  deposits  found  between  the  shore  and  the  edge  of  the  con- 
tinental shelf,  estimated  by  Murray  as  covering  at  present  some 
10,000,000  square  miles,  consist  mainly  of  the  finer  sands,  and  more 
rarely  of  the  muds  which  are  washed  out  from  the  shores,  together 
with  organic  and  chemical  deposits  of  the  type  outlined  in  previous 
chapters.  As  we  have  seen,  however,  on  coasts  which  fall  off 


550      Deposition  of  Clastic  Material  in  the  Sea 


rapidly,  and  which  are  bordered  by  high  mountain  areas,  coarse 
stream-transported  material  may  be  deposited  beyond  the  shore 


zone. 


Since  this  shallow  portion  of  the  sea  is  the  great  theater  of  organic  activity,- 
the  regions  where  most  of  the  shell-bearing  and  other  hard-structure  secreting 
organisms  live,  it  is  evident  that  all  deposits  formed  here  are  likely  to  inclose 
the  remains  of  these  animals  and  will  therefore  be  highly  fossiliferous.  Again, 
large  portions  of  the  floor  may  be  covered  by  sands  consisting  of  the  fragments 
of  shells  and  other  calcareous  organic  structures,  as  is  the  case  on  the  floor  of 
large  parts  of  the  shallow  sea  which  surrounds  the  British  Isles.  In  warmer 
waters  coral  sands  and  muds  worn  from  the  reefs  there  building  will  form  an 
important,  if  not  the  whole  of  the  bottom  deposit. 

In  general,  deposits  formed  in  these  shallower  waters  are  well 
stratified,  but  cross-bedding  and  irregularity  of  structure  are 
absent.  Around  coral  reefs,  however,  the  slope  of  the  layers  is 
often  steep,  being  sometimes  as  high  as  60  degrees,  and  inter- 
fingering  of  the  clastic  and  organic  deposits  produce  an  irregular 

structure.  (See 
p.  304.)  Ripple 
marks  are  com- 
mon in  the  shallow 
parts  and  may 
even  be  developed 
at  considerable 
depths.  These 

FIG.  461.  —  Diagram  illustrating  the  forms  of  ripple-  ripples  differ  in 
marks  and  of  the  reverse  impressions  of  the  same,  character  from 
a,  reverse  of  current  ripple,  original  position  inverted ; 
6,  form  of  same;  c,  form  of  oscillation  ripple;  d,  re- 
verse of  same,  original  position  inverted.  Note  con- 
trasting form  of  two  ripples  in  center  (6,  c).  Also 
note  general  resemblance  of  a  and  c,  and  b  and  d,  ex- 
cept for  asymmetry  of  a  and  b. 


those   formed   on 
the     shore     (Fig. 

4S3»  P-    537)  and 
in    terrestrial   de- 


posits (Fig.  371, 
P-  453)  >  consisting  of  sharp,  symmetrical  ridges  between  regularly 
concave  troughs  (Fig.  461  c).  They  represent,  indeed,  the  form 
of  the  wave  on  a  small  scale,  and  are  due  to  the  oscillatory  move- 
ment of  the  water  caused  by  wave  motion.  Such  oscillation 
ripples,  as  they  are  called,  are  readily  distinguished  from  the 
current  ripples  by  their  form,  but  when  a  relief  impression  of 
them  is  formed,  as  in  the  bottom  of  a  layer  of  sediment  deposited 
over  them  and  perserved  by  hardening,  this  relief  is  not  so 


Types  of  Clastic  Deposits  in  the  Sea          551 

readily  distinguished  from  the  normal  current  ripples,  as  the 
difference  in  angle  of  slope  of  the  two  sides  of  the  latter  is  not 
always  very  great  (Fig.  461  b,  d}.  The  significance  of  this  will  ap- 
pear in  the  discussion  of  the  structure  of  ancient  sediments. 


Deposits  in  the  Bathyal  Zone 

Beyond  the  edge  of  the  continental  shelf  we  meet  the  zone  of 
mud  deposits,  though  where  cpnditions  are  favorable  this  may 
extend  into  shallower  water  as  well.  The  principal  clastic  de- 
posits of  terrigenous  origin  in  the  bathyal  zone  are  blue  muds,  red 
jnuds,  green  muds,  and  green-sands.  Volcanic  sands  and  muds, 
either  derived  from  volcanoes  on  the  land  or  from  submarine  vol- 
canoes, also  cover  wide  areas,  and  besides  these  there  are  frequently 
deposits  of  coral  sands  and  coral  muds  or  elastics  of  oceanic  origin. 

The  Blue  Muds.  —  These  are  more  often  slate-colored,  and  they  are  the 
most  widely  distributed  of  the  mud  deposits,  covering  an  area  estimated  at 
14,500,000  square  miles  in  the  several  oceans  of  the  world,  including  the  Arctic 
Ocean.  (See  map,  Fig.  198,  p.  277.)  In  the  Gulf  of  Naples  the  mud  begins 
at  a  depth  of  15  meters,  but  its  greatest  distribution  is  below  the  2oo-meter  (or 
loo-fathom)  line,  while  the  greatest  depth  from  which  it  has  been  obtained  is 
5120  meters.  Its  upper  part  is  commonly  stained  red  or  brown  by  iron  oxide 
or.  hydrate,  but  the  lower  layers  are  bluish.  In  composition  it  varies  greatly ; 
sometimes  as  much  as  97  per  cent  is  clay,  while  at  other  times  this  may  fall  to 
1 6  per  cent.  In  like  manner  the  content  of  carbonate  of  lime  varies  greatly, 
ranging  from  almost  nothing  to  35  per  cent.  Quartz  in  fine  grains  or  as  rock 
flour  is  a  characteristic  constituent,  and  other  rock-forming  minerals  are  also 
present.  Some  parts  of  this  clay  are  rich  in  shells  and  other  hard  structures  of 
marine  organisms,  especially  Foraminifera,  while  in  others  these  are  compara- 
tively rare.  The  fresh  mud  generally  smells  strongly  of  sulphureted  hydrogen, 
due  apparently  to  the  decomposition  of  organic  matter.  By  combination  with 
the  iron  in  the  mud,  iron  sulphide  is  formed,  which  may  separate  out  as  crystals 
or  specks  of  the  mineral  pyrite. 

The  Red  Muds.  —  Opposite  the  mouths  of  tropical  rivers  such  as  the  Ama- 
zon and  the  Yang-tse-Kiang,  the  floor  of  the  sea  is  covered  by  a  red  mud  which 
is  derived  from  the  lateritic  soil  of  the  tropical  regions.  (See  p.  403.)  Its  red 
color  is  due  to  the  high  proportion  of  iron  oxide  present,  a  constituent  charac-^ 
teristic  of  the  laterite.  As  in  the  case  of  the  blue  mud,  the  composition  of  the 
red  mud  varies,  pure  clay  forming  from  28  to  68  per  cent  of  its  mass,  while 
the  content  of  carbonate  of  lime  ranges  from  6  to  60  per  cent.  Quartz  is  also 
a  characteristic  constituent.  The  area  covered  by  this  mud  in  the  present  oceans 
is  about  100,000  square  miles.  As  in  the  case  of  the  blue  mud,  the  red  mud  also 
contains  the  remains  of  marine  organisms,  and  with  these  may  readily  be  mingled 
organic  bodies  that  were  brought  from  the  land  by  the  rivers. 


55 2      Deposition  of  Clastic  Material  in  the  Sea 

Green  Mud  and  Green-sand.  —  In  the  upper  slope  of  the  bathyal  zone,  from 
the  edge  of  the  continental  shelf  to  a  depth  of  1000  fathoms  or  over  (180-2300 
meters),  is  found  a  deposit  of  mud  of  a  character  similar  to  that  of  the  two  pre- 
ceding types,  except  that  it  is  colored  green  by  the  mineral  glauconite,  which 
is  a  hydrous  silicate  of  potassium  and  iron,  often  with  some  aluminum.  This 
mineral  forms  in  the  sea  under  the  influence  of  the  decaying  organic  matter 
from  the  elements  of  the  terrigenous  muds  brought  there.  Glauconite  forms 
abundantly  in  the  shells  of  dead  Foraminifera,  which  it  fills  completely,  and  on 
solution  of  the  shell  it  remains  behind  as  grains  of  glauconite  sand.  This  con- 
stitutes the  green-sand ;  between  this  and  the  green  muds  the  difference  is  mainly 
one  of  fineness  of  grain.  The  green  muds  contain  from  24  to  48  per  cent  of  clay, 
and  their  content  of  carbonate  of  lime  may  be  as  high  as  56  per  cent. 

Green-sands  sometimes  constitute  extensive  formations  in  the  older  rock 
series  of  the  earth,  especially  those  of  the  Cretaceous  period.  They  are 
probably  not  always  formed  in  the  same  way  nor  always  near  the  edge  of 
the  continental  shelf.  Some  of  them  contain  an  abundance  of  coarse  clastic 
quartz  and  feldspar  grains  and  have  the  appearance  of  having  been  formed  in 
shallow  water. 

Volcanic  Sands  and  Muds.  —  These  are  especially  abundant  in  the  neigh- 
borhood of  volcanic  islands  and  off  those  coasts  which  are  characterized  by 
strong  volcanic  activities.  They  grade  into  other  types  of  deposits  and  may  be 
mingled  with  calcareous  sands  and  muds  to  such  an  extent  that  on  hardening 
they  form  impure  limestone  deposits. 

Coral  Sands  and  Muds.  —  As  these  are  derived  by  the  erosion  of  coral  reefs, 
their  distribution  is  chiefly  in  the  neighborhood  of  such  structures.  They 
merely  represent  the  finer  products  of  reef  erosion  carried  into  the  deeper  water, 
where  they  mostly  form  beds  of  lime-mud,  which  has  much  the  characteristics 
of  the  other  muds  except  for  the  difference  in  composition.  Such  muds  and 
fine  sands  are  especially  abundant  on  the  floor  of  the  tropical  portions  of  the 
Pacific  Ocean. 


Deposits  of  the  Deep  Sea 

Prevailing  Types.  —  We  have  already  seen  that  the  deeper  parts 
of  the  sea,  the  abyssal  regions,  are  chiefly  characterized  by  the 
accumulation  of  organic  structures  (foraminifers,  pteropods,  cocco- 
liths,  diatoms,  radiolarians,  etc.),  derived  from  organisms  which 
spend  their  lives  in  a  floating  manner  in  the  upper  or  pelagic  dis- 
trict of  the  sea.  With  these  may  be  mingled  the  bones  and  teeth  of 
aquatic  animals  (whales,  sharks,  etc.)  and  the  volcanic  dust  and 
pumice  fragments  which  are  carried  out  over  such  portions  of  the 
ocean  and  eventually  reach  the  bottom.  Clastic  material  is,  how- 
ever, not  common  in  the  deep  sea,  though  some  of  the  sands  or 
muds  deposited  in  the  bathyal  zone  may  be  carried  to  the  deeper 
waters. 


Summary  of  Structures  of  Marine  Clastics      553 

Red  Clay.  —  In  some  of  the  deeper  parts  of  the  oceans,  however,  below  depths 
of  2400  to  2600  fathoms,  there  is  a  peculiar  deep-sea  red  clay  which  covers  an 
area  aggregating  51,500,000  square  miles.  (See  map,  Fig.  198,  p.  277.)  In  color 
this  material  ranges  from  brick-red  to  chocolate  tints,  though  some  samples 
have  a  bluish  color.  When  fresh  it  is  soft,  plastic,  and  greasy,  but  it  becomes 
very  hard  on  drying.  It  is  chiefly  derived  from  the  decomposition  of  volcanic 
dust  which  has  slowly  settled  to  these  depths,  along  with  minute  quantities  of 
earthy  matter  left  on  solution  of  the  foraminiferan  shells,  which  never  reach  this 
depth  but  are  dissolved  before  they  sink  so  far.'  Volcanic  glass  in  minute  frag- 
ments is  an  abundant  constituent,  and  the  silicious  structures  of  the  radiolarians 
constitute  an  important  element  of  its  mass.  Manganese  nodules  and  those 
of  other  minerals  are  also  present. 

This  clay  accumulates  with  exceeding  slowness  upon  the  floor  of  the  deep  sea, 
so  that  the  teeth  of  sharks  which  have  lain  upon  the  ocean  floor  since  Tertiary 
time  have  in  many  cases  not  been  so  deeply  covered  by  it,  but  that  by  dredging 
they  may  be  obtained  alongside  of  those  of  species  still  living  in  these  oceans. 

SUMMARY  or  STRUCTURES  OF  MARINE  CLASTICS 

All  marine  elastics  are  well  stratified,  but  cross-bedding  structure 
is  found  only  in  the  deposits  of  steep  beaches,  bars,  and  spits. 
There  is,  however,  an  inclined  stratification  in  the  calcareous  de- 
posits around  coral  reefs  (Fig.  228,  p.  304).  Ripple  marks  are 
characteristic  of  all  the  shallower  deposits. '  They  are  commonly 
of  the  oscillation  type  with  symmetrical  troughs  separated  by  sharp 
ridges  (Fig.  461  c).  The  current  type  of  ripple  occurs,  how- 
ever, on  beaches  and  in  very  shallow  water  where  the  influence 
of  currents  is  manifested  (Fig.  453,  p.  537) .  On  the  beaches  we  also 
find  the  cusps,  which  are  sharp  ridges  projecting  at  right  angles 
to  the  shore  and  separated  by  concavities  (Fig.  451,  p.  535). 

Rill  marks  formed  by  the  running  off  of  waters  are  common  upon 
beaches  and  tidal  flats,  but  their  character  is  generally  the  reverse 
of  those  formed  on  inland  mud  surfaces.  On  the  coast,  rill  marks 
are  formed  by  the  union  of  many  minor  rills  into  larger  ones, 
being  thus  more  of  the  nature  of  miniature  river  systems,  while 
the  rill  marks  on  inland  flats  are  comparable  to  the  distributaries 
of  rivers  on  the  delta.  This  type,  however,  also  occurs  upon  the 
beach  (Fig.  462).  Other  structures  restricted  to  the  beaches  are 
wave  marks,  sand-ridges  behind  shells  and  pebbles,  and  shallow 
gougings  of  the  sands  and  muds  around  various  objects.  Certain 
markings  due  to  shore  ice  and  ice  crystal  growth,  and  channels 
due  to  dragging  of  seaweeds  and  other  objects  over  the  floor  of 
shallow  water  may  also  be  mentioned.  Mud-cracks,  raindrop  and 


554      Deposition  of  Clastic  Material  in  the  Sea 


FIG.   462.  — Rill  marks   upon   the   coast    of 
Rockaway  Beach.     (M.  O'Connell  photo.) 


footprint  impressions,  on  the  other  hand,  so  characteristic  of 
river  flood-plains  and  playa  surfaces,  are  rarely  preserved  in  sea- 
coast  deposits,  though 
this  may  be  the  case  in 
special  regions  where 
muddy  surfaces  are  ex- 
posed for  long  intervals 
between  highest  tides. 

Above  all,  however, 
marine  elastics  are  char- 
acterized by  the  presence 
of  organic  remains  (shells, 
etc.)  which  are  seldom 
absent  over  wide  areas 
or  through  successive 
layers.  Indeed,  such  or- 
ganic remains  may  be  re- 
garded as  the  surest  in- 
dication of  the  marine  origin  of  older  sediments  and  very  often 
the  only  reliable  one.  If  the  remains  of  marine  organisms  are 
absent  from  a  formation  of  wide  extent  and  great  thickness,  it 
may  reasonably  be  concluded  that  that  formation  was  not  de- 
posited under  normal  marine  conditions. 

LATERAL  CHANGES  IN  FACIES  AND  OVERLAP  RELATIONS 
OF  MARINE  CLASTICS 

Change  in  Fades 

When  we  consider  the  deposits  formed  in  the  sea  at  a  given  time, 
it  is  evident  that  they  cannot  be  of  uniform  lithological  character 
throughout.  Close  to  the  shore,  pebbles  may  accumulate,  while 
at  the  same  time  sands  are  deposited  farther  out  to  sea,  and  at  a 
still  greater  distance  muds  are  forming.  At  another  place  the  shore 
deposit  may  be  wholly  quartz  sand,  but  as  we  pass  seaward,  es- 
pecially in  tropical  regions,  the  sands  may  become  more  calcareous, 
for  the  material  worn  from  a  coral  reef  at  some  distance  from  shore 
may  become  mingled  with  the  shore-derived  terrigenous  quartz 
sands.  Finally,  at  greater  distances  from  shore,  the  quartz  grains 
may  disappear  and  lime-sands  and  lime-muds  will  alone  be  de- 
posited. Thus  when  we  consider  the  deposits  formed  in  a  given 


Lateral  Changes  in  Fades 


555 


short  period  of  time  as  a  whole,  we  find  a  lateral  gradation  in  facies 
from  the  shore  outward.  Either  the  change  is  from  coarse  terrig- 
enous deposits  at  the  shore  to  fine  ones  at  a  distance  from  shore, 


FIG.  463  a.  — Diagram  Illustrating  change  in  facies  of  marine  elastics,  from 
sands  and  muds  near  the  shore  to  calcareous  beds  near  coral  reefs,  etc.  Note 
also  the  overlap  of  the  successive  formations.  (From  Principles  of  Stratig- 
raphy.) 

or  it  is  from  terrigenous  sands  to  ocean-derived  (coral)  sand  and 
muds  (Fig.  463  a).  Such  a  lateral  gradation  in  character  of 
material  or  in  facies  is  practically  universal  for  marine  deposits. 

Overlaps  and  Off-Laps 

Progressive  Overlap.  —  When  the  sea  slowly  advances  upon  a 
sinking  land,  it  is  obvious  that  from  period  to  period  of  deposition 
there  is  a  shifting  of  the  facies  in  the  direction  of  transgression. 
In  the  case  of  varying  terrigenous  deposits,  the  pebble  zone  of  the 
second  stage  of  deposition  comes  to  rest  farther  up  on  land  than 


FIG.  463  b.  —  Section  to  show  normal  progressive  overlap  of  the  formations 
a-c  and  their  shoreward  change  from  muds  to  sands  and  pebbles.  (From 
Principles  of  Stratigraphy.) 

that  of  the  first,  while  the  sand  zone  of  the  second  stage  is  shifted 
so  as  to  rest  more  or  less  upon  the  pebble  zone  of  the  first,  and  the 
mud  zone  of  the  second  in  part  upon  the  sand  zone  of  the  first. 
With  further  advance  of  the  sea,  a  continued  shifting  of  the  zones 
in  the  same  direction  takes  place,  until  the  mud  zone  of  the  last 
stage  may  rest  vertically  above  the  pebble  zone  of  the  first,  being 
separated  from  it,  however,  by  a  sandy  zone  (Fig.  463  b).  Similar 


556      Deposition  of  Clastic  Material  in  the  Sea 

conditions  occur  when  the  lateral  change  in  facies  is  from  quartz 
sand  to  coral  sands  or  muds  (Fig.  463  c). 

It  is  further  evident  that  if  we  separate  the  deposit  formed  simul- 
taneously at  the  different  periods  or  stages  of  deposition  we  shall 
find  not  only  a  lateral  change  in  facies  in  each,  and  essentially  the 
same  type  of  lateral  change,  but  we  shall  also  find  that  the  deposits 
of  each  successive  period  or  stage  extend  farther  up  on  the  old  land 
than  did  those  of  the  preceding  period  or  stage.  In  other  words, 
the  deposits  of  successive  stages  progressively  overlap  one  another 
in  the  direction  of  sea  advance.  This  is  illustrated  in  the  diagrams 
(Figs.  463  b,  c),  in  which  the  periods  of  deposition  are  lettered  from 
below  upward,  or  in  the  order  of  formation. 

It  must  be  recognized,  however,  that  such  overlaps  are  of  signif- 
icance only  when  they  extend  over  wide  areas  and  long  time  inter- 
vals, for  though  now  and  then  such  structures  can  actually  be  seen 


FIG.  463  c.  —  Section  to  show  normal  progressive  overlap  of  marine  forma- 
tions a-c,  and  their  shoreward  change  from  limestones  to  sandstones. 

in  small  sections,  it  usually  requires  the  comparison  of  sections  at 
a  distance  from  one  another  to  bring  out  this  relationship.  More- 
over, it  is  necessary  to  have  some  means  by  which  the  different 
divisions  of  the  series  can  be  separated  from  one  another,  and  by 
which  the  same  division  can  be  traced  from  point  to  point.  In  a 
series,  the  formation  of  which  occupied  a  considerable  period  of  time, 
the  organic  remains  or  fossils  serve  such  a  purpose  of  identification, 
for  the  character  of  the  organisms  undergoes  a  progressive  change 
from  period  to  period,  and  at  each  period  the  organisms  peculiar 
to  it  are  distributed  over  much  if  not  the  whole  of  the  area  in  which 
the  sediments  of  such  an  overlapping  series  are  formed.  Thus,  in 
the  following  illustration  (Fig.  464) ,  we  may  assume  that  each  one 
of  the  divisions  from  a  to  c  had  its  own  peculiar  organic  remains 
which  form  its  "  index  fossils,"  and  that  the  index  fossils  of  each  are 
distributed  horizontally  over  the  entire  extent  of  the  division. 
If  we  now  examine  the  deposits  of  such  an  overlapping  series, 


Overlaps  of  Marine  Clastics 


357 


which  we  will  assume  have  been  consolidated  into  rock  and  have 
been  subjected  to  considerable  erosion,  we  may  find  the  following 
conditions  along  a  line  at  three  points,  each  separated  from  the  other, 
we  will  assume,  by  an  interval  of  fifty  miles.  The  only  exposure  at 
locality  A  is  a  cliff  of  sandstone  resting  with  a  basal  pebble  bed 
upon  the  crystalline  rocks  (granite  or  gneiss).  At  B,  fifty  miles 
distant,  we  find  a  river  valley  cut  into  horizontal  beds  down  to  the 
underlying  crystalline  rocks.  Here  we  find  again  a  sandstone  and 
basal  pebble  bed  resting  upon  the  crystallines,  and  we  might  as- 
sume that  this  was  the  same  sandstone  which  we  found  resting  upon 
the  crystallines  in  section  A.  This  assumption  would,  however, 
be  erroneous,  as  we  shall  see.  Overlying  this  lowest  series  are 
sandy  muds  and  finally  beds  of  clay  rock.  Fifty  miles  farther  in 


FIG.  464.  —  Natural  and  columnar  sections  at  each  end  and  at  the  center  of 
a  line  100  miles  long  and  at  right  angles  to  the  original  trend  of  the  shore.  The 
progressive  overlap  and  change  in  facies  is  shown.  Note  that  in  each  section 
the  clastic  formation  next  above  the  crystallines  is  a  sandstone,  but  that  it 
belongs  to  different  formations  in  each. 

the  same  direction  we  meet  with  the  outcrops  at  C,  where  the  beds 
have  been  disturbed  (by  faulting  and  tilting;  see  Chapter  XIX), 
and  here  we  find  a  still  more  extensive  series  of  stratified  beds 
resting  upon  the  crystallines,  but  beginning  again  with  a  sand  and 
pebble  rock*  followed  by  a  succession  of  beds  very  similar  to  those 
seen  at  B,  but  with  an  additional  formation  at  the  top  which  con- 
tains more  or  less  lime. 

If  we  now  study  the  index  fossils  of  the  successive  divisions  a 
to  c  at  C  and  then  compare  them  with  the  index  fossils  found  in  the 
two  divisions  shown  in  the  river  gorge  at  B,  we  shall  recognize  that 
the  two  divisions  at  B  correspond  only  to  the  upper  two  (b  and  c) 
at  C,  although  in  their  physical  characters,  that  is,  their  petrology, 
they  resemble  the  lower  two.  In  other  words,  the  lowest  division 
at  B  has  the  same  fossils  as  the  second  division  at  C,  although 


558      Deposition  of  Clastic  Material  in  the  Sea 

in  its  general  character  it  resembles  the  first  division  at  C.  In 
like  manner  the  second  division  at  B  corresponds  to  the  third 
division  at  C  in  its  index  fossils,  though  in  its  physical  characters 
it  corresponds  more  closely  to  the  second  division  at  C.  Now  ex- 
tensive studies  have  shown  that  index  fossils  are  more  reliable 
guides  than  are  the  physical  (lithologic)  characters  of  the  forma- 
tions, and  from  this  we  conclude  that  the  formations  exposed  at 
B  represent  divisions  b  and  c  as  shown  at  C,  formation  a  being  ab- 
sent at  B  through  overlap,  while  the  physical  characters  or  facies 
of  b  and  c  have  changed  laterally.  From  the  character  of  the  index 
fossils  we  further  conclude  that  the  only  bed  shown  at  locality  A 
corresponds  to  bed  c  at  C  and  B,  though  in  its  physical  characters 
it  resembles  bed  b  at  B  and  bed  a  at  C.  Beds  a  and  b  are  thus  en- 
tirely absent  at  A,  having  been  overlapped  by  bed  c.  In  the 
chapters  on  historical  geology,  in  the  later  part  of  this  book,  we 
shall  give  several  examples  of  such  overlaps. 

Progressive    Offlapping.  —  In   the   foregoing   section   we  have 
dealt  with  the  arrangement  of  strata  produced  in  a  transgressing 


FIG.  465.  —  Section  to  illustrate  the  arrangement  and  change  in  facies  in  an 
offlapping  series  formed  during  a  retreatal  movement  of  the  sea.  Note  that 
at  A,  only  the  lowest  bed  (a)  is  present  as  a  sand-facies,  this  being  limestone 
farther  out  at  sea.  At  B  all  the  formations  are  present,  the  highest  (d)  being 
sandy,  the  lowest  calcareous.  (From  Principles  of  Stratigraphy.) 

sea.  The  reverse  condition  obtains  in  a  retreating  sea,  forming  a 
regressive  series.  Here  each  succeeding  division  will  cover  a  lesser 
area  than  the  preceding  one,  and  a  portion  of  the  older  one  is 
always  exposed  beyond  the  younger.  This  is  illustrated  in  the  pre- 
ceding diagram  (Fig.  465)  in  which  the  sea  is  assumed  to  have 
successively  retreated  from  A  to  B.  As  the  shore  line  retreats,  the 
types  of  deposits  will  change  with  it,  and  the  zone  of  sands  will 
migrate  seaward  with  the  retreat,  and  come  to  rest  upon  the  finer 
muds,  etc.,  which  were  previously  deposited  here  while  the  shore 
stood  at  A. 

If  now  we  have  natural  exposures  of  such  a  series  (Fig.  466)  we 
shall  find  that  the  sandstone  bed  at  A  has  the  fossils  of  division  a, 


Overlaps  of  Marine  Clastics 


559 


while  the  beds  set  B  are  known  by  their  fossils  to  be,  respectively, 
beds  a  and  b  instead  of  b  and  c,  as  in  the  transgressive  series  (Fig. 
464).  At  C  all  three  beds  are  exposed.  Moreover,  it  will  be  seen 
that  in  general  the  change  in  the  physical  characters  of  the  suc- 
cessive beds  is  the  reverse  of  that  seen  in  the  previous  series,  the 


FIG.  466.  —  Natural  and  columnar  sections  at  each  end  and  in  the  center  of 
a  line  100  miles  long  and  extending  at  right-angles  to  the  original  shore-line. 

finer  ones  (muds  and  calcareous  beds)  being  at  the  bottom  and  the 
sands  at  the  top.  The  top  sandstone  is  one  of  emergence,  whereas 
in  the  previous  series  of  sections  the  bottom  sandstone  was  one  of 
submergence.  Conditions  of  this  kind,  though  rarely  as  simple  as 
here  outlined,  are  found  in  nature  and  some  of  them  will  be  discussed 
in  later  chapters. 

Compound  Regressive  and  Transgressive  Series 

Offlaps  Followed  by  Overlap.  —  It  is  obvious  that  if,  after  a 
retreatal  movement  of  the  sea  and  the  formation  of  an  offlapping 
series  as  outlined  in  the  preceeding  section,  a  transgressive  move- 

£ 


FIG.  467.  —  Section  to  illustrate  the  relationships  of  strata  formed  by  a 
compound,  transgressing,  retreatal  and  transgressive  movement  of  the  sea  with 
the  resulting  overlapping,  offlapping  and  overlapping  series  separated  by  a 
compound  sandstone  of  emergence  and  submergence.  (From  Principles  of 
Stratigraphy.} 

ment  with  overlapping  of  formations  should  follow,  the  sandstone 
bed  of  emergence  would  be  in  part  reworked  by  the  advancing 
waters  and  transformed  into  a  sandstone  of  submergence,  at  least 


560      Deposition  of  Clastic  Material"  in  the  Sea 

in  its  upper  part.  Thus  a  complex  series  would  be  produced,  sepa- 
rated by  this  compound  sandstone.  The  resulting  relationships 
of  the  strata  are  shown  in  the  diagram  (Fig.  467,  p.  559),  which 
represents  an  actual  case  of  a  transgressive,  followed  by  a  regres- 
sive, and  again  by  a  transgressive  series.  The  appearance  in  the 
exposures  of  such  a  series  is  shown  in  the  next  figure  (Fig.  468). 


FIG.   468.  —  Natural    and    columnar    sections   to    illustrate    compound   off- 
lapping  and  overlapping  series. 

At  locality  A  the  sandstone  of  emergence,  a,  is  followed  by  the 
sandstone  of  submergence,  cr.  At  locality  B  these  two  sandstones 
are  represented  by  formations  b  and  bf,  respectively,  and  at  locality 
C  by  c  and  a'.  In  both  localities  B  and  C,  this  compound  sand- 
stone will  occupy  the  middle  of  the  series  and  the  other  beds  will 
become  finer  grained  both  downward  and  upward. 

Replacing  Overlap.  —  When  a  series  of  delta  or  other  clastic 
beds  is  built  outward  from  the  land,  while  a  series  of  marine  beds 


FIG.  469  a.  —  Diagrammatic  section  to  illustrate  replacing  overlap  of  shore 
or  continental  sands  on  the  right  and  marine  shales,  sandstones  and  calcareous 
beds  on  the  left.  (From  Principles  of  Stratigraphy.) 

such  as  limestones  is  forming  farther  out  to  sea,  the  advancing 
deposits  from  the  land  will  progressively  overlap  the  purer  marine 
deposits,  successively  replacing  them.  This  relationship  is  shown 
in  the  preceding  diagram  (Fig.  469  a).  Instead  of  delta  or  other 
clastic  seashore  deposits  (of  terrigenous  origin),  deposits  of  purely 
continental  character  may  progressively  replace  those  of  marine 


Overlaps  of  Marine  Clastics 


561 


character,  the  resulting  relationships  being  essentially  similar  (Fig. 
469  b).  The  replacement  may  be  abrupt  or  gradual,  or  on  account 
of  oscillations  the  two  series  may  interfinger  in  various  degrees 
along  the  line  of  contact. 


FIG.  469  b.  —  Diagrammatic  section  to  illustrate  the  overlap  of  continental 
beds  over  a  retreatal  marine  series.     (From  Principles  of  Stratigraphy.) 

In  the  following  two  diagrams  such  replacing  series  of  various 
types  are  shown  (Figs.  469  c,  469  d).  These  represent  interpreta- 
tions of  actual  cases,  as  are  also  the  simpler  ones  shown  in  the 


FIG.  469  c.  —  East-west  section  across  the  state  of  New  York,  showing  the 
various  types  of  overlap  characteristic  of  the  formations.  The  depression,  at 
Utica  is  due  to  subsequent  erosion.  The  strata  were  formerly  continuous. 
(From  Principles  of  Stratigraphy.) 


Interior  Region 


FIG.  469  d.  —  Restored  section  from  the  Appalachians  to  the  Cincinnati 
region,  to  show  the  overlaps  of  continental  and  marine  strata.  (From  Prin- 
ciples of  Stratigraphy.) 


562       Deposition  of  Clastic  Material  in  the  Sea 

preceding  figures.     In  the  next  figure  (Fig.  470)  non-marine,  a, 
and  marine,   b,  overlaps  are  compared,  the  former  overlapping 


FIG.  470.  —  Sections  to  illustrate  progressive  overlap ;  a,  in  a  continental 
series,  away  from  the  source  of  supply ;  b,  in  a  marine  series  towards  the  source 
of  supply.  (From  Principles  of  Stratigraphy.) 

primarily  away  from  the  source  of  supply  of  terrigenous  material, 
the  latter  towards  it,  the  source  being  indicated  by  the  arrows. 


CHAPTER  XVIII 


CONSOLIDATION   OF   CLASTIC   MATERIAL;   TYPES 
OF   CLASTIC   ROCKS 

CONSOLIDATION  OF  CLASTICS 

WITH  the  passage  of  time  clastic  material  is  in  most  cases  bound 

together  into  solid  rock,  but  the  degree  of  such  solidification  does 

not    always    correspond    to   the 

length  of  time  during  which  these 

deposits  have  existed.     Thus  in 

the   Baltic   provinces  of    Russia 

there  are  sands  and  muds  which 

belong  to  the  oldest  deposits  of 

the  Palaeozoic  era,  deposits  which 

everywhere  else  have  been  com- 
pletely consolidated  into  rock,  or, 

as  it  is  technically  expressed,  have 

become    thoroughly     indurated. 

The    Russian    deposits,    on    the 

other  hand,  are  for  the  most  part 

still    unconsolidated,   though    in 

some  cases  this  may  be  the  result        FIG.  471  a.  —  Roadway,  cut  along 

of  later  removal  of  the  binding  the  side  of  a  cliff  of  consolidated 

glacial  gravels  (Monchsberg).  The 
rock,  which  is  called  Nagelfluh,  be- 
cause of  the  resemblance  of  the  de- 


substance    which   once    had    in- 
durated them.     On  the  Atlantic 


coast  of  New  Jersey  and  Mary-    Passions  left  by  the  pebbles  when 

.       ,  ,          j       *     removed,  to  that  formed  by  a  large 

land  are  soft  clays  and  sands  of    nail_head>  forms  partiy  overhanging 

the  same  age  as  the  consolidated    cliffs  and  is  in  places  tunneled  by 
chalk  beds  of  England  and  France    roadways.    Salzburg,  Austria, 
and  of  the  hard  limestones  and 

other  rocks  of  the  Alps.  Finally,  some  comparatively  young  de- 
posits, formed  during  the  ice  age,  are  already  solidified  in  certain 
regions,  the  most  noted  example  being  the  great  pebble  beds 
called  "  Nagelfluh  "  which  form  some  of  the  remarkable  hills  in 
the  city  of  Salzburg,  Austria  (Fig.  471  a).  These  are  honey- 

563 


564  Consolidation  of  Clastic  Material 

combed  in  places  by  catacombs  which  date  back  to  Roman  times 
(Fig.  471  b).     Deposits  of  lime-sands  and  muds  are  frequently 


FIG.  471  b. — Maximus  Chapel,  in  the  Catacombs  of  Salzburg  (Roman 
Juvavum).  These  catacombs  are  hewnjout  of  the  Nagelfluh,  or  consolidated 
glacial  gravels.  They  date  back  to  the  third  century. 

bound  together  very  shortly  after  their  formation,  especially  in  the 
sea,  where  cementing  lime  is  deposited  by  precipitation  from  solu- 
tion in  the  sea-water. 


Causes  and  Agents  of  Induration 

Clastic  material  may  become  consolidated  or  indurated  in  a  vari- 
ety of  ways,  the  chief  of  which  are  through  pressure,  by  infiltration 
of  a  cementing  medium  into  the  pores,  or  by  recrystallization  and 
the  formation  of  new  minerals  which  bind  the  grains  together.  The 
last  method  belongs  to  the  changes  due  to  metamorphism,  and 
will  be  more  fully  discussed  in  Chapter  XX.  The  others  may  be 
briefly  considered  here. 

Induration  by  Pressure.  —  When  an  older  is  buried  beneath 
younger  sediments,  the  pressure  of  the  latter  will  tend  to  bring  the 
grains  of  the  older  more  closely  together,  forcing  out  the  air  and 
water  from  the  pores.  This  will  result  in  a  certain  amount  of  inter- 
locking of  the  grains  and  cause  them  to  adhere  to  one  another  more 
or  less  firmly.  When  sand  grains  are  thus  pressed  together  a  soft 


Consolidation  of  Clastics  565 

sandstone  is  produced,  which  because  of  its  ready  yielding  to  the 
stone-cutter's  tools  is  called  a  freestone.  Finer-grained  clastic  de- 
posits are  more  strongly  solidified  in  this  manner  than  those  of 
coarser  grain,  and  thus  strata  of  clay  and  rock-flour  may  be  changed 
into  a  rock  called  shale  (Fig.  473,  p.  570).  There  is  probably  in  all 
cases  a  certain  amount  of  cementation  by  mineral  matter,  and  in 
the  older  deposits  there  is  generally  some  recrystallization  as  well. 
Induration  by  Cementation.  — The  common  cements  which  bind 
clastic  fragments  together  are  carbonate  of  lime,  silica,  and  iron 
oxide.  Carbonate  of  lime  is  of  course  the  chief  cementing  agent  of 
all  calcareous  deposits,  but  it  also  binds  quartz  grains  and  pebbles 
into  a  calcareous  sand  or  conglomerate,  and  it  may  be  an  impor- 
tant cementing  agent  of 
muds  of  various  kinds. 
When  sand  and  pebble  beds 
are  penetrated  by  waters 
which  carry  lime  in  solution, 
either  rising  from  below  or 
descending  through  a  soil 

rich  in  lime  or  through  an 

FIG.  472.  —  Well-rounded  quartz  grains 
overlying     calcareous     de-      from   ancient   eolian  rock  (shaded),  en- 

posit,  the  grains  and  pebbles  larged  by  secondary  addition  of  silica  in 
are  quickly  cemented  into  JP^  continuity  with  the  old  quartz 

Sandstone   Dike,   Buffalo,  N.  Y.      (Much 
a  solid  rock,  just  as  sand      enlarged.) 

grains,   pebbles,  and  large 

blocks  are  cemented  into  an  artificial  rock  mass  by  mortar,  which 
is  mainly  carbonate  of  lime.  Silica  brought  in  solution  by  water 
will  cement  many  kinds  of  fragmental  material,  but  is  especially 
effective  when  the  clastic  material  consists  of  pure  quartz  grains. 
In  such  cases,  the  silica  will  be  deposited  around  the  grains  in 
such  a  way  that  the  physical  (especially  crystallographic)  charac- 
ters of  the  new  quartz  will  be  continuous  with  the  quartz  grain  of 
the  clastic  sands.  This  is  called  secondary  enlargement  of  the 
clastic  particles  and  is  of  common  occurrence  (Fig.  472).  When 
the  process  has  gone  so  far  that  the  grains  are  no  longer  dis- 
tinguishable a  bed  of  quartzite  is  produced. 

Iron  oxide  is  a  common  cementing  agent  especially  of  impure 
sands.  Such  sands  are  then  colored  yellow  or  brown,  forming 
the  familiar  brownstone  so  much  used  in  the  past  for  dignified 
buildings. 


566  Types  of  Clastic  Rocks 

CLASSIFICATION  OF  CLASTIC  ROCKS 

Important  Characters.  —  When  we  remember  that  the  clastic 
material  from  which  the  clastic  rocks  are  produced  is  the  product 
of  rock  destruction  or  fragmentation,  it  is  apparent  that  in  any 
natural  classification  the  agent  causing  this  fragmentation  demands 
first  consideration,  just  as  the  medium  from  which  the  various 
chemical  (endogenetic)  rocks  were  separated  out,  and  the  agent 
active  in  the  separation,  demanded  our  first  attention  among  the 
non-clastic  rocks.  In  those  non-elastics,  the  formation  of  the  rock 
from  the  magma  or  the  state  of  solution  and  vapor,  was  primarily 
a  chemical  process,  and  therefore  the  chemical  composition  of  the 
medium  and  of  the  resulting  rock  was  "of  next  importance.  In  the 
case  of  the  clastic  rocks,  however,  the  chemical  composition  of  the 
material  from  which  the  clastic  matter  is  derived  (that  is,  the  older 
rocks)  is  of  less  significance,  since  it  plays  generally  only  a  subor- 
dinate part  in  the  process  of  clastation.  Of  much  greater  signifi- 
cance as  indicative  of  the  kind  and  amount  of  destructive  work 
performed,  is  the  coarseness  of  the  fragments  produced,  and  it  is 
this  degree  of  coarseness  which  in  the  consolidated  rock  produces 
its  grain  or  texture.  Hence,  in  the  clastic  rocks,  texture  may  be  con- 
sidered of  more  importance  than  chemical  composition,  whereas 
in  the  non-elastics  the  reverse  was  true.  Chemical  composition,  it 
is  true,  may  play  a  very  important  part  in  clastic  rocks  as  well,  as 
for  example  in  limestones  and  in  clay  rocks,  but  this  composition 
is  never  constant  over  more  than  very  small  areas,  because 
clastic  rocks,  more  than  any  other,  are  subject  to  the  mechanical 
mixture  of  materials  of  varying  composition.  We  shall  then,  in 
the  classification  of  clastic  rocks,  consider  the  basis  of  subdivision 
in  the  following  order :  (i)  agents  of  clastation,  transportation,  and 
deposition ;  (2)  size  of  grain  or  texture ;  (3)  chemical  composi- 
tion ;  (4)  other  characters. 

Classification  according  to  Agents  of  Clastation,  Transportation, 
and  Deposition 

It  is  obvious  that  these  activities  may  not  always  be  due  to  the 
same  agent,  though  there  are  examples  where  this  is  undoubtedly  the 
case.  Thus  sand  and  larger-grained  fragments  worn  by  the  waves 
of  the  sea  from  a  cliff  of  older  rock,  transported  by  waves  and  cur- 
rents, and  deposited  in  quieter  water  in  the  ocean,  are  throughout 


Classification  of  Clastic  Rocks  567 

the  product  of  sea  erosion,  transportation,  and  deposition.  Frag- 
ments broken  from  rocks  by  a  river  current,  transported  by  it,  and 
deposited  upon  the  flood-plain  or  alluvial  fan,  form,  throughout,  a 
river-made  deposit.  So  too,  a  rock  is,  throughout,  a  wind-formed 
or  eolian  rock  if  the  fragments  are  removed  from  the  older  rock  by 
the  erosive  action  of  the  wind,  and  if  they  are  transported  and  de- 
posited by  it.  While  such  pure  types,  as  they  may  be  called,  are 
perhaps  not  at  all  uncommon,  actual  purity  of  origin  will  be  very 
difficult  to  determine  in  any  but  the  simplest  cases.  Thus  the 
sands  transported  by  and  deposited  in  the  sea  may  have  been 
furnished  to  it  by  rivers,  —  a  very  common  occurrence.  The  sands 
washed  away  by  the  rivers  or  blown  away  by  the  wind  may  origi- 
nally have  been  the  product  of  disintegration  and  decomposition 
under  the  atmosphere  or  they  may  be  the  product  of  glacial  erosion. 
Indeed,  it  is  probably  true  that  clastic  deposits  and  the  clastic  rocks 
formed  from  them  have,  as  a  rule,  a  varied  history  and  are  the 
product  of  the  interaction  of  various  agencies.  Only  in  the  con- 
solidated residual  soils,  —  the  product  of  atmospheric  decay  or 
weathering  without  transport,  —  can  we  be  reasonably  sure  that 
but  one  agent,  the  atmosphere,  was  active  in  their  production. 
To  state  it  in  another  way,  most  clastic  rocks  are  of  multiple  par- 
entage —  they  are  polygenetic  —  though  some  are  of  single  parent- 
age, or  monogenetic. 

If  it  is  not  possible  to  determine  all  the  agents  active  in  the  pro- 
duction of  a  given  rock,  we  are  constrained  to  give  prominence  to 
the  agent  which  has  impressed  its  most  characteristic  features  upon 
the  deposit,  and  this  is  commonly  the  last  of  the  agents,  namely, 
that  effecting  deposition,  or  the  medium  by  and  in  which  the  material 
was  deposited.  Thus  an  eolian  rock  is  one  deposited  by  wind  upon 
dry  land,  whatever  the  origin  of  the  material.  If  the  origin  can 
be  determined,  this  adds  to  the  precision  of  the  classification.  Thus 
certain  eolian  beds  may  consist  wholly  of  volcanic  fragments,  i.e. 
pyroclastic  material.  Others  again,  like  the  loess,  are  believed  to 
be  in  part  of  glacial  origin. 

In  general,  it  may  be  said  that  the  origin  of  rocks  according  to 
agent  is  a  matter  primarily  for  field  determination,  and  one  only 
rarely  indicated  in  small  fragments.  True,  a  marine  rock  may  al- 
ways be  recognized  from  its  fossils,  and  so  may  a  lake  or  pond- 
formed  rock.  But  when  fossils  are  absent,  as  they  may  be  even 
iii  rocks  thus  formed,  there  is  no  positive  indication  of  origin.  In- 


568  Types  of  Clastic  Rocks 

deed,  some  rocks  may  be  wholly  composed  of  minute  shells  of  marine 
organisms  and  yet  be  wind-laid  deposits,  as  in  the  case  of  the  Milio- 
litic  limestone  of  India,  already  referred  to  (p.  278).  Again,  a  rock 
may  show  all  the  purity,  roundness  of  grains,  and  uniformity  of 
their  size,  characteristic  of  wind  deposits,  and  yet  may  be  reworked 
by  marine  waters  and  so  be  a  typical  marine  rock. 

Taking  first,  then,  the  agent  or  medium  which  has  given  the  clas- 
tic rock  its  most  distinctive  character,  we  may  distinguish  the  fol- 
lowing types : 

1.  Residual  Rocks.  — Those  formed  in  place  by  disintegration 
and  decomposition  with  little  or  no  transport  (Atmoclastics) . 

2.  Eolian  Rocks.  — •  Those  transported  and  deposited  by  wind 
upon  dry  land  (Anemoclastics) . 

3.  Water-laid  Clastic  Rocks.  —  Those  deposited  by  and  in  water 
(Hydroclastics).     Among  these  we  may  distinguish: 

(a)  Marine  Clastics.  —  Those  deposited  in  the  open  sea. 

(b)  Estuarine  Clastics.  —  Those  deposited  in  estuaries. 

(c)  Lacustrine  Clastics.  —  Those  deposited  in  lakes,  ponds,  and 
playas. 

(d)  Fluviatile  Clastics.  —  Those  deposited  on  river  flood-plains, 
alluvial  fans,  and  deltas. 

4.  Volcanic  Clastics.  —  Those  produced  by  volcanic  explosions, 
and  primarily  settling  down  without  much  reworking  by  wind  or 
water  (Pyroclastics) . 

5.  Fault-Crush  and  Glacial  Clastics.  —  Those  produced  by  the 
movement  of  one  rock  mass  past  or  over  another,  including  those 
produced  by  ice  erosion  (Autoclastics) . 

6.  Organically  Produced  Clastics.  — Those    produced  by  the 
rock-breaking  activities  of  organisms,  a  small  class  except  for  arti- 
ficial rock-masses  produced  by  man  (Bioclastics). 


The  Texture  of  Clastic  Rocks 

The  texture  of  clastic  rocks  is  their  most  obvious  character  and 
the  one  most  readily  determined  in  hand  specimens  or  other  frag- 
ments which  may  give  no  indication  concerning  the  agent  active  in 
their  formation.  Hence  clastic  rocks  are  more  often  classified  by 
their  textures  than  by  any  other  character. 

Three  main  types  of  texture  may  be  recognized,  the  coarse,  inter- 
mediate, and  fine,  according  as  the  material  consists  primarily  of 


Classification  of  Clastic  Rocks  569 

coarse  fragments  (rubbly  material),  of  sand  grains,  or  of  impal- 
pable rock-flour  or  clay.  Three  terms  will  here  be  used  to  express 
these  textures. 

1.  Rudaceous,  for   the   coarse   rubbly  texture    (Latin,   nidus , 
rubble). 

2.  Arenaceous,  for  the  sandy  texture  (Latin,  arena,  sand). 

3.  Lutaceous,  for  the  rock-flour  and  clay-size  texture  (Latin 
lutum,  mud). 

Accordingly  three  textural  types  of  rock  may  be  recognized: 

(1)  the  rubble-rock  or  rubble-stone,  or  rudyte,  which  when  the  frag- 
ments are  rounded  is  a  conglomerate  and  when  angular  a  breccia; 

(2)  the  sand-rock  or  sandstone  or  arenyte ;  and  (3)  the  mud-rock 
or  mud-stone,  or  lutyte. 

Several  distinct  types  can  be  recognized  under  each  division,  but 
as  these  are  further  modified  by  the  composition,  this  will  be  con- 
sidered first. 

Composition  of  Clastic  Rocks 

All  clastic  rocks  which  have  been  subject  to  prolonged  transport 
and  sorting  of  material  before  consolidation  will  be  relatively  pure, 
or  of  more  or  less  uniform  composition.  With  material  derived 
from  crystalline  rocks  (igneous  or  metamorphic)  quartz  will  pre- 
dominate, and  in  exceptional  cases  the  rock  may  consist  wholly  of 
quartz,  as  in  that  of  the  Sylvania  sandstone  of  Ohio  and  Michigan 
already  referred  to  (p.  440),  and  of  the  Shawangunk  and  Olean 
conglomerates  of  New  York  state.  In  other  cases  the  rock  may 
contain  large  quantities  of  feldspar  because  there  has  been  little 
transport  and  sorting.  Such  are  the  arkose  sandstones  of  New 
Jersey  (Triassic)  and  the  arkose  Torridon  breccia  or  fragment  rock 
of  Loch  Marie  and  other  regions  of  West  Scotland  (pre-Cambrian), 
which  looks  superficially  like  a  coarse  granite,  having  been  produced 
by  the  disintegration  of  such  rock  and  the  recementation  of  the 
material. 

When  the  materials  of  a  clastic  rock  are  derived  from  the  erosion 
of  a  coral  reef,  or  shell  accumulations,  or  older  limestones,  the 
grains  and  larger  fragments  are  apt  to  be  pure  calcium  carbonate 
with  little  or  no  admixture  of  other  material.  This  produces  a 
lime  sandstone  (calcarenyte}  —  such  as  that  found  on  Bermuda,  or 
a  limestone  breccia  (calcirudyte) ,  of  which  the  Point  of  Rocks  "  Mar- 
ble "  of  Maryland  is  an  example. 


570 


Types  of  Clastic  Rocks 


The  mineral  glauconite  is  often  a  characteristic  constituent  of 
sands,  as  we  have  seen  (p.  552),  and  thus  glauconitic  sandstones  may 
be  produced.  When  iron  oxide  abounds  as  a  cementing  agent, 
ferruginous  clastic  rocks  (sandstones,  conglomerates)  are  produced. 
When  the  material  of  the  rock  is  largely  clay,  its  texture  is  that  of 
a  mud-rock,  or  lutaceous,  and  the  rock  becomes  a  claystone  or  argillite 
(Latin,  argillum,  clay).  When  clay  is  one,  but  not  the  only  constit- 
uent, the  rock  is  called  argillaceous.  Rocks  of  all  textures  may  be 


FIG.  473.  —  A  bank  of  typical  shale  (Hamilton  formation,  Devonian)  on 
the  shore  of  Lake  Erie.  The  shale  splits  into  small  chips  on  exposure,  and 
rapidly  weathers  to  clay.  It  is  full  of  marine  fossils.  (Photo  by  the  author.) 

argillaceous,  those  of  rubbly  (rudaceous)  texture  and  those  of 
arenaceous  texture  generally  carrying  the  clay  as  an  admixture  or 
as  part  of  the  cement.  When  carbon  is  present,  the  rock  is  called 
carbonaceous. 

While  there  are  other  composition  types,  the  silicious,  calcareous, 
and  argillaceous  are  the  most  common,  and  from  a  purely  chemical 
point  of  view  these  are  called  quartz-stones,  limestones,  and  clay- 
stones.  Other  substances  are  present  chiefly  in  smaller  quantities 
and  form  modifications  of  these  primary  types.  It  must,  however, 
be  remembered  that  there  are  quartz  rocks  and  limestones  of  other 
than  clastic  origin. 


Structural  and  Other  Characters 


STRUCTURAL  AND  OTHER  CHARACTERS  USED  IN  DEFINING 
CLASTIC  ROCK  TYPES 

Shaly  and  Slaty  Structures.  —  Certain  structures  are  so  commonly 
associated  with  special  composition  and  textural  types  of  clastic 
rocks,  that  they  have  been  made  the  chief  basis  of  classification. 
Mud-rocks  (lutytes)  in  which  much  clay  occurs  (argillaceous)  gen- 
erally show  a  peculiar 
mode  of  breaking  in 
small  plates  parallel  to 
the  bedding,  these  plates 
having  curved  surfaces 
(gently  convex  on  one 
side  and  concave  on  the 
other)  which  give  the 
fragment  a  superficial 
resemblance  to  a  piece 
of  a  shell.  On  this  ac- 
count the  name  shale 
has  been  given  to  such 
rocks  and  the  structure 
is  spoken  of  as  shaly 
(shelly).  When  exposed, 
typical  shales  quickly 
split  into  thin  chips,  and 
weather  into  clay  (Fig. 
473) .  The  term  shale  is, 


FIG.  474.  —  A  bank  of  fissile  shale  (Genesee 
Shale,  Upper  Devonian)  exposed  in  the  gorge 
of  Eighteen  Mile  Creek  in  western  New  York. 
Note  the  perfect  jointing.  The  rock  splits 
into  thin  slate-like  sheets  parallel  to  the  bed- 
ding. (Photo  by  the  author.) 


however,  loosely  used 
for  all  kinds  of  mud-rocks  which  split  into  thin  layers,  whether 
these  are  shaly,  i.e.  have  curved  surfaces,  or  are  smooth-surfaced. 
To  the  latter  type  the  term  fissile  shale  is  sometimes  applied,  and 
the  name  slate  is  also  popularly  used  in  some  cases  where  this 
smooth  splitting  is  parallel  to  the  bedding  surfaces  (Genesee 
Shale,  Fig.  474).  Such  rock  is  generally  well  jointed  or  separated 
by  vertical  planes  with  smooth  faces.  True  slate,  however,  is  a 
metamorphic  mud-rock,  the  splitting  into  thin  plates  having,  as 
a  rule,  no  reference  to  the  original  bedding,  but  being  secondarily 
produced.  (See  Fig.  562.) 

Platey  Structure.  —  When  rocks  split  into  layers  a  few  milli- 
meters or  centimeters  in  thickness,  and  with  smooth  surfaces  which 


572 


Types  of  Clastic  Rocks 


are  generally  the  bedding  planes,  they  are  called  platey  rocks.  The 
most  common  platey  rocks  are  calcareous  and  fine-textured  or  lime- 
mud-rocks  (calcilutytes). 
Such  are  the  famous 
platey  layers  of  the 
Solnhofen  region  in  Ba- 
varia (Plattenkalke) 
(Fig.  475),  which  are 
used  for  roofing  purposes 
in  that  country.  Many 
other  limestones  in  our 
own  and  other  countries 
split  into  thin  layers, 
producing  suitable  ma- 
terials for  "  tablets  of 
stone  "  on  which  inscrip- 
tions can  easily  be  en- 
graved on  account  of 
their  softness.  When 
seen  in  sections,  such 
thin-bedded  limestones 

often  present  a  banded  aspect,  and  they  are  then  called  ribbon 
limestones  (Fig.  476).     They  abound  among  the  older  formations. 

Concretionary  Structure  and  Con- 
cretions. —  The  name  concretion  is  ap- 
plied to  a  stony  mass  included  in 
stratified  rocks  and  which  has  been 
formed  as  the  result  of  segregation  of 
material  either  during  the  deposition 
of  the  rock  or  afterwards  under  the 
influence  of  circulating  waters  or  other- 
wise. Concretions  are  very  common 
in  clay  rocks  or  unconsolidated  clay 
beds,  and  generally  consist  of  clay 
bound  together  by  carbonate  of  lime  or 
carbonate  of  iron.  In  form  they  vary 
greatly  from  spherical  to  disk-shaped 
(Fig.  477  a),  often  compound  (Figs. 
477  b  and  c),  cylindrical,  tubular,  or 
irregular  (Loess piippchen),  etc.,  fre- 


FIG.  475.  —  Deep  quarry  in  the  lithographic 
and  platey  limestone  deposits  (Jurassic)  of 
Solnhofen  in  Bavaria.  Most  of  the  rock  con- 
sists of  thin  layers  (platey  strata),  but  be- 
tween these  are  the  thicker,  purer  layers 
which  furnish  the  lithographic  stone.  (Photo 
by  author.) 


FIG.  476.  —  A  piece  of 
"ribbon  limestone"  or  thin- 
bedded,  fine-grained  lime- 
mud-rock  (calcilutyte)  seen 
in  section  at  right  angles 
to  the  bedding.  Cambro- 
Ordovician  beds  of  Penn- 
sylvania, slightly  reduced. 
(Photo  by  Bela  Hubbard.) 


Structural  and  Other  Characters 


573 


quently  simulating  organic  forms  (Fig.  47 7  d).  Sometimes  the 
entire  rock  may  be  a  mass  of  such  concretions,  as  are  certain  beds  of 
the  Magnesian  Limestone  on  the  coast  of  Durham,  England,  already 


FIG.  477.  —  Clay-stone  concretions  from  the  glacial  clays  of  the  Connecticut 
valley.  (After  J.  M.  Arms  Sheldon.)  a,  single  disk-shaped  form,  about  three 
fifths  natural  size ;  b,  confluence  of  two  disk-shaped  forms,  one  half  natural  size ; 
c,  three  confluent  disks,  about  one  half  natural  size ;  d,  forms  imitative  of 
animal  shapes  (Loesspuppchen)  less  than  half  natural  size. 

referred  to  (p.  221).  Thin  beds  of  limestone  in  shales  may  be 
formed  from  confluent  concretions.  Sometimes  concretions  are 
full  of  radiating  veins  which,  when  the  outer  layer  of  the  concretion 


574 


Types  of  Clastic  Rocks 


FIG.  478.  —  Large  concretion  of  the  Sep- 
tarium  or  "Turtle  Stone  "  type,  weathered 
out  of  the  shale  banks  (Upper  Devonian)  on 
the  Genesee  River.  Others  are  seen  forming 
bands  in  the  shale  bank.  (See  also  Figs. 
161  a,  6,  p.  222.) 


p.  224),  and  in  sandstones  they  often 
sand  (Fig.  479). 


is  worn  away,  may 
weather  out  in  relief. 
Such  concretions  are 
called  septaria  (Fig.  478). 
The  oolites  and  pisolites, 
previously  discussed, 
form  aggregates  of  small, 
regular  concretions  oc- 
curring as  separate  de- 
posits. Not  infrequently 
concretions  are  formed 
around  a  nucleus,  which 
may  be  a  plant  or  a 
shell  or  other  organic 
structure  (Fig.  181, 
p.  257).  In  chalk  and 
limestones  the  concre- 
tions are  mostly  of  silica, 
flint,  and  chert  (Fig.  162, 
consist  of  iron-cemented 


FIG.  479.  —  Concretionary  masses  of  sandstone  (Laramie  formation),  south- 
west of  New  Castle,  Wyo.     (Darton,  photo ;  from  U.  S.  G.  S.) 


Varieties  of  Clastic  Rocks 


575 


VARIETIES  OF  CLASTIC  ROCKS 

As  we  have  seen,  the  texture  of  clastic  rocks  is  their  most  evident 
characteristic,  as  seen  in  fragments  or  hand  specimens.  Hence  it 
is  usual  to  divide  clastic  rocks  first 
upon  a  textural  basis,  and  this  is 
threefold  as  shown  above.  The  more 
common  types  of  each  of  these  tex- 
tural groups  may  now  be  reviewed. 


Rubble  Rocks  or  Rudytes 
(Texture  Rudaceous) 

These  are  the  clastic  rocks  in  which 
the  prevailing  size  of  the  fragments 
is  larger  than  that  of  a  sand  grain,  or 
in  general  larger  than  2  or  2.5 
millimeters.  Two  main  varieties  are  recognized,  as  follows: 


FIG.  480.  —  Conglomerate  (re- 
duced).   (B.  Hubbard,  photo.) 


Conglomerates  (Fig.  480).  —  These  are  composed  predominantly  of  rounded 
fragments,  generally  well  water-worn,  and  most  commonly  of  river  or  seashore 

origin.  In  size  the  fragments 
may  range  from  pebbles  of 
the  dimensions  of  peas  to 
large  boulders.  Some  parts 
of  the  Old  Red  Sandstone  of 
Scotland  are  composed  almost 
wholly  of  well-rounded  bould- 
ers, mostly  of  the  size  of  a 
man's  head.  Such  rocks  are 
generally  spoken  of  as  boulder 
conglomerates  (Fig.  481). 
When  the  pebbles  are  few  and 
scattered  among  sand  grains, 
the  rock  is  called  a  pebbly 
sandstone.  When  the  pebbles 
are  very  small  the  rock  is 
called  a  grit.  In  the  decom- 
position of  certain  basic 
igneous  rocks,  such  as  ba- 
salts, etc.,  a  mass  of  rounded 
residual  boulders  is  left  (see 
p.  397),  and  these  may  remain 
FIG.  481.  —  Boulder  conglomerate.  Old  piled  together  in  their  original 
Red  Sandstone  (Devonian)  at  Oban,  Scotland.  position  separated  by  de- 
(Photo  by  author.)  composition  sand.  When 


FIG.  482.  —  Roxbury  Pudding-stone,   Franklin  Park,  Boston,  Mass.      (P.  L. 

Grabau,  photo".). 


FIG.  483. — A  breccia.  Devonian  limestone  which  has  been  broken  into 
angular  fragments  which  have  then  become  recemented  into  a  solid  rock  —  a 
typical  breccia.  Iowa.  (After  Diller,  U.  S.  G.  S.) 

576 


Varieties  of  Clastic  Rocks 


577 


close-pfled,  their  structure  is  spoken  of  as  a  cannon-ball  structure,  and  the  rock 
is  called  a  residual  boulder  conglomerate  (atmoclastic)  (Fig.  33 1  a) .  Conglomerates 
of  small  pebbles  may  be  well  stratified,  and  may  show  cross-bedding  and  other 
structures.  Boulder  conglomerates  generally  show  little  structure. 

According  to  the  composition  of  the  pebbles,  we  may  have  quartz  conglomer- 
ates, arkose  conglomerates,  limestone  conglomerates,  etc.  Examples  of  these  have 
already  been  cited.-  •  Many  conglomerates  are,  however,  heterogeneous ;  that  is, 
the  pebbles  are  composed  of  a  variety  of  material  —  granite,  gneiss,  sandstone, 
etc.  Sometimes. conglomerates  are  formed  mostly  of  one  kind  of  such  material. 
Thus  we  may  have  granite  con- 
glomerate, volcanic  (lava)  con- 
glomerate, sandstone  conglomer- 
ate, etc.  According  to  the  nature 
of  the  cementing  material,  we 
may  have  quartz-sand  conglomer- 
ates, calcareous  conglomerates, 
argillaceous  conglomerates,  fer- 
ruginous conglomerates,  etc. 
When  the  pebbles  (generally 
large)  and  the  cementing  mixture 
between  them  show  little  grada- 
tion and  a  marked  contrast,  the 
name  pudding-stone  is  often  ap- 
plied (Fig.  482).  If  the  pebbles 
are  formed  of  worn  organic  struc- 
tures, we  may  have  coral-con- 
glomerates, shell-conglomerates, 
etc.  Finally,  there  is  the  artificial 
conglomerate  or  rubble  concrete, 
so  extensively  produced  by  man. 

Breccias.  —  This  term  is  ap- 
plied   when    the    fragments    are 

angular,  showing  little  or  no  evidence  of  water  wear  (Fig.  483).  The  principal 
types  of  breccia  are  the  fault  breccia  (an  autoclastic  rock)  (Fig.  32,  p.  80),  the 
talus  breccia  (an  atmoclastic  rock),  and  the  volcanic  breccia  or  volcanic  ag- 
glomerate (Fig.  484),  formed  around  and  in  volcanic  vents  (pyroclastic).  A 
glacial  breccia  may  also  be  produced,  but  generally  the  fragments  of  a  glacial 
till  are  scratched  and  polished.  Such  a  rock  when  consolidated  forms  a  tillite 
(p.  508).  In  deposits  formed  from  glacial  moraines  and  outwash  material,  the 
pebbles  and  boulders  are  commonly  well  worn,  forming  a  glacial  conglomerate. 


FIG.  484.  Projecting  cliff  of  volcanic 

agglomerate,  tunneled  by  waves  along  a 

joint  fissure.  North  shore,  Bay  of  Fundy, 

Nova  Scotia.  (G.  W.  Stose,  photo.) 


Sandstones  or  Arenytes 
(Texture  Arenaceous) 

When  the  prevailing  size  of  grain  of  a  clastic  rock  falls  between 
2.5  and  0.05  millimeters,  the  rock  is  said  to  have  an  arenaceous  tex- 
ture, and  is  called  a  sandstone  or  arenyte.  Generally  the  term  sand- 


578 


Types  of  Clastic  Rocks 


stone  is  restricted  to 
such  rocks  in  which  the 
grains  are  chiefly  quartz. 
For  this  reason  the  term 
arenyte  is  preferable, 
since  it  implies  nothing 
but  textural  character. 
According  to  origin, 
composition,  and  struc- 
ture a  number  of  com- 
mon types  are  recog- 
nized. 

Quartz  Sandstone.  — 

This  is  the  common  type  in 
which  quartz  grains  prevail. 
Many  varieties  occur  deter- 
mined by  the  nature  of  the 
cementing  material.  When 
this  is  oxide  or  hydrate  of 
iron,  &  ferruginous  sandstone 
is  produced ;  when  carbonate 
of  lime,  a  calcareous  sand- 
stone is  formed ;  when  much  clay  is  present,  the  rock  is  an  argillaceous  sand- 
stone. When  pure  it  is  valued  for 
glass-making,  as  are  the  Sylvania  and 
the  Oriskany  sandstones  (Fig.  485). 

Quartzite.  —  This  is  a  quartz  sand- 
stone in  which  the  grains  have  become 
enlarged  by  the  deposition  around 
them  of  new  quartz  in  such  a  way  as 
to  form  a  crystallographic  continuation 
of  the  older  quartz.  The  pores  are 
completely  filled  and  the  rock  becomes 
a  hard,  very  resistant  mass.  (Ex- 
ample :  Potsdam  quartzite  of  Ausable 
Chasm,  N.  Y.)  Quartzites  are  fre- 
quently placed  among  metamorphic 
rocks.  (See  Fig.  373,  p.  455.) 

Freestone.  —  This  name  is  given  to 
a  quartz  sandstone  in  which  the  grains 
are  loosely  held  together,  so  that  they 
will  readily  yield  to  the  graver's  tools. 
This  is  commonly  due  to  the  fact  that 
the  grains  are  bound  together  only  by 
pressure.  (Example:  Ohio  freestone.) 


FIG.  485.  —  Ledge  of  Oriskany  sandstone, 
Pennsylvania.  The  rock  is  a  pure  quartz  sand- 
stone covered  by  talus  from  higher  sandstone 
masses.  It  is  quarried  here  for  glass-making 
under  the  name  Juniata  glass  sand. 


FIG.  486.  —  Micro-photograph  of  a 
thin  section  of  Hudson  River  Blue- 
stone.  The  clear  grains  are  quartz, 
the  rest  of  the  field  is  made  up  chiefly 
of  secondary  derivatives  from  the 
original  feldspars  and  ferromagnesian 
minerals,  and  of  clastoliths.  (Photo 
made  and  contributed  by  C.  P. 
Berkey.) 


Varieties  of  Clastic  Rocks 


579 


Brownstone.  —  This  is  a  highly  ferruginous  quartz  sandstone  in  which  the 
grains  are  generally  coated  with  iron  oxide.  It  has  been  extensively  used  for 
building  purposes.  (Example  :  Connecticut  Valley  brownstone.) 

Bluestone,  Flagstone.  —  This  is  a  highly  argillaceous  sandstone  of  even  tex- 
ture and  bedding.  Commonly  the  argillaceous  material  of  the  cement  has  been 
altered  to  the  mineral  sericite  (Fig.  486).  It  is  probably  in  most  cases  the  prod- 
uct of  ill-assorted  sediments  in  lagoons  and  lakes  near  the  mouth  of  a  river. 
Such  rock  is  much  used  for  flags  and  sidewalk  curbings.  (Example :  Hudson 
River  Bluestone.)  "  (Fig.  487.) 


FIG.  487.  —  Ledge  of  Hudson  River  Bluestone,  showing  solid  layers  of  blue- 
stone  alternating  with  shaly  beds.  Near  Kingston,  N.  Y.  (Photo  by  C.  P. 
Berkey.) 

Graywacke.  —  This  name  is  applied  to  an  impure,  highly  argillaceous  sand- 
stone of  variable  composition,  texture,  and  structure.  Generally  fragments 
of  other  clastic  rocks  are  present.  The  name  has  been  widely  used  in  the  past 
for  all  the  harder  argillaceous  sandstones  of  the  older  geological  systems.  (Ex- 
ample :  Silurian  graywackes  of  England.) 

Arkosic  Sandstone.  —  A  sandstone  in  which  much  feldspar  is  present.  This 
may  range  from  unassorted  products  of  granular  disintegration  of  fine  or  medium 
grained  granite  (recomposed  granites  or  pure  arkoses),  to  a  partly  sorted  river- 
laid  or  even  marine  arkosic  sandstone.  (Example:  Triassic  arkose  of  New 
Jersey,  some  beds  of  the  Potsdam  sandstone  of  New  York,  etc.) 

Glauconitic  Sandstone.  —  (Green-sand.)  This  is  a  quartz  sandstone  or  an 
arkosic  sandstone  rich  in  glauconite  grains.  (Example :  Green-sand  of  England, 
and  of  New  Jersey  and  Maryland.) 


580  Types  of  Clastic  Rocks 

Lime  Sandstone  (Calcarenyte).  —  Arenytes  in  which  the  grains  are  largely 
or  wholly  composed  of  carbonate  of  lime  are  called  calcarenytes.  They  are 
common  around  coral  reefs,  and  many  of  the  common  "  limestones  "  of  the 
various  geological  formations  are  really  calcarenytes.  (Example :  Coral  sand- 
stone of  Bermuda.) 

Eolian  Sandstone.  —  Sandstones  in  which  the  eolian  origin  can  be  recognized 
in  small  specimens,  either  by  structure  or  by  character  of  grains,  are  referred  to  as 
eolian  sandstones.  They  may  be  pure  quartz  (Example  :  Sylvania  sandstone) 
or  pure  lime.  (Example :  eolian  coral  sandstone  of  Bermuda.) 

Volcanic  Sandstone  or  Ash  Rock  (Volcanic  Tuff).  —  This  consists  of  the 
material  of  arenaceous  texture  produced  by  volcanic  explosions.  It  is  recog- 
nizable both  by  its  structure,  which  is  often  porous,  and  by  the  composition  and 
form  of  the  grains.  The  rock  is  generally  called  a  volcanic  tuff,  but  this  term 
is  also  applied  to  the  finer-grained  varieties,  which  in  reality  are  lutytes  (Fig.  488). 


FIG.  488.  —  Fragments  of  volcanic  glass  in  tuff  as  seen  under  the  microscope 
X5o.  a,  minute  particle  of  pumice;  b,  curved  wall  of  a  broken  bubble; 
c,  small  vesicle  still  complete  and  surrounded  by  tubular  glass ;  d,  interstitial 
glass  between  bubbles  not  tubular.  (After  Diller,  U.  S.  G.  S.) 


Mud-stones  or  Lutytes 
(Texture  Lutaceous) 

When  the  dominant  material  of  a  clastic  rock  consists  of  grains 
less  than  0.05  mm.  in  diameter,  the  rock  is  a  mud-stone  or  lutyte. 
Clay  is  a  common  constituent  of  such  a  rock,  but  may  be  largely 
or  wholly  wanting,  and  the  rock  may  consist  of  quartz-flour  or  in 
part  or  entirely  of  limestone  flour,  or  of  lime  mud.  It  may  also 
consist  of  very  small  wind-transported  or  water- worn  shells  of 
Foraminifera,  etc.,  and  of  fragments  of  these.  Some  of  the  common 
types  are : 

Claystones  (Argillutytes).  —  Rocks  in  which  much  clay  is  present  or  which 
are  largely  composed  of  clay.  Sometimes  these  are  bound  together  by  iron 
carbonate,  forming  clay  iron-stones,  generally  of  a  concretionary  character. 
In  large  typical  masses  clay  rocks  show  no  regularity  of  splitting. 

Shale.  —  Mud-rocks  containing  much  clay  and  splitting  into  thin  layers  with 
curved  surfaces  parallel  to  the  bedding  are  called  shales.  Many  shales  or  similar 
thin-bedded  mud-rocks  contain  much  carbonaceous  material,  in  the  form  of 
disseminated  coaly  matter,  or  they  are  saturated  with  oil.  Such  shales  are 
called  carbonaceous  shales,  coal  shales,  oil  shales,  or  black  shales.  When  extremely 
rich  in  carbonaceous  material  they  are  called  pyroschists  and  form  a  transition 
to  coal.  Carbonaceous  shales  generally  split  with  smooth  surfaces.  Shales 
may  also  DC  highly  fossiliferous,  when  they  are  generally  calcareous.  (See  Fig. 


Varieties  of  Clastic  Rocks 


581 


473,  p.  570).  Pyritiferous  shales  contain  much  scattered  iron  pyrite,  which 
weathers  to  iron  hydrate  on  exposure,  and  causes  the  splitting  of  the  shale. 
Silicious  shales  contain  much  quartz-flour  and  are  apt  to  split  into  irregular 
pencil-like  and  other  forms  of  fragments  on  weathering.  Red  shales  are  colored 
by  finely  disseminated  iron  oxide. 

Slates.  —  These  are  more  or  less  metamorphosed  mud-rocks,  the  metamor- 
phism  being  due  to  compression.  As  a  result  the  slates  split  into  thin  layers  on 
weathering  or  under  properly 
applied  force,  but  this  split- 
ting is  seldom  parallel  to  the 
bedding.  (Example :  roofing 
slate.) 

Lime  Mud-rocks  (Cal- 
cilutytes).  —  These  are  com- 
mon around  coral  reefs,  and 
many  old  limestones  are  of 
this  character,  having  been 
formed  from  lime-flour  gen- 
erally deposited  in  the  sea  or 
near  its  border.  Sometimes 
such  rocks  show  mud-bracks, 
ripple-marks,  and  other 
structures.  When  alumina 
and  silica  are  present  a  water- 
lime  or  natural  cement  rock 
is  produced  (Fig.  489).  Pure 
lime  mud-rocks  form  the 
famous  lithographic  stone  of 

Solnhofen,  Bavaria.  Rocks  of  this  type  often  preserve  organic  remains  in  won- 
derful perfection,  as  is  seen  in  the  ancient  birds  with  their  feathers  preserved 
and  in  the  dragon  flies  and  other  organisms  for  which  the  lithographic  stone 
has  become  famous. 

Quartz-flour  Rock  (Silicilutyte).  —  Rocks  composed  apparently  largely  or 
wholly  of  quartz-flour  are  known.  They  have  a  very  fine  and  uniform  grain, 
and  furnish  smooth  and  hard  surfaces.  They  are  excellent  for  hone-stones  and 
for  polishing,  etc.  The  Arkansas  Novaculite  appears  to  be  such  a  deposit,  but 
may  be  a  chemical  precipitate. 


FIG.  489.  —  An  abandoned  quarry  in  natural 
cement  rock  or  water-lime.  The  pillars  which 
support  the  roof  are  remnants  of  the  water- 
lime.  Rondout,  N.  Y. 


CHAPTER  XIX 

DEFORMATION    OF   THE   ROCKS   OF   THE 
EARTH'S   CRUST 

ALL  rocks  are  subject  to  deformation  after  they  have  come  into 
existence.  These  deformations  disturb  the  original  arrangement 
of  the  material  of  the  rock,  and  modify  the  original  structure  which 
the  rock  assumed  in  formation.  Such  changes  are  of  a  superinduced 
nature,  and  they  are  produced  by  forces  other  than  those  operative 
in  the  formation  of  the  rock,  or  by  these  forces  acting  upon  it  in 
a  new  way.  Deformation  structures  are  therefore  secondary,  as 
compared  with  the  primary  or  original  structures. 

EFFECTS  OF  DEFORMATION 

The  effects  of  deformation  are  of  several  kinds,  the  most  impor- 
tant being:  (i)  production  of  new  structures  in  the  rock  masses; 
(2)  change  in  the  character  of  the  material  (metamorphism) ;  (3) 
superficial  disturbances  of  the  earth's  surface  (earthquakes) ;  (4) 
changes  in  topography  (formation  of  mountains,  etc.).  In  the  pres- 
ent chapter  we  will  consider  only  the  changes  in  structure  and 
incidentally  their  topographic  expression.  Metamorphism  and 
earthquakes  will  be  discussed  in  subsequent  chapters. 

TYPES  OF  DEFORMATION  STRUCTURES 

In  general  we  may  consider  four  types  of  deformations;  viz. 
(i)  Foldings  and  warpings  of  the  rocks;  (2)  fracturing  with  dis- 
placement of  rocks,  or  faults;  (3)  fracturing  without  displacement, 
or  joints;  (4)  slaty  cleavage.  Although  all  rocks  are  affected  by 
deformation  the  effects  are  most  readily  recognized  in  the  stratified 
series,  and  for  that  reason  the  illustrations  will  be  taken  from 
these. 

582 


Deformation  by  Folding 

DEFORMATION  BY  FOLDING 
Inclined  Strata 


583 


In  the  mountain  regions  of  the  earth  it  is  generally  seen  that  the 
stratified  rocks  lie  in  positions  deviating  markedly  from  the  hori- 
zontal (Fig.  490),  the  latter,  as  we  have  learned,  being  the  approxi- 
mate position  in  which  they  are  deposited,  barring  minor  local 
variations  as  in  deltas,  around  coral  reefs,  etc.  It  is  true  that  in 
some  mountains,  as  in  the  Catskills,  parts  of  the  Rocky  Mountains, 


FIG.  490.  —  Folded  strata  south  of  Heaven's  Peak,  Livingston  Range, 
Montana.  (Limestones  and  argillites  of  Algonkian  age.)  Vertical  range  2000 
feet.  (U.  S.  G.  S. ;  courtesy  of  D.  W.  Johnson.) 

and  elsewhere,  the  strata  may  still  appear  horizontal,  but  this  is 
rather  the  exception  than  the  rule.  Again,  there  are  regions  of  the 
earth,  such  as  eastern  New  York  and  New  England,  numerous  areas 
in  Great  Britain,  Ireland,  and  parts  of  the  northwestern  continent 
of  Europe  (Belgium,  Northwest  Germany,  etc.)  (Fig.  491),  where 
the  strata  are  not  horizontal,  although  the  regions  are  not  now 
mountainous.  Such  regions  may  generally  be  considered  as  the 
sites  of  former  mountain  ranges,  which  have  long  since  been  worn 
away,  leaving  only  their  stumps  exposed  in  what  is  now  level  or  un- 
dulating country.  In  such  regions  the  most  general  aspect  of  the 
strata  is  that  of  rising  at  an  angle  or  vertically  from  the  surface  and 
ending  abruptly  in  the  air,  and  their  appearance  may  be  compared 


584     Deformation  of  Rocks  of  the  Earth's  Crust* 


with  that  of  the  leaves  of  a  book  standing  on  end  or  resting  in  an 
inclined  position.  This  comparison  is,  however,  apt  to  mislead, 
because  the  leaves  of  the  book  are  unrelated  to  the  base  on  which 


FIG.  491. — The  Lorelei  rock  on  the  Rhine,  formed  by  inclined  Devonian 
shales  and  sandstones.  The  summit  of  the  cliff  is  a  part  of  the  peneplane  which 
has  beveled  all  the  strata  of  this  region.  (See  Fig.  6oO 

the  book  rests.  A  better  illustration  of  the  simplest  form  of  in- 
clined strata  is  furnished  by  the  metal  sheeting  of  a  nearly  flat  roof, 
when  this  sheeting  is  bent  up  at  one  end  to  form  a  leader  or  gutter 
for  the  raia  water.  The  inclined  strata  represent  the  bent-up  por- 
tion of  this  sheeting,  the  continuation  in  the  horizontal  or  slightly 
sloping  part  being  covered.  For  it  must  be  clearly  understood  that 
inclined  strata  do  not  continue  downwards  into  the  earth  with  the 
„,--—..  same  inclination  for 

an  indefinite  dis- 
tance, but  that  be- 
low the  surface  they 
either  bend  upward 
again,  or  in  the 
simplest  case  as- 

FIG.  492.  —  Diagram  to  illustrate  the  underground 
extension  of  inclined  strata  which  crop  out  on  the  sur- 
face, and  the  former  relationship  of  the  two  inclined 
beds  A  and  B ;  S,  soil. 


sume  a  nearly  hori- 
zontal position. 
This  is  illustrated 


in    diagrammatic 

manner  in  Figure  492  at  A ,  where  the  inclined  bed,  rising  above  the 
surface  of  soil  (s),  is  seen  to  be  merely  the  bent-up  portion  of  the 
bed  which  beneath  the  surface  changes  again  to  the  horizontal. 


Deformation  by  Folding 


585 


Moreover,  it  is  obvious  that  the  upper  edge  of  the  inclined  bed, 
which  now  ends  in  the  air,  was  not  its  original  end  but  that  it  is 
the  cut  or  eroded  edge  of  a  formerly  much  more  extensive  bed  which 
continued  beyond  the  present  point  of  outcrop  (in  our  diagram  to 
the  right)  for  an  unknown  distance,  as  suggested  by  the  dotted  lines. 
Indeed,  it  often  happens  that  the  continuation  of  the  interrupted 
bed,  recognized  by  its  character,  its  fossils,  or  otherwise,  is  found 
again  at  another  point  (in  our  diagram  farther  to  the  right),  where 
it  may  be  represented  by  another  inclined  layer  descending  into  the 
earth  in  the  opposite  direction.  In  that  case  it  is  evident  that  the 


FIG.  493.  —  Diagram  illustrating  the  relationships  of  dip  and  strike  of  the 
inclined  strata  of  an  outcropping  series  of  ledges.  The  strike  is  represented 
by  the  line  parallel  to  the  horizon  (horizontal  line) ;  the  dip  by  the  line  at  right 
angles  to  the  strike. 

two  outcrops  of  inclined  layers  are  parts  of  an  arch,  the  top  of  which 
has  been  cut  away  by  erosion.  Inclined  layers  ending  in  the  air 
may  be  produced  in  other  ways  (faulting,  etc.),  but  in  practically 
all  such  cases  the  inclined  part  visible  is  only  a  portion  of  a  much 
more  extensive  bed  which  continues  horizontally  or  otherwise  in 
one  direction  and  the  cut-off  edge  of  which  was  formerly  continuous 
in  the  other  direction. 

Dips.  —  In  the  study  of  the  deformational  structures  of  rocks  it 
becomes  necessary  to  measure  the  amount  of  inclination  of  the 
strata  and  the  changes  from  place  to  place.  The  angle  of  inclina- 
tion from  the  horizontal  is  called  the  angle  of  dip  (Fig.  493),  and 
it  is  measured  by  an  instrument  -called  the  clinometer. 

A  simple  form  of  clinometer  can  easily  be  made  by  any  one.  A  piece  of  card- 
board or  wood  with  parallel  sides,  or  the  inside  of  a  firm  note-book  cover,  forms 
the  foundation.  A  graduated  semicircle  of  paper  or  metal  is  then  fastened  to 
this,  so  that  the  ends  of  the  semicircle  are  on  a  line  parallel  with  the  upper  and 


586     Deformation  of  Rocks  of  the  Earth's  Crust 


lower  e.dges  of  the  supporting  board.  In  the  center  of  this  line  the  end  of  a 
thread  is  fastened  (by  a  pin  or  through  a  hole  in  the  board  and  a  knot  in  the 
thread),  and  a  small  weight  is  suspended  at  the  other  end  in  such  a  way  that 


FIG.  494  a.  —  A  home-made  clinometer.     (For  description  see  text.) 

it  hangs  below  the  arc  but  still  above  the  lower  edge  of  the  supporting  board 
(Fig.  494  a).  If  the  work  is  properly  done,  the  thread  should  cut  the  middle  of 
the  graduated  semicircle  when  the  lower  edge  of  the  board  rests  tipon  a  hori- 
zontal surface.  This  point  is  marked  zero.  When  the  lower  edge  of  the 

board  (or  the  upper,  which 
is  parallel  to  it)  rests  against 
a  vertical  plane,  the  string 
should  cut  either  end  of  the 
arc,  and  this  is  marked  90°. 
If  the  surface  on  which  the 
clinometer  is  placed  slopes 
halfway  between  these  two 
extremes,  the  string  cuts  the 
semi-circle  at  a  corresponding 
point,  and  this  is  marked 
45°.  The  other  divisions  are 
marked  accordingly.  Those 

FIG    494  b.  —  Diagram   illustrating   gently      who  desire  may  obtain  dabo_ 
inclined  strata,  with  the  clinometer  in  proper  j       constructed    clinom- 

position  to  measure  the  dip  at  right  angles  to 

the  strike.     (Drawn  by  Mary  Welleck.)  eters>    but    the    Beater    ac- 

curacy  of  observation   with 

them  will  scarcely  be  of  much  value  because  of  the  almost  constant  variation 
in  dip  from  point  to  point. 

Strike.  —  The  intersection  of  an  inclined  bed  with  a   horizontal 
surface  is  called  the  strike  of  the  outcrop  (Fig.  493),  and  its  direc- 


Deformation  by  Folding  587 

tion  is  measured  by  a  compass,  the' dial  of  which  is  graduated  to 
degrees.  Thus  if  this  line  of  intersection  extends  in  a  direction 
30  degrees  east  of  north,  the  strike  is  recorded  as  N.  30°  E.  (or  in 
exceptional  cases  S.  30°  W.).  The  dip  is  then  recorded  as  either 
east  or  west,  though  in  the  case  cited  it  would  really  be  north  of 
west  or  south  of  east,  because  it  is  at  right  angles  to  the  line  of  strike. 
Thus  a  record  of  strike  N.  30°  E.  and  dip  25°  W.  means  that  the 
angle  of  dip  is  25°  to  the  westward,  but  that  the  actual  direction 
of  the  dip  (which  is  at  right  angles  to  the  strike)  is  W.  30°  N.  (or 
N.  60°  W.).  Upon  a  map  these  facts  are  recorded  by  the  symbol 
T,  the  cross-arm  of  which  is  placed  in  the  direction  of  the  strike  and 
the  stem  in  the  direction  of  the  dip.  As  the  position  of  the  cross- 
arm  upon  the  map  indicates  the  direction  of  strike,  it  is  not  neces- 
sary to  record  this  in  figures  (except  for  greater  accuracy),  but  the 
angle  of  dip  is  recorded  opposite  the  stem  of  the  T-  Thus  if  the 
strike  is  due  north  and  south,  a  direction  represented  by  the  edge 
of  this  page,  and  the  dip  is  25°  to  the  west,  the  symbol  would  have 
this  position:  25°— f 

Deflection  of  Strike.  —  If  the  surface  on  which  an  inclined  stra- 
tum crops  out  is  not  horizontal,  but  inclined,  it  becomes  evident, 
on  reflection,  that  the  direction  which  the  outcrop  takes  upon 
such  a  surface  is  not  that  of  the  true  strike,  but  a  deflection  of  the 
same,  the  amount  of  deflection  depending  on  the  angle  of  dip  of  the 
stratum  and  the  angle  of  slope  of  the  surface.  Only  if  the  dip  of 
the  bed  is  90°,  or  vertical,  will  the  line  of  outcrops  be  the  same, 
whatever  the  slope  of  the  surface.  The  mode  of  deflection  is  illus- 
trated in  the  following  diagram  (Fig.  495).  A  BCD  represents 


FIG.  495.  —  Diagram  illustrating  the  deflection  of  outcrop  of  an  inclined 
stratum,  a,  b,  c,  d,  from  the  true  strike  be,  on  a  horizontal  surface  (EFGH)  to 
a  false  strike  (cV)  on  an  inclined  surface,  A  BCD.  , 


588       Deformation  of  Rocks  of  the  Earth's  Crust 

the  sloping  surface  of  the  ground,  EFGH  a  horizontal  plane  ;  abed 
is  the  inclined  bed,  the  direction  of  true  dip  of  which  is  indicated 
by  the  sides  of  this  bed.  It  is  evident  that  the  true  strike  is  the 
line  be,  the  intersection  with  the  horizontal  plane,  whereas,  the 
actual  outcrop  of  this  stratum  on  the  sloping  surface  is  represented 
by  the  line  b'c. 

The  degree  of  deflection  of  the  outcrop  from  the  true  strike  can  be  calcu- 
lated from  the  following  formula: 

tan  ^  =  cot  6  tan  <£,  or  ^  =  tan-1  (cot  0  tan  <£). 

when  ^  is  the  angle  of  deflection  ;  6  the  angle  of  dip  of  the  stratum,  and  <£ 
the  angle  of  inclination  of  the  sloping  surface  from  the  horizontal.1  When  it  is 
possible  to  get  the  true  strike  on  a  horizontal  surface  and  its  deflection  on  a 
sloping  surface,  the  angle  of  dip  (  6}  may  be  calculated  by  the  following  formula  : 
tan  0=tan<£  cot  ^,  or  6  =  tan-1  (tan  <£  cot^). 

The  illustrations  in  Fig.  496,  a,  b  show  the  manner  in  which 
the  outcrops  of  inclined  beds  are  deflected  along  the  sides  of 
valleys  with  different  slopes. 


FIG.  496.  —  Models  showing  deflection  of  outcrop  of  dipping  strata  on  sloping 
surfaces,  a,  slope  of  valley  40°,  dip  of  strata  20° ;  b,  slope  of  valley  20°,  dip 
of  strata  50°.  (After  Lyell.) 

Relation  between  Dip  and  Width  of  Outcrop.  —  When  a  stratum 
of  rock  is  tilted  to  the  vertical  and  ends  in  a  horizontal  erosion  sur- 
face, the  width  of  the  outcrop  on  the  surface  is  equal  to  the  thickness 
of  the  stratum.  If  the  tilting  is  less  than  vertical,  the  width  of  the 
outcrop  on  the  horizontal  surface  will  be  greater  than  the  thickness 
of  the  bed,  and  will  increase  in  proportion  as  the  dip  of  the  stratum 
decreases.  From  the  width  of  the  outcrop  and  the  angle  of  dip, 
the  thickness  of  the  bed  may  be  ascertained  by  simple  mathematical 
calculation. 

1  For  fuller  discussion,  see  A.  W.  Grabau,  Principles  of  Stratigraphy,  pp.  800-806. 


Deformation  by  Folding 


589 


Thus  in  the  following  diagram  (Fig.  497)  the  width  of  the  outcrop  is  shown 
oy  AB,  and  the  thickness  of  the  bed  to  be  ascertained  by  AC,  which  is  a  line 
at  right  angles  to  the  upper  and 
lower  surfaces.  The  dip  is  the 
angle  ABC,  which  is  one  angle 
of  a  right-angled  triangle  of  which 
the  length  of  the  hypotenuse, 
AB,  is  known  and  the  side  AC 
to  be  ascertained.  This  is  found 
by  the  following  formula : 


or 


FIG.  497.  —  Diagram  illustrating  width 
of  outcrop  AB  of  the  bed,  the  thick- 
ness of  which  is  AC;  its  depth  at  X  is 
measured  by  the  length  of  XY. 


AC=sin  ABCXAB. 
If  the  angle  ABC,  that  is,  the 
dip,  is  30  degrees,  and  the  width 

of  the  outcrop  AB  100  feet,  we  have  AC  =  sin  30° X  100  =  0.5X100  =  50  feet. 
In  like  manner,  if  the  thickness  of  a  bed  and  its  dip  are  known  while  the  outcrop 
is  mostly  covered  by  soil,  its  width  for  purposes  of  mapping  can  be  determined 


by  the  formula  :  AB  = 


AC 


sin^BC 


,  if  the  thickness,  AC,  is  50  feet,  and  the 


dip  30°,  gives  A  B  =  ^-  =  ioo  feet.     If  the  bed  in  question  is  a  coal  seam  or  a 

o-5 

water-bearing  stratum,  its  depth  below  the  surface  at  any  point  within  the 
belt  of  outcrop  may  be  determined  by  noting,  the  distance  from  the  outcrop  and 
the  dip.  Thus  at  X  the  depth  X  Y  to  the  bed  is  to  be  ascertained.  The  dis- 
tance A  X  is  measured,  and  the  dip  XA  Y  determined.  Then  we  apply  the 

formula  :  tan  XA  Y  =  —  or  X  Y  =  A  X  tan  XA  Y.     With  the  dip  30°  and  the 

yl    J\. 

distance  AX  300  feet,  we  have  XF  =  3ooXtan  3o°  =  3ooXo.5774=i73.22 
feet.  If  the  dip  of  a  water-bearing  stratum  is  i°  and  the  distance  from  the 
point  of  outcrop  50  miles,  while  the  elevation  of  the  outcrop  is  1000  feet  above 
the  point  at  which  the  well  is  to  be  sunk,  the  depth  beneath  the  surface  will  be 
(tan  i° X 50 X S 280)  — 1000  =  (.01 75  X 5oX  5 280)  — 1000  =  4620  — 1000  =  3620  feet. 
This  assumes  that  the  dip  is  constant,  which  for  so  low  an  angle  in  a  region  of 
little  disturbance  is  likely  to  be  the  case. 


Types  of  Folds 

Three  principal  types  of  simple  folds  are  recognized :  (a)  anti- 
clines, (b)  synclines,  and  (c)  monoclines.  Each  of  these  has  various 
modifications. 

The  Anticline.  —  Arched  folds  are  called  anticlines  (Fig.  498), 
and  their  sides,  which  are  called  the  limbs,  dip  away  from  the  crest- 
line  or  axis  of  folding.  Such  folds  may  vary  from  broad  and  gentle 
to  sharply  ridged  arches.  If  both  limbs  dip  at  the  same  angle, 


590     Deformation  of  Rocks  of  the  Earth's  Crust 


the  anticline  is  called  symmetrical,  and  this  type  is  most  character- 
istic of  the  Jura  Mountains  of  Switzerland  (Fig.  499,  a),  though  ex- 

amples are  also  found 
in  the  Appalachians 
(Fig.  500)  and  else- 
where. If  one  limb 
has  a  steeper  dip  than 
the  other,  the  anti- 
cline is  asymmetrical 
(Figs.  499,  b,  501). 
In  the  Appalachian 
Mountains,  the  folds 
of  which  have  their 
axes  extending  in  a 
northeasterly  direc- 


FIG.  498.  —  A  symmetrical  anticlinal  fold,  the  top 
of  which  has  been  eroded.  Near  St.  Abb's  Head, 
Scotland.  (After  Geikie.) 


tion,  the  western  limb 
of  the  anticline  is 
generally  steep,  often 
vertical  or  even  overturned,  while  the  eastern  limb  is  more  gentle. 
Overturned  anticlines  (Fig.  499,  c)  are  characteristic  of  some  strongly 
folded  regions,  and  in  these  one  limb  comes  to  lie  under  the  other, 
and  the  beds  of  this  limb  appear  in  the  reversed  order.  If  the 
overturning  is  so  extreme  that  the  limbs  lie  nearly  or  quite  hori- 


FIG.  499.  —  Types  of  folds,  a,  symmetrical  anticline ;  b,  asymmetrical  anti- 
cline ;  c,  overturned  anticline ;  d,  recumbent  fold ;  e,  fan- fold. 

zontally,  the  folds  are  said  to  be  recumbent  (Fig.  499,  d).  Over- 
turned strata,  can  often  be  recognized  as  such  by  the  position  on 
their  surfaces  of  ripple-marks,  rain-prints,  foot-prints,  rill  marks, 
mud-cracks,  and  other  structures.  These  will  be  seen  on  the  under 
side  of  overturned  strata,  while  their  reverse  impressions  will  ap- 
pear on  the  upper  side,  both  being  the  reverse  of  the  normal  posi- 
tion. 

Fan-shaped  folds  (Fig.  499,  e)  are  produced  when  the  lower  parts 
of  the  limbs  of  the  anticline  are  so^  compressed  that  the  upper  part 
bulges  beyond  them  on  both  sides.  Such  folds  are  found  in  the 
Alps  (Fig.  523,  p.  606). 


Deformation  by  Folding 

When  the  limbs  of  the  fold  are  parallel  or  nearly  so,  an  isoclinal 
folding  is  produced  (Fig.  502).     It  will  sometimes  be  difficult  to 


FIG.  500.  —  Classic  arch  or  symmetric  anticline  of  Upper  Silurian  red  sand- 
stone (in  Wills  Creek  formation),  Roundtop,  Md.  Banks  of  the  Chesapeake 
and  Ohio  Canal.  The  center  of  the  arch  has  crumbled  away,  leaving  a  cavern. 
(Photo  by  Stose,  from  U.  S.  G.  S.) 


FIG.  501.  —  A  small  asymmetric  anticline  in  Upper  Silurian  shales  and  sand- 
stones, High  Falls,  N.  Y. 

determine  the  structure  of  an  isoclinal  series  of  folds  if  the  top  has 
been  eroded  away.  In  such  a  case  all  the  beds  may  appear  to  be- 
long to  one  series  (Fig.  502,  a),  unless  it  can  be  shown  by  the  charac- 


592      Deformation  of  Rocks  of  the  Earth's  Crust 

• 

ter  of  the  beds  and  by  their  fossils  and  otherwise  that  there  is  a 
repetition  of  the  same  strata  in  the  series.  In  the  diagram  (Fig. 
502)  such  a  regular  repetition  of  strata  is  shown,  but  even  here  it 


FIG.  502.  —  Isoclinal  folds,  a,  outcrop  of  strata  in  which  there  is  a  certain 
repetition  of  similar  beds,  which  by  their  fossils  or  otherwise  are  recognized 
to  be  repetitions  of  the  same  strata;  b,  restoration  as  two  anticlines  and  a 
syncline ;  in  this  case,  4,  4  is  the  oldest  bed ;  c,  restoration  as  two  synclines 
separated  by  an  anticline ;  in  this  case,  4,  4  is  the  youngest  bed.  (From  Prin- 
ciples of  Stratigraphy.) 

is  possible  to  reconstruct  the  series  in  two  ways,  as  anticlines  with 
a  narrow  syncline  between  (Fig.  502,  £),  or  as  synclines  separated 
by  an  anticline  (Fig.  502,  c).  The  correct  reconstruction  depends 
upon  the  recognition  of  the  relative  ages  of  the  beds,  the  older  being 
at  the  centers  of  the  anticlines,  and  the  younger  at  the  centers  of 
the  synclines.  If  this  cannot  be  determined,  recourse  may  be 


FIG.  503.  —  Eroded  folds,  a,  Symmetrical  anticline,  from  the  axis  of  which 
the  higher  beds  have  been  eroded  down  to  a  harder  stratum;  b,  breached  asym- 
metrical anticline,  the  hard  stratum  forming  two  opposing  uniclines. 

had  to  structures  such  as  ripple-marks,  mud-cracks,  etc.,  which 
will  indicate  the  upper  and  under  sides  of  the  strata.  It  is  apparent 
that  the  upper  sides  of  the  strata  will  be  on  the  inside  of  the  syn- 
cline but  on  the  outside  of  the  anticlines. 


Deformation  by  Folding 


593 


In  general,  when  folding  is  pronounced,  the  beds  in  the  axial 
part  of  the  anticline  as  in  that  of  the  syncline  (see  beyond)  are 
thickened,  while  those  of  the  limbs  become  thinner.  This  reaches 
its  extreme  in  fan-folds,  but  is  also  observable  in  others.  It  im- 


FIG.  504.  —  Block-diagram  of  the  uniclinal  ridges  of  the  Appalachian  type; 
the  original  anticline  from  which  the  uniclines  are  cut  was  asymmetric,  with 
the  steeper  limb  on  the  west. 

plies  a  certain  amount  of  transference  of  the  material  of  the  beds 
from  the  limbs,  to  the  axes. 

Small  anticlines  may  be  complete,  but  the  larger  structures 
of  this  type  have  commonly  suffered  erosion  along  their  tops. 
This  is  comparatively  slight  in  the  Jura  Mountains,  where  large 
anticlines  are  still  nearly  complete  with  only  their  upper  layers 
partly  removed  from  the  axis  (Fig.  503,  a).  In  the  Appalachian 
Mountains,  on  the  other  hand,  the  anticlines  have  been  for  the  most 
part  deeply  dissected  so  that  a  depression  or  valley  lies  at  their  axes 
and  the  cut  ends  of  the  limbs  end  in  the  air  (Figs.  503,  b,  504).  If 


FIG.  505.  —  Uniclinal  ridges  formed  by   the  erosion  of  an  anticline.     Utah. 
(F.  J.  Pack,  Photo.) 

the  folds  of  the  original  anticlines  consist  of  alternating  hard  and 
soft  strata,  a  series  of  parallel  ridges  formed  by  the  hard  layers,  and 
valleys  formed  on  the  soft  layers  may  be  produced  as  the  result  of 
erosion.  The  beds  of  corresponding  ridges  will  dip  in  opposite 
directions.  This  is  of  common  occurrence  in  the  Appalachians, 


594     Deformation  of  Rocks  of  the  Earth's  Crust 

and  to  such  erosion  remnants  of  anticlinal  folds,  the  name  unicline 
is  applied  (Fig.  505).     (See  further,  Chapter  XXII.) 

Pitch  of  Axis  of  Anticline.  —  The  axis  of  the  anticline  may  con- 
tinue in  a  horizontal  position  for  a  long  distance  (sometimes  for 


FIG.  506.  —  Diagram  showing  pitching  folds  and  the  topography  formed  by 
their  erosion.     (Drawn  by  F.  K.  Morris,  Military  Geology.) 

more  than  a  hundred  miles),  but  eventually  it  descends  or  pitches 
into  the  .ground  and  the  fold  dies  away  (Fig.  506).  When  the 
pitching  axis  of  an  anticline  is  planed  across  horizontally  by  erosion, 


FIG.  507.  —  Eroded  anticlines  with  horizontal  and  with  pitching  axes.  In 
the  first  case  the  strata  crop  out  in  parallel  series,  the  oldest  at  the  center;  in 
the  second  case  they  converge  in  the  direction  of  pitch.  (From  Principles  of 

Stratigraphy.) 

the  upper  ends  of  the  several  beds  will  be  seen  to  converge  in 
the  direction  of  the  pitching  and  eventually  to  meet  (Fig.  507). 
When  some  of  these  beds  form  ridges,  these  too  meet  and  enclose 
a  semicircular  canoe-shaped  valley  (Fig.  508). 


Deformation  by  Folding 


595 


Domes.  —  Anticlines  with  very  short  axes  are  called  domes.  The 
length  of  the  axis  may  be  several  times  the  transverse  diameter  of  the 
dome,  or  it  may  nearly  equal  it,  but  domes  are  seldom  quite  circular 
in  basal  outline.  Such  domes  may  be  pronounced,  with  steeply 
dipping  sides,  as  in  the  case  of  the  Black  Hills  of  South  Dakota,  or 


FIG.  508.  — Development  of  concentric  ridges  in  anticlinal  structures.  A .  An 
anticlinal  or  cigar-shaped  mountain  results  from  the  position  of  the  hard  rock. 
B.  The  anticlinal  mountain  A  is  replaced  by  an  anticlinal  valley  because  of 
the  position  of  the  strata.  (After  A.  K.  Lobeck.)  Note  that  the  cut  faces  of  the 
ridge-forming  strata  face  inwards.  These  ridges  are  "monoclines  of  erosion" 
or  "uniclines." 

they  may  be  so  gentle  that  the  dip  of  the  strata  is  scarcely  perceived. 
The  latter  are  by  far  the  more  common,  and  of  them  the  Cincinnati 
dome  may  serve  as  an  example.  On  account  of  their  large  size 
and  gently  dipping  strata  they  are  recognized  only  when  plotted 
upon  a  geological  map. 

Anticlinoria.  —  A  mountain  mass  composed  of  a  number  of  anti- 
clines may  have  these  so  arranged  that  they  form  a  part  of  a  larger 
arch.  This  compound  anticlinal  series  is  called  an  anticlinorium. 


FIG.  509.  —  Anticlinorium.      Generalized  section  in  the  Alps.      (After  Heim.) 

Such  an  anticlinorium  may  constitute  a  part  of  a  mountain  mass,  as 
in  the  Alps  (Fig.  509) ;  or  the  central  beds  may  be  eroded  because 
these  are  soft,  and  so  a  valley  is  produced.  The  Wealden  district 
of  southeastern  England  is  such  an  eroded  anticlinorium,  the  minor 
foldings  being  seen  in  the  variable  dips  of  the  strata  in  the  center, 
which,  because  of  the  softness  of  the  strata,  has  generally  been 
transformed  by  erosion  into  a  valley. 


596     Deformation  of  Rocks  of  the  Earth's  Crust 

Synclines.  —  When  two  or  more  anticlines  occur  in  a  folded  sys- 
tem such  as  the  Jura  or  Appalachian  Mountains,  they  are  separated 
by  trough  folds  or  synclines  (Figs.  510,511).  These  are  symmetrical 


FIG.  510. —Natural  section,  showing  two  anticlines  and  a   syncline.     The 
surface  is  a  peneplane  cutting  across  the  folds.     Scotland.     (After  Geikie.) 


FIG.  511.  —  Synclinal  fold  near  Banff,  Scotland.     (Geikie.) 

in  the  Jura  but  asymmetrical  in  the  Appalachians,  corresponding  in 
character  to  the  anticlines.  In  the  Appalachians  the  eastern  limb 
(or  arm)  of  the  syncline  is  steep,  vertical,  or  overturned,  being  in 


Deformation  by  Folding 


597 


FIG.  512  a.  —  Rapid  change  in  thickness  of  beds,  due  to  strong  folding,  w1, 
Neocomian ;  n2,  Schrattenkalk ;  g,  Gault ;  Kr,  Seewer  limestone  (Upper  Cre- 
taceous). After  Alb.  Heim.  Santis.  (From  Kayser's  Lehrbuch.) 


FIG.  512  b.  —  Synclinal  fold  of  Knox  dolomite,  showing  thinning  on  the 
limbs  of  the  fold  and  thickening  in  the  axis.  One  half  mile  north  of  Embree- 
ville,  Tenn.  (Photo  by  Keith,  from  U.  S.  G.  S.) 


598     Deformation  of  Rocks  of  the  Earth's  Crust 


fact  the  western  limb  of  the  anticline  next  east.  The  western  limb 
(or  arm)  of  the  syncline,  which  is  also  the  eastern  limb  of  the  anti- 
cline next  west,  has  a  moderate  inclination.  In  the-  center  of  the 
syncline  the  beds  are  usually  thickened,  as  is  well  shown  in  the  sec- 
tion of  Alpine  folds  in  Fig.  512  a.  It  is  also  seen  in  the  Appala- 
chian Mountains  (Fig.  51 26).  This  portion  of  the  fold  may  also 
be  characterized  by  numerous  smaller  folds,  this  complex  when 


FIG.  513.  —  Synclinorium.     Section  of  Mt.  Greylock,  Mass.     (After  Dale.) 

occurring  on  a  sufficiently  large  scale  being  called  a  syndinorium 
(Fig.  513).  Synclines  may  also  be  overturned  and  recumbent,  and 
their  limbs  may  be  parallel,  forming  a  part  of  an  isoclinal  series  of 
folds  as  in  Fig.  502,  p.  592. 

Topographically,  the  beds  of  a  much  eroded  syncline  may  also 
form  parallel  ridges,  but  the  cut  edges  of  these  face  outward,  instead 


FIG.  514. — Development  of  concentric  ridges  in  synclines.  A,  a  region 
with  synclinal  or  canoe-shaped  folds.  In  B  the  synclinal  valley  shown  in  A 
is  replaced  by  a  mountain  because  of  the  occurrence  of  a  hard  stratum  in  the 
center.  (A.  K.  Lobeck.)  Compare  with  Fig.  508,  p.  595.  Note  that  the  cut 
edges  of  the  hard  ridge-forming  strata  face  outward.  The  ridges,  too,  are 
"monoclines  of  erosion"  or  uniclines. 

of  inward  as  in  the  case  of  the  dissected  anticline  (Fig.  514).  For 
such  ridges  the  term  unicline  is  used.  They  are  also  called 
"  monoclinal  ridges  of  erosion." 

As  the  syncline  dies  out,  the  axis  rises,  and  in  eroded  synclines 
the  cut  edges  of  the  strata  and  the  ridges  which  they  form  converge 
and  finally  unite.  Thus  another  series  of  rimming  ridges  and  canoe- 
shaped  valleys  is  produced. 


Deformation  by  Folding 


599 


Chief  Min 


Lewis  Range 


By  deep  erosion  of  the  centers  of  adjoining  anticlines,  the  center 
of  the  syncline  may  be  left  standing  in   relief  and  a  synclinal 
mountain    is    produced    (Fig. 
515,    see   also    Figs.    513    and 
514,  B).     Such  mountains  are 
not  uncommon  in  the  Appala- 
chian and  other  deeply  eroded 
mountain  systems. 

Basins.  —  A  syncline  with 
much  shortened  axis  produces  a 
basin,  which  is  the  reverse  of 
the  dome.  The  majority  of 
basins  have  their  strata  so 
gently  inclined  that  they  appear 
to  be  practically  horizontal. 
They  can  be  recognized,  how- 
ever, in  eroded  regions  by  the 
fact  that  the  edges  of  the  older 
or  lower  strata  project  progres- 
sively beyond  those  of  the 
younger  or  upper,  while  their 
outcrops  form  a  more  or  less 
completely  encircling  series  of 
bands.  Where  hard  strata  are 
involved,  these  form  rimming 
ridges  with  their  cut  ends  facing 
outward.  Such  a  series  of  rim- 
ming cliffs  partly  encircles  the 
Paris  basin,  in  the  center  of 
which  lies  the  city  which  gives 
it  its  name.  The  basin  in  the 
center  of  which  London  lies  is 
another,  though  less  perfect  ex- 
ample. In  our  own  country, 
the  Michigan  basin,  which  com- 
prises the  whole  of  the  Lower 
Peninsula,  is  a  typical  example. 

On  the  west,  north,  and  east  of  this  basin  the  lowlands  cut  on  the 
softer  strata  are  partly  occupied  by  the  waters  of  some  of  the  Great 
Lakes  and  their  embayments  (Figs.  516,  517). 


Livingston 


6oo     Deformation  of  Rocks  of  the  Earth's  Crust 


The  Monocline.  —  A  simple  flexure  in  the  rocks  in  which  the  beds 
bend  up  or  downward  and  then  continue  in  the  same  direction,  is 
called  a  monocline.  Such  monoclines  are  found  on  the  margins  of 

folded  areas  and   in 

'  VJ     /5^^r\  regions  of  minor  dis- 

turbance as  in  the 
Colorado  Plateaus 
(Figs.  690  a,  b),  where 
they  form  important 
structural  features. 
A  monocline  may 
pass  into  a  fault  as 
shown  in  the  diagram 
(Fig.  518  a).  The 
monoclines  of  the 
Colorado  Plateau 
region  (Grand  Canon 
district)  commonly 
pass  into  what  may 
be  termed  an  in- 
verted fault,  where 
the  upflexed  side 
becomes  the  down- 
throw side  of  the 
fault,  and  the  down- 
flexed,  the  upthrow 
side  of  the  fault 

(Fig.  5186).  In  these  faults  the  beds  of  the  upthrow  block  bend 
upwards  as  they  approach  the  fault  plane  and  those  of  the  down- 
throw block  bend  downward.  This  probably  indicates  two  move- 
ments, a  flexing  to  the  breaking  point,  and  a  settling  back  with 
faulting,  of  the  elevated  block. 

Relation  of  Folded  Mountain  Series  to  Geosyndines  of  Deposition 

All  the  larger  folded  mountain  systems  appear  to  be  located  along 
regions  which  were  formerly  geosynclines  of  deposition  (p.  518). 
This  is  shown  by  the  fact  that  in  such  folded  areas  the  formations 
are  all  of  much  greater  thickness  than  elsewhere.  Several  of  these 
may  be  noted. 


FIG.  516.  —  Geological  map  of  the  Michigan 
Basin,  showing  the  successive  rims  formed  by  the 
outcrops  of  the  strata  around  the  central  coal 
basin.  (Mich.  Geol.  Survey.) 


Deformation  by  Folding 


60 1 


The  Appalachian  Mountains.  —  These  were  'formed  by  the  fold- 
ing of  the  strata  which  accumulated  in  the  Appalachian  geosyncline 
to  a  thickness  of  perhaps  40,000  feet.  This  geosyncline  was  bounded 


on  the  east  by  a  high  more  or  less  mountainous  land  mass,  the  an- 
cient Appalachia,  from  which  the  clastic  material  which  makes  up 
a  large  part  of  the  strata  was  chiefly  derived.  The  series  comprises 
alternating  marine  and  continental  beds,  but  on  the  whole  is 
rather  uniform,  and  constant  subsidence  is  indicated  for  the  old 


602     Deformation  of  Rocks  of  the  Earth's  Crust 


FIG.  518  a.  —  Monocline  replaced  in 
the  lower  strata  by  a  fault. 

the  steep  or  overturned  limb  of 
would  seem  that  the  movement 
which  produced  the  folds  came 
from  the  east,  or  from  the  Ap- 
palachian old  land.  The  most 
intense  folding  is  on  the  east,  the 
next  upon  the  western  border 
(Fig.  519).  There  are  also  pro- 
nounced overthrusts  (see  p.  626). 
Beyond  the  strongly  folded  west- 
ern part  minor  folds  occur,  but 
soon  die  out,  and  the  strata  be- 
come approximately  horizontal. 
The  force,  however,  appears  to 
have  been  transmitted  over  a  great 
part  of  eastern  North  America, 
with  the  development  of  an  ex- 
tensive series  of  low  domes  and 
shallow  basins,  such  as  the  Cin- 
cinnati dome  and  Michigan  basin. 
Some,  or  perhaps  most,  of  these 
had  begun  to  form  at  an  earlier 
time,  probably  during  one  or  more 
of  the  earlier  periods  of  folding 
but  their  most  pronounced  struc- 
ture was  given  to  them  during 
the  Appalachian  folding.  It  must 
be  remembered  that  these  domes 
and  basins  are  so  large  and  so 
gentle  that  the  strata  appear  hori- 
zontal and  their  dome-  and  basin- 
character  can  be  seen  only  on  the 


geosynclinal  trough.  Folding 
occurred  locally  at  several 
periods,  but  the  great  folding 
took  place  toward  the  end  of 
the  Palaeozoic  era.  This  pro- 
duced the  asymmetrical  anti- 
clines and  synclines  character- 
istic of  this  system.  Because 
the  anticlines  is  on  the  west,  it 


FIG.  5186.  —  Diagram  showing 
early  stages  in  the  development  of 
the  Hurricane  fault  in  the  Grand 
Canon  region,  —  a  monoclinal 
fold  develops  into  an  "inverted" 
fault,  the  upflexed  side  becomes 
the  downthrow  side,  the  down- 
flexed  the  upthrow.  Note  that 
the  strata  at  the  border  of  the 
fault  plane  bend  in  the  reverse" 
direction  of  that  due  to  drag  along 
a  simple  fault.  The  upper  dia- 
gram represents  the  monoclinal 
flexure;  the  middle,  the  faulting; 
and  the  lower,  the  appearance  after 
erosion  (peneplanation).  This  was 
followed  by  further  movements 
in  the  same  direction  along  the 
fault  plane.  East  on  the  left. 
(After  D.  W.  Johnson.) 


Deformation  by  Folding 


603 


study  of  extended  areas.  Wherever  two  basins  adjoin  each  other, 
there  is  a  sharp  but  small  anticline  or  series  of  anticlines  formed 
between  them,  as  shown  in  the  lower  sections  on  this  page  (Fig. 


FIG.  519.  —  Restoration  of  the  Appalachian  folds  of  Pennsylvania  between 
Harrisburg  on  the  east  (right)  and  Tyrone  on  the  west  (left),  showing  the  differ- 
ent intensities  of  folding,  and  the  portion  of  the  earth's  crust  (ABC)  affected 
by  the  compressive  movement.  (After  R.  D.  Chamberlin.) 

520).  In  some  cases  these  anticlines  have  proved  favorable  for  the 
accumulation  of  oil  and  gas.  In  like  manner,  a  syncline  or  group 
of  synclines  lies  between  adjoining  domes. 

The  trend  of  the  Appalachian  folds  conforms  more  or  less  to  the 
position  of  the  domes  and  basins  adjoining  them,  being  bowed  out- 
ward around  the  basins  and  inward  between  them  (Fig.  521). 


a  0 

FIG.  520.  —  Cross-sections  of  shallow  basins,  separated  (a)  by  a  simple  anti- 
cline, (6)  by  two  anticlines  and  a  syncline. 

The  Carpathians.  —  As  a  second  example,  the  Carpathian 
Mountains  of  Europe  may  be  cited.  These  form  a  semicircular 
arc,  enclosing  the  basins  of  Hungary  and  Transylvania  on  the 
north,  east,  and  south,  and  they  are  a  part  of  the  Alpine  system  of 
folds  (map,  Fig.  522).  They  are  of  much  more  recent  origin  than 
the  Appalachians,  having  been  formed  in  Tertiary  time.  The  out- 
line of  this  arc  appears  to  have  been  determined  by  the  form  of 


604     Deformation  of  Rocks  of  the  Earth's  Crust 

the  geosyncline  in  which  the  strata,  which  became  involved  in  the 
folds,  were  deposited.  The  old  land  from  which  the  clastic  material 
was  derived  lay  to  the  north  and  the  east  of  the  present  mountain 
chain,  and  the  sediments  were  carried  toward  the  Hungarian  basin, 
most  of  them  lodging  in  the  geosyncline,  around  its  outer  (eastern 
and  northern)  margin.1  The  first  folding  took  place  at  the  begin- 
ning of  Mid-Tertiary  (Miocene)  time,  and  resulted  in  the  formation 
of  mountain  chains  from  the  deposits  in  the  geosyncline.  The 
movement  was  probably  toward  the  Hungarian  basin.  During 


FIG.  521.  —  Map  of  the  domes  and  basins  of  eastern  North  America,  and  of 
the  outline  of  the  Appalachian  chains.     1-6,  land  lobes ;   i  a-6  a,  sea  lobes. 

this  folding,  or  probably  as  a  part,  if  not  a  cause  of  it,  a  new  geo- 
syncline came  into  existence  by  the  sinking  of  a  part  of  the  old  land 
which  had  supplied  the  sediments.  This  sinking  occurred  along  a 
belt  parallel  to  and  outside  of  (nqrth  and  east  of)  the  newly  formed 
semicircle  of  mountains.  In  this  new  geosyncline,  later  sediments, 
to  the  extent  of  several  thousand  feet,  were  deposited,  these  being 
in  large  part  derived  from  the  newly  formed  Carpathians.  In  this 
geosyncline  were  also  formed  many  of  the  important  salt  deposits 
of  that  region.  Then  came  a  second  folding,  this  time  with  a  move- 
ment toward  the  new  geosyncline  (eastward  and  northward)  so  that 
the  strata  of  this  geosyncline  were  thrown  into  asymmetric  anti- 


1  A.  W.  Grabau,  Geology  of  the  Non-Metallic  Mineral  Deposits,  Vol.  II,  Chapter 
XXXII,  McGraw-Hill  Book  Co. 


Deformation  by  Folding 


605 


>  -r       '-  ' 


+J    <-i    I— I     -M      t/1     <U      0"^     V 


606     Deformation  of  Rocks  of  the  Earth's  Crust 


clines,  with  steep  limbs  on  the  east  and  north  or  away  from  the 
mountains.     At  the  same  time,  the  already  folded  strata  of  the 

earlier  Carpathians  were  folded 
more  intensely,  and  portions  were 
thrust  over  toward  the  Russian 
regions  on  the  east.  Thus  the 
horizontal  rocks  of  Russia  and  the 
coal  fields  of  Silesia  on  the  north 
pass  under  the  folded  rocks  of  the 
Carpathians,  which  have  over- 
ridden them. 

The  movement  toward  the  Hun- 
garian plain  during  the  first  fold- 
ing appears  to  have  affected  the 
rocks  of  this  plain  also,  by  break- 
ing them  into  a  series  of  blocks  in- 
stead of  developing  domes  and 
basins  as  in  North  America. 

The  Alps.  —  These  are  among 
the  most  complex  mountain  sys- 
tems known.  The  main  line  forms 
an  arc  extending  around  the  plains 
of  North  Italy,  in  general  north 
and  south  along  the  Franco-Italian 
border,  and  turning  eastward  in 
Switzerland  and  the  Tyrol  (see 
map,  Fig.  522,  p.  605).  As  in  the 
case  of -the  Carpathians,  this  arc 
was  outlined  by  the  old  geosyncline 
in  early  Tertiary  time  and  the  form 
of  the  old  land  on  the  west  and 
north.  After  deposition  of  great 
thicknesses  of  clastic  material,  de- 
rived from  this  old  land,  had  taken 
place  in  the  geosynclines,  the  first 
folding  occurred  at  the  beginning 
of  Mid-Miocene  time.  The  move- 
ment there  was  apparently  toward 
the  Italian  region,  which  was 
broken  into  blocks,  while  the 


Deformation  by  Folding 


607 


strata  were  overturned  toward  that  region, 
thians,  another  new  geosyncline  came  into 
land,  north  and  west 
of  the  newly  formed 
chain,  and  in  this 
geosyncline  great 
masses  of  clastic  ma- 
terial, derived  from 
.the  newly  formed 
Alps,  were  deposited. 
Then  came  the  sec- 
ond folding  toward 
the  new  geosyncline 
(west  and  north)  so 
that  the  strata  were 
folded  over  in  the 
other  direction,  while 
at  the  same  time 
great  blocks  were 
thrust  over  in  this 
direction.  In  this 
manner  was  probably 
formed  the  peculiar 
fan  structure  of  the 
Alpine  folds  (Fig. 
523)- 

Structures  Due  to  Re- 
peated Folding 

In  the  Alps,  Car- 
pathians, and  other 
great  mountain  sys- 
tems of  Europe  and 
elsewhere,  at  least 
two  periods  of  folding 
and  perhaps  more  are 
recognized,  following 
each  other  with  com- 
paratively short  in- 
tervals. These  two 


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608     Deformation  of  Rocks  of  the  Earth's  Crust 


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Deformation  by  Folding 


609 


foldings  appear  to  have  been  in  opposite  directions,  the  last  move- 
ment being  toward  the  west  and  northwest  in  the  French  Alps 
(Fig.  524),  and  toward  the  north  in  the  Swiss-Italian  and  Austrian 
Alps  (Fig.  525) ;  while  in  the  Carpathians  it  was  northward  in  the 
northern,  eastward  in  the  eastern,  and  southward  in  the  southern 
portion  of  the  arc.  The  results  of  this  compound  folding  have 
produced  extremely  complex  mountain  systems,  the  complexity 


FIG.  527.  —  Photograph  of  the  unconformity  between  the  Silurian  and 
Ordovician  formations  on  the  Vlightberg  above  Rondout  (Kingston),  N.  Y.  On 
the  right  are  the  Hudson  River  sandstones  and  shales  with  a  strike  of  N.  30°  W. 
and  a  dip  of  51°  to  the  northeast.  Against  the  eroded  surface  of  these  rest 
the  Upper  Silurian  limestones  (Cobleskill)  which  strike  N.  56°  E.  and  dip  72° 
to  the  northwest.  The  large  inclined  masses  of  limestone  in  the  left  half  of 
the  view  are  the  highest  beds  of  the  Silurian  (Manlius)  which  originally  had 
the  same  inclination  as  the  Cobleskill,  the  space  between  these  two  series  being 
occupied  by  the  Rondout  water  lime  which  has  been  quarried  out,  after  which 
the  Manlius  fell  over  into  its  present  position.  (Photo  by  M.  O'Connell.) 
(See  Fig.  528  a.) 

being  increased  by  great  overthrusts.  In  the  Caucasus  the  folding 
was  very  intense,  so  that  all  the  beds  are  either  vertical  or  steeply 
inclined  (Fig.  526). 

In  the  Appalachians,  where  more  than  one  period  of  folding  is 
recognized,  the  foldings  appear  to  have  been  in  the  same  general 
direction,  though  in  part,  at  least,  with  a  slight  change  in  strike. 
Moreover,  the  movements  of  at  least  two  of  these  foldings  of  which 
we  have  positive  evidence  occurred  at  long  intervals,  one  near  the 
close  of  the  Ordovician  period,  the  other  toward  the  close  of  the 


6io     Deformation  of  Rocks  of  the  Earth's  Crust 


Permian,  although  there  were,  in  some  localities  at  least,  others 
between  these.  In  the  intervals  between  the  foldings,  extensive 
erosion  of  the  older  folded  series  took  place,  after  which  more  strata 
were  deposited  upon  these  eroded  surfaces,  and  the  whole  complex 
folded  again.  Thus  a  complicated  structure  was  produced,  but  the 
complication  is  less  than  in  the  Alps.  The  illustration  on  page 
609  (Fig.  527)  presents  the  appearance  of  an  outcrop  in  a  hillside 
above  Rondout,  N.  Y.,  which  shows  strikingly  the  complexity 
resulting  from  such  double  folding.  (See  also  Fig.  528  a.) 

A  bed  of  limestone,  S,  of  Upper  Silurian  (Monroan)  age  is  seen  to 
have  a  nearly  vertical  position.     It  is  part  of  a  thick  series  which  has 

the  same  position 
and  represents  the 
western  vertical 
limb  of  an  asym- 
metric anticline  as 
shown  in  other  sec- 
tions. This  belongs 
to  the  last  period  of 
folding,  which  was 
in  general  toward 
the  northwest  as  the 
strike  of  this  bed  is 
to  the  northeast. 
The  exposed  side  of 
this  bed  is  its  upper  surface.  Resting  against  it  on  the  right  is 
a  series  of  sandy  beds  (O),  which  apparently  dip  at  an  angle  of 
about  50°  to  the  northeast,  or  in  the  direction  of  strike  of  the  lime- 
stone bed.  These  belong  to  the  Middle  Ordovician  series.  From 
observation  of  ripple  marks,  mud  cracks,  and  raindrop  impressions, 
on  the  overhanging  portions  of  the  cliff  and  other  exposures  near 
by,  it  appears,  however,  that  these  beds  are  completely  overturned ; 
that,  in  fact,  the  overhanging  surface  is  the  top  of  the  beds  (see  p. 
550).  This  apparent  complication  is  simplified  when  we  attempt  to 
restore  the  conditions  which  existed  here  before  the  last  folding. 
Turning  the  bed  (S)  back  to  the  horizontal,  we  find  that  the  ap- 
parent dip  of  the  Ordovician  beds  (O)  is  really  their  strike  as  it  ex- 
isted after  the  first  folding,  and  that  their  dip  was  vertical,  this 
being  transformed  into  the  present  strike  of  these  beds  by  the  second 
folding  (Fig.  5286).  By  this  return  to  the  conditions  before  the 


FIG.  528  a. — Diagram  showing  the  relationships 
of  the  Ordovician  sandstones  (0)  to  the  Silurian 
limestones  (S)  on  the  Vlightberg  at  Rondout,  N.  Y. 
Shown  in  Fig.  527.  . 


Folding,  Erosion,  and  Deposition 


6n 


second  folding,  it  is  seen  that  the  strike  of  the  first  set  of  folds  was 
more  nearly  north  and  south  instead  of  northeast,  as  is  that  of  the 
later  folding  in  this  region. 

If  the  beds  are  returned  to  a  vertical  position,  it  becomes  apparent 
that  the  present  under  side  was  the  western  side  of  this  series 
after  the  first  folding.  As 
we  have  seen,  this  represents 
the  upper  sides  of  the  strata, 
and  it  thus  becomes  evident 
that  we  are  dealing  here 
with  the  western  limb  of  an 
anticline  of  the  first  period 
of  folding.  Since  the  west- 
ern limb  was  vertical,  the 
eastern  one  must  have  had 


FIG.  528  b.  —  Relation  of  the  Ordovician 
sandstones  and  Silurian  limestones  of  the 
Vlightberg  region  at  Kingston,  N.  Y., 
before  the  last  folding. 

a  gentler  dip  unless  the  folds 

were  isoclinal.  Observation  of  other  sections  in  this  region  con- 
firms the  supposition  of  an  asymmetric  anticline,  which  thus  ap- 
pears to  represent  folding  in  the  same  general  direction,  i.e.  toward 
the  west,  as  was  that  of  the  later  foldings.  These  conditions  be- 
fore the  second  folding  are  restored  in  the  section  (Fig.  528  b), 
which  covers  a  larger  field  than  is  shown  in  the  preceding  figure. 
It  is  by  such  analysis  of  the  facts,  and  reasonings  therefrom,  that 
the  geologist  is  enabled  to  reconstruct  the  conditions  which  existed 
at  a  given  period  in  the  earth's  history  at  the  locality  under  ob- 
servation, and  from  many  such  observations  over  wide  areas  he  is 
enabled,  by  combinations,  to  write  a  moderately  complete  chapter 
of  the  history  of  his  country  at  a  given  period  in  the  far  distant  past. 


STRUCTURES  DUE  TO  FOLDING,  EROSION,  AND  RENEWAL 
OF  DEPOSITION 

Unconformities  and  Disconformities 

Unconformities.  —  In  the  example  of  complex  structure  de- 
scribed in  the  last  section  (Fig.  528  b),  we  have  seen  that  horizontal 
beds  (S)  are  deposited  upon  the  eroded  edges  of  an  older  folded 
series.  The  two  series  of  beds  thus  have  a  discordant  relation,  their 
dips  being  in  different  directions  and  of  different  degrees,  in  one 
part  at  right  angles  to  each  other.  Such  a  relationship  is  termed 
an  unconformity,  a  term  sometimes  qualified  by  prefixing  the  word 


6i2     Deformation  of  Rocks  of  the  Earth's  Crust 


angular.  When  such  an  unconformity  exists,  it  always  points 
to  the  folding  and  erosion  of  the  older  series  before  the  deposition 
of  the  younger,  which  rests  unconformably  upon  the  older.  Such 


FIG.  529.  —  Typical  unconformity  between  inclined  sandstones  ("Laramie") 
below  (dipping  to  the  left)  and  horizontal  conglomerates  (Basal  Wasatch)  above. 
Near  Meeteetsie,  Wyo.  (Photo :  C.  A.  Fisher,  from  U.  S.  G.  S.) 

a  relation  implies  the  existence  of  a  time-interval  between 
the  deposition  of  the  two  series,  during  which  the  folding  of 
the  older  series  and  its  partial  erosion  took  place.  In  the  example 
cited,  where  the  older  beds  are  of  Middle  Ordovician  age  and  the 

younger  of  Upper 
Silurian,  the  in- 
terval comprises 
the  Upper  Or- 
N  x  dovician  and  the 


•Lower  and  Mid- 
dle Silurian  eras. 
It  is  probable  that 
the  folding  of  the 
older    series    oc- 
curred in  Upper  Ordovician  time,  which  would  leave  the  Lower 
and   Middle   Silurian  periods   for   the   accomplishment    of    the 
erosion. 


Fir..  530.  —  Unconformity  at  Siccar  Point,  Berwick, 
Scotland,  a,  folded  and  truncated  Siluro-Ordovician 
beds;  d,  Old  Red  Sandstone  beds  (Devonian).  (After 
Lyell.)  See  Figs.  531  a,  b. 


Folding,  Erosion,  and  Deposition 


613 


Unconformities  are  met  with  in  all  geological  formations  in  regions 
where  folding  of  strata  has  occurred.  Sometimes  the  later  series  has 
not  been  folded  and  then  it  represents  horizontal,  or  nearly  hori- 


FIG.  531  a.  —  Irregular,  unconformable  contact  between  the  vertical  (on 
right)  or  steeply  inclined  (on  left)  Siluro-Ordovician  shales,  and  the  gently  dip- 
ping Old  Red  Sandstone  beds  in  the  center  and  foreground.  Note  that  the  older 
beds  often  project  into  the  basal  beds  of  the  Old  Red  series,  which  were  deposited 
in  the  hollows  between.  The  higher  Old  Red  beds  have  been  removed  by  sub- 
sequent erosion.  (Photo,  M.  I.  Goldman.) 


zontal,  strata  resting  abruptly  upon  inclined  beds  (Fig.  529).  This 
is  well  shown  in  the  unconformity  between  the  Siluro-Ordovician 
beds  and  the  Old  Red  Sandstone  at  Siccar  Point,  near  St.  Abb's 
Head  (Berwick),  Scotland  (Figs.  530-5316),  and  at  Banff  on  the 


FIG.  531  b.  —  Near  view  of  the  unconformity  at  Siccar  Point,  Scotland, 
showing  horizontal  basal  Old  Red  conglomerates  resting  on  vertical  Siluro- 
Ordovician  shales  and  sandstones.  (Photo,  M.  I.  Goldman.) 


614     Deformation  of  Rocks  of  the  Earth's  Crust 

south  shore  of  the  Moray  Firth  (Fig.  532).     Other  illustrations  of 
unconformities  will  be  cited  in  the  chapters  on  Historical  Geology. 


FIG.  532.  —  Unconformity  and  disconformity  exposed  on  the  coast  of  Banff- 
shire,  south  shore  of  Moray  Firth,  Scotland,  q,  quartzite  (Siluro-Ordovician) ; 
s,  Old  Red  Sandstone ;  d,  drift  resting  disconformably  upon  the  Old  Red  beds, 
and  unconformably  against  the  older  series.  (After  Geikie.) 


FIG.  533.  —  Diagram  showing  the  relationship  of  the  strata  in  five  successive 
sections,  in  a  compound  regressive-transgressive  series.  The  intercalated  sand- 
stone xy  encloses  the  hiatus.  (From  Principles  of  Stratigraphy.} 


FIG.  534.  —  Diagram  showing  the  relative  magnitude  of  the  hiatus  in  the 
various  sections  shown  in  Fig.  533.     (From  Principles  of  Stratigraphy.) 


Folding,  Erosion,  and  Deposition  . 


615 


Disconformities.  —  Where  the  surface  of  a  low  dome,  such  as 
that  of  the  Cincinnati  dome,  or  the  margins  of  a  shallow  basin,  such 


FIG.  535.  —  Diagram  showing  the  relationships  of  a  transgressive-regres- 
sive-transgressive  series.  A  hiatus  is  found  at  the  base  of  the  transgressive 
series  (xz)  and  between  the  regressive  and  upper  transgressive  series  (xy).  Ow- 
ing to  the  vertical  exaggeration  of  the  scale  the  essential  parallelism  of  the  strata 
is  not  expressed.  (From  Principles  of  Stratigraphy.) 

as  the  Michigan  basin,  are  eroded  and  later  strata  deposited  upon 
the  eroded  surfaces,  the  difference  in  the  dip  of  these  two  series 
will  be  so  slight  that  it  cannot  be  measured,  much  less  seen  by  the 


FIG.  536.  —  Disconformable  contact  between  the  Bedford  shales  below  and 
the  Berea  sandstone  above,  the  latter  resting  in  erosion  hollows  in  the  former. 
Near  Cleveland,  O.  (C.  S.  Prosser,  photo.) 

eye.  Nevertheless,  there  may  be  a  pronounced  break  in  continuity 
between  the  two  apparently  concordant  series,  and  this  break  may 
represent  a  long  time  interval.  A  similar  break  in  continuity  may 


616     Deformation  of  Rocks  of  the  Earth's  Crust 


be  produced  when  a  series  of  strata  left  exposed  on  the  retreat  of  the 
sea,  after  a  long  time  interval  is  again  covered  by  the  advancing 
sea  in  which  strata  of  much  younger  age  are  deposited  in  the  manner 
illustrated  in  the  preceding  diagrams  (Figs.  533-535).  When  such 
a  break  in  continuity  is  established  between  two  successive  forma- 
tions which  have  a  parallel  or  concordant  position,  it  is  designated 

a  disconformity.  The  break 
and  disconformity  may  be 
indicated  in  many  ways.  The 
older  series  may  show  evi- 
dence of  erosion  on  its  upper 
surface  (Fig.  536) ;  pebbles 
of  the  older  series  may  be  in- 
cluded in  the  base  of  the 
upper  one;  an  old  soil  bed, 
or  a  bed  of  eolian  sands, 
may  separate  two  marine 
formations  (Fig.  535),  this 
bed  indicating  an  emergence 
followed  by  a  submergence. 
Sometimes,  however,  no  such 
detailed  characters  are  shown, 
and  then  the"  disconformity 
and  break  are  indicated  only 
by  the  great  difference  in  age 
FIG.  537.  —  Disconformable  contact  be-  of  the  two  formations  and  the 
tween  the*  Cobleskill  limestone  (Upper  absence  of  an  intermediate 
Silurian)  above,  and  the  Brayman  shales  .  ,  ,  ,  ,  . 

(Middle  Ordovician)  below.  Near  Howe's  senes  tnat  should  intervene 
Cave,  N.  Y.  (C.  C.  Mook,  photo.)  (Fig.  537).  This  difference 

in  age  is  shown  primarily  by 

the  fossils  which  the  rocks  contain  and  by  the  known  succession 
of  the  series  elsewhere.  In  some  cases  the  line  of  disconformity 
is  marked  by  springs  (Fig.  538).  In  Fig.  532,  a  section  is  given 
showing  both  unconformity  and  disconformity. 

Diastems.  —  This  term  has  been  applied  to  minor  breaks  in  the  continuity 
of  a  series  of  strata  due  to  a  temporary  cessation  in  sedimentation  followed  by 
its  resumption  at  a  later  period,  but  as  a  rule  without  erosion  or  even  emergence 
of  the  older  strata  (if  they  are  marine),  as  is  always  the  case  in  a  disconform- 
ity (Barrell). 


The  Causes  of  Folding 


617 


FIG.  538.  —  Disconformable  contact  between  the  Akron  dolomite  (Upper 
Silurian)  and  the  Onondaga  limestone  (Middle  Devonian),  at  Williamsville, 
N.  Y.  •  The  contact  is  marked  by  the  line  of  springs.  (Photo  by  author.) 


THE  CAUSES  or  FOLDING 

Folding  of  the  rocks  of  the  earth's  crust  always  implies  compres- 
sion, and  in  a  folded  region  the  crust  has  become  correspondingly 
shortened.  The  monoclinal  flexure  must,  however,  be  excepted, 


FIG.  539.  —  Machine  for  producing  folded  strata.     Baily  Willis, 
tonne,  Geographic  Physique.) 


(From  Mar- 


for  this  is  not  necessarily  due  to  compression.  That  folds  are  pro- 
duced by  lateral  compression  of  horizontal  layers  of  material  can 
easily  be  shown  by  experiment.  It  is  only  necessary  to  press  the 


618     Deformation  of  Rocks  of  the  Earth's  Crust 


leaves  of  a  book  against  the  baqk  of  the  cover  to  produce  folds  in 
the  paper.  More  elaborate  experiments  with  layers  of  Wax  and 
other  substances  to  represent  the  strata  have  been  made  in  a  ma- 
chine permitting  slow  and  regular  compression.  In  such  an  experi- 
ment the  strata  to  be  folded  are  generally  weighted  by  a  load  of 

shot,  on  the 
supposition 
that  folding 
is    best   ac- 
complished in 
nature  beneath  a 
load    of    surface 
rock    material,    and 
that  in   the  absence 
of  such  a  load   the 
layers   will    fracture    and 
become  dislocated.    A  ma- 
chine of  this  type  is  illus- 
trated in  figure  539,  and 
by  its  use  many  of  the  charac- 
teristic types  of  foldings  found 
in  the  strata  of  mountain  re- 
gions   have    been    reproduced 
(Fig.  54o). 

It  should  be  noted,  however, 
that  under  lateral  pressure  not 
all  strata  undergo  folding.  Some 
of  them  become  merely  com- 
pacted or  thickened,  and  their 
material  flows  into  any  spaces 
released  under  the  pressure. 
Such  strata,  among  which  clay 
rocks  are  found,  have  been  called 
"  incompetent  beds  "  in  contradistinction  to  the  "  competent  beds," 
which  are  thrown  into  folds  by  the  compression.  In  many  cases 
thrust  faulting  accompanies  folding,  the  one  passing  into  the  other 
(Fig-  54i). 

In  the  folding  of  the  Appalachian  Mountains,  the  crust  of  the 
earth  has  been  shortened  to  an  amount  estimated  at  from  40  to  50 
miles  and  in  some  portions  probably  more.  In  the  Swiss  Alps 


FIG.  540.  —  Folds  made  by  com: 
pression  of  layers  of  wax  and  other 
substances  in  the  machine  illustrated 
in  Fig.  539.  (U.  S.  G.  S.) 


Deformation  by  Faulting 


619 


the  foreshortening  was  originally  estimated  at  74  miles,  but  more 
recent  studies  have  led  to  the  conclusion  that  the  original  Alpine 
geosyncline  was  from  400  to  750  miles  broad,  and  that  it  has 
been  reduced  by  folding  and  mashing  to  100  miles.  In  the 


FIG.  541.  —  Diagram  showing  passage  of  thrust  fault  into  fold. 
(After  de  Martonne.) 

Rocky  Mountains  of  British  Columbia  an  area  originally  50 
miles  wide  has  been  reduced  to  a  width  of  25  miles  by  such 
compression. 

There  are  a  number  of  theories  which  aim  to  explain  the  causes  of  the  com- 
pressive  movement.  Among  these  the  one  held  longest,  appeals  to  the  gradual 
shrinking  of  the  earth's  interior  through  loss  of  heat,  and  the  inward  pressure 
of  the  cool  crust,  with  the  result  that  tangential  strains  are  set  up  within  this 
crust.  Other  causes  appealed  to  are :  the  transference  of  subcrustal  molten 
material  to  the  surface ;  isostatic  readjustment ;  escape  of  volatile  substances ; 
changes  in  the  oblateness  of  the  spheroid  toward  a  more  spherical  form ;  migra- 
tion of  the  equatorial  bulge  with  change  in  polar  position,  etc.  The  subject  is 
too  complex  for  elementary  treatment,  nor  are  all  of  the  elements  of  the  problem 
as  yet  fully  understood. 

DEFORMATION  BY  FAULTING 

Definition  of  Terms 

A  fault  is  a  displacement  of  the  strata  of  the  earth's  crust  on 
opposite  sides  of  a  fracture  plane  or  surface.  Faults  affect  all 
kinds  of  rocks,  but  are  most  readily  recognized  in  the  stratified 
deposits,  and  the  illustrations  will  therefore  be  taken  from  them. 
The  surface  along  which  the  movement  has  taken  place  is  called 
the  fault-plane,  or  better,  fault  surface,  as  it  is  not  a  perfect  plane 
except  for  short  distances,  but  curved,  warped,  irregularly  broken, 
and  even  with  offsets.  Sometimes  there  are  a  number  of  more 
or  less  parallel  surfaces  along  each  of  which  a  small  amount  of 
slipping  has  taken  place.  In  such  cases  the  entire  series  is  re- 


620     Deformation  of  Rocks  of  the  Earth's  Crust 


ferred  to  as  the  fault  zone,  and  the  displacement  itself  is  called  a 

step-fault  (Fig.  542). 

Owing  to  the  friction  along  the  fault  surfaces  by  the  movement 

against  each  other  of  the  rock  masses,  these  surfaces  are  often  pol- 
ished and  striated  in  the  direction 
of  movement.  Such  surfaces  are 
said  to  be  slickensided.  The  line 
of  intersection  of  the  fault  sur- 
face with  a  horizontal  surface  is 
the  fault-line  (Fig.  543,  F.  L.\ 
and  its  direction  is  called  its 
trend.  (Strike  is  also  used,  but 
this  is  properly  restricted  to  the 
intersection  of  the  strata  with 
FIG.  542.— Transverse  section  of  the  surface.)  The  normal  posi- 

a  piece  of  banded  sandstone,  showing 

series  of   small  faults.     (Photo,    B.       tlon  of  a  fault  Plane  or  sulface 

Hubbard.)  is  assumed  to  be  vertical,  and  this 

is  called  its  zero  (Fig.  543,  F^). 

Any  departure  from  this  (F2)  is  measured  with  reference  to  a  verti- 
cal plane  passed  through  the  fault  line  and  is  called  the  hade  (Fig. 
543,  F2  ti).  It  will  be  seen  that  this  is  the  complement  of  the  dip, 
which  is  the  departure  from  the  horizontal,  but  the  term  dip  should 
be  restricted  to  the  inclination  of  strata.  The  dip  of  the  fault 
plane  may  be  measured  with  the  clinometer  and  the  amount  sub- 


FIG.  543.  —  Diagram  illustrating  various  types  of  fault.  F.  L.,  fault-line; 
FI,  F/,  normal  fault  with  vertical  fault  plane,  the  fault  scarp  or  cliff  has  been 
worn  back  from  the  fault-line ;  F2,  normal  fault  with  inclined  fault  plane,  with 
hade  h ;  F.W.,  foot  wall ;  H.W.,  hanging  wall ;  dk,  dike  showing  offset  because 
of  inclination ;  F.  S.,  fault  scarp,  uneroded;  F3,  thrust  fault;  F.  L.  S.,  fault- 
line  scarp  formed  by  erosion ;  F4,  fault  without  vertical  but  with  horizontal 
displacement  or  shove  as  shown  by  offset  in  dike  (dk)  at  yz. 


Deformation  by  Faulting  621 

tracted  from  90°,  the  difference  being  the  angle  of  hade.  With  an 
inclined  fault-plane  or  surface  the  side  projecting  below  is  called 
the  foot  wall  (Fig.  543,  F.  W.),  while  the  overhanging  side  is  called 
the  hanging  wall  (Fig.  543,  H.  W.).  In  a  vertical  fault  plane 
neither  is  present.  Most  faults  are  best  shown  in  vertical  sections. 


FIG.  544.  —  Section  of  the  Island  of  Helgoland  in  the  North  Sea,  showing 
numerous  faults  in  the  inclined  strata,  and  the  decrease  in  the  size  of  the  island, 
and  formation  of  a  submarine  platform,  by  wave-erosion.  (After  Walther.) 

The  vertical  cliff  section  rof  the  island  of  Helgoland  cut  by  the 
waves  of  the  North  Sea  shows  a  number  of  faults  not  otherwise 
noticeable  (Fig.  544). 

Types  of  Faults 

Nature  of  Movement  in  Faulting.  —  The  movement  of  the  two 
sides  of  a  broken  mass  in  faulting  is  relative.  Both  may  undergo 
a  change  in  position  of  the  same  or  of  different  extent,  or  either  may 
remain  stationary  while  the  other  moves.  In  any  case  a  displace- 
ment results,  which  is  all  that  is  visible  of  the  fault.  This  displace- 
ment is  best  measured  by  the  change  in  position  of  the  two  parts  of 
a  definite,  recognizable  stratum,  such  as  a  coal  bed,  a  limestone  layer, 
or  some  other  bed  (x,  Fig.  543),  which  can  be  recognized  by  its  lithic 
character  and  thickness.  The  movement  may  be  vertical,  hori- 
zontal, or  oblique,  or  it  may  be  a  rotary  one,  the  displacement 
being  of  a  corresponding  character.  Only  the  simpler  types  will 
be  considered. 

Normal  Faults.  —  If  the  displacement  or  slipping  along  the  fault 
plane  is  in  a  direction  at  right  angles  to  the  fault  line,  two  distinct 
cases  may  be  recognized,  the  normal  and  the  reverse  fault.  With 
reference  to  an  assumed  stationary  footwall,  the  hanging  wall  may 
slip  down,  thus  producing  a  normal  fault.  This  is  best  recognized 
upon  a  vertical  face  cut  at  right  angles  to  the  fault  line  (abed,  Fig. 
543),  where  it  will  be  seen  that  the  guide  or  index  stratum  has  been 
vertically  displaced  (Fig.  543,  FI,  FZj).  The  extent  of  vertical  dis- 


622     Deformation  of  Rocks  of  the  Earth's  Crust 

placement  is  called  the  throw,  this,  inF2,  Fig.  543,  being  measured 
by  the  vertical  distance  s-t.  There  is,  however,  also  a  horizontal 
displacement  at  right  angles  to  the  fault  line,  and  this  is  called  the 
heave.  In  the  present  case  this  is  a  separation  or  positive  displace- 
ment of  the  beds,  and  its  amount  is  measured  by  the  distance  r-s. 
It  is  evident  that  in  this  case  there  has  been  a  lengthening  of  the 
earth's  crust  by  the  amount  of  the  heave,  and  this  implies  a  stretch- 
ing movement  such  as  may  exist  at  the  top  of  an  anticlinal  arch. 
If  the  fault  plane  is  vertical  the  heave  is  zero.  (Fig.  543,  F/.) 
Normal  faults  may  also  develop  upon  a  monoclinal  flexure,  where 
the  upflexed  side  becomes  the  downthrow  side,  and  the  down- 
flexed,  the  upthrow  side.  (See  p.  602.) 

Reverse  Faults.  —  If  the  hanging  wall  moves  upward  with  ref- 
erence to  an  assumed  stationary  footwall,  the  movement  again 
being  only  in  a  direction  at  right  angles  to  the  fault  line,  a  reverse 
fault  is  produced  (Fig.  543,  F3).  Here,  too,  we  meet  with  a  throw, 
but  in  the  opposite  direction,  and  if  the  fault  plane  is  inclined,  with 
a  heave  which  is  also  in  the  opposite  direction,  the  result  being  an 
overlapping  or  negative  displacement.  Here  again  the  amount  of 
throw  is  measured  by  the  vertical  line  s-t,  and  the  amount  of 
heave  by  the  horizontal  line  t-r,  as  seen  in  a  vertical  section  at 
right  angles  to  the  fault  plane  (bcfe).  It  is  clear  that  in  this  case 
there  has  been  a  shortening  of  the  earth's  crust  by  the  amount  of 
the  heave,  which  would  imply  lateral  compression  of  the  type 
which  produces  folded  structures. 

Surface  Expression  of  Normal  and  Reverse  Faults.  —  Both  nor- 
mal and  reverse  faults  would  be  characterized  by  a  cliff  or  initial 
fault  scarp  upon  the  surface  immediately  after  faulting  has  taken 
place,  if  this  were  more  rapid  than  the  erosion  which  would  tend  to 
level  the  inequalities  (Fig.  543,  F.  5.).  Fault  scarps  of  this  type  are 
sometimes  seen  in  the  modern  topography  where  faulting  has  been 
so  rapid  as  to  be  accompanied  by  an  earthquake  (Fig.  582,  p.  677). 
Such  scarps  and  the  modifications  which  they  undergo  by  erosion 
will  be  considered  fully  in  the  chapter  on  surface  sculpture  (Chap- 
ter XXIII).  Two  examples,  an  eroded  fault  scarp  (of  Ft)  and  the 
revived  fault-line  scarp  F.  L.  S. ,  are  indicated  in  the  diagram  (Fig. 
543).  In  normal  faulting  the  initial  fault  scarp  is  formed  by  the 
footwall,  in  reverse  faulting  by  the  hanging  wall. 

Horizontal,  Oblique  and  Rotary  Faults.  —  Instead  of  vertical 
movements  up  or  down  the  fault  plane,  the  movement  may  be  con- 


Deformation  by  Faulting  623 

ceived  as  merely  a  horizontal  one  along  the  fault  plane,  though  this 
is  probably  rare.  Such  a  fault  would  produce  no  initial  fault  scarp, 
though  a  secondary  one  might  be  developed  .later  by  erosion.  The 
extent  of  horizontal  displacement  along  the  fault  plane  is  called 
the  shove,  and  if  the  fault  plane  cuts  across  strata  or  across  a  dike, 
vein,  pebble,  or  other  guiding  structure,  the  amount  of  the  shove 
along  the  fault  plane  can  be  measured  (yz  in  F4,  Fig.  543).  An  ap- 
parent horizontal  displacement  of  the  dike  is  produced  by  a  simple 
vertical  fault,  because  of  the  inclination  of  the  dike  (Fig.  543,  dk 
at  F,). 

Probably  most  faultings  involve  a  more  or  less  oblique  movement  down  or 
up  the  fault  plane,  comprising  at  the  same  time  lateral  displacement  along  that 
plane,  or  the  movement  may  be  a  double  one,  partly  up  or  down  the  plane  and 
partly  parallel  to  it,  or  it  may  be  rotary,  one  block  being  twisted  with  reference 
to  the  other.  In  any  case  the  total  amount  of  displacement  between  any  two 
points,  originally  in  contact,  can  be  measured  by  a  direct  but  oblique  line,  and 
this  is  called  the  slip  of  the  fault.  It  may  be  resolved  into  the  three  components, 
the  throw,  the  heave,  and  the  shove,  from  which  measurable  units  the  amount 
of  slip  may  be  calculated.  If  the  fault  plane  is  vertical  there  is  no  heave,  but 
the  line  of  slip  forms  the  hypotenuse  of  a  right-angled  triangle,  the  other  sides 
of  which  represent  the  throw  and  shove,  respectively.  Rotary  faults  may  some- 
times be  recognized  by  the  difference  in  dip  or  strike  of  the  strata  on  opposite 
sides  of  the  fault  plane. 

Strike  Faults.  —  Faults  in  tilted  stratified  rocks  may  have  their 
trend  essentially  parallel  to  the  strike  of  these  rocks.  Such  a  fault 
is  called  a  strike  fault,  and  it  may  be  either  normal  or  reverse,  and 
the  inclination  of  the  fault  plane  may  coincide  with  the  dip  of  the 
strata,  or  may  have  any  angle  with,  reference  to  it.  This  is  the 
common  type  of  fault  found  in  folded  beds  where,  moreover,  fault- 
ing is  commonly  reverse.  Whenever  the  angle  of  inclination  of 
the  fault  plane  and  the  dip  of  the  strata  differ  in  a  strike  fault,  there 
will  be  either  a  concealment  or  a  duplication  of  the  beds  in  the  sur- 
face exposures  as  the  result  of  faulting.  The  following  diagrams 
(Fig.  545)  illustrate  these  results  with  varying  angles  of  inclination 
of  the  fault  plane  and  constant  dip  of  strata,  both  in  normal  faults 
(A  to  G)  and  in  reverse  faults  (H  to  J) .  It  will  be  seen  that  in  figures 
A,  F,  I,  and  /  certain  beds  are  concealed  at  the  surface,  while  in 
figures  B,  C,  £,  G,  and  H  certain  beds  are  repeated.1 

Dip  Faults,  Oblique  Faults.  —  When  the  trend  of  the  fault  line 
is  at  right  angles  to  the  strike  of  the  strata  or  nearly  so,  the  fault  is 

1  See  further,  A.  W.  Grabau,  Principles  of  Stratigraphy,  pp.  817-818. 


624     Deformation  of  Rocks  of  the  Earth's  Crust 

spoken  of  as  a  dip  fault,  while  faults  whose  trend  makes  an  angle 
of  approximately  45°  with  the  strike  are  called  oblique  faults.  Such 
faults  are  commonly  indicated  by  an  offset  or  shifting  of  the  strata 
on  either  side  of  the  fault  line  (Fig.  546). 


21  23456.1          21  2  3456 


FIG.  545.  —  Strike  faults.  A-F,  Normal  or  gravity  faults,  the  arrows 
indicating  the  downthrow  side ;  H-J,  thrust  faults,  the  arrows  indicating 
the  upthrust  side.  The  diagrams  represent  the  conditions  after  erosion  of 
the  surface.  In  A,  F,  7,  and  /  certain  strata  are  eliminated,  in  B,  C,  E,  G, 
and  H  certain  strata  are  repeated.  (From  Principles  of  Stratigraphy.') 

Gravity  Faults.  —  While  the  terms  heretofore  used  have  refer,- 
ence  to  the  nature  of  the  movement  (normal,  reverse,  horizontal, 
rotary,  etc.),  or  to  the  relation  of  the  fault  and  the  strata  (strike, 
dip,  and  oblique  faults),  there  are  two  other  terms  in  common  use 
which  refer  to  the  agent  or  cause  of  the  movement.  These  are 
gravity  and  thrust  faults.  The  term  gravity  fault  implies  that  one 


Deformation  by  Faulting  625 

side  slipped  down  as  the  result  of  its  own  weight,  and  normal  faults 
are  generally  assumed  to  be  of  such  character,  the  terms  being  some- 
times used  interchangeably.  A  normal  fault  may,  however,  be 
produced  by  the  up-pushing  of  the  footwall,  rather  than  the  down- 
slipping  of  the  hanging  wall,  as  is  illustrated  by  the  fault  formed 
during  the  New  Zealand  earthquake  of  1855  (pp.  675-677,  Figs. 
581-582). 

Thrust  Faults.  — These,  on  the  other  hand,  are  of  common  occur- 
rence, and  they  are  generally  of  the  reverse  type,  but  are  also  much 
more  complicated.  They  occur  on  the  margins  of  folded  areas  and 


FIG.  546.  —  Diagram  illustrating  the  offset  of  strata  produced  by  vertical 
slipping  along  an  inclined  plane,  when  the  fault  is  oblique  with  reference 
to  the  strata.  The  dotted  outline  restores  the  fault  scarp  and  gives  the 
appearance  immediately  after  faulting,  before  erosion  has  removed  this  por- 
tion of  the  block. 

sometime^  within  them.  Indeed,  within  the  same  area  an  over- 
turned fold  at  one  locality  may  be  seen  to  pass  into  an  overthrust 
fault  in  another  (Fig.  541,  p.  618). 

Magnitude  of  Thrusts.  —  Thrust  faults  may  range  in  magni- 
tude from  minute  slippings  of  a  few  feet  or  less  to  cases  in 
which  the  movement  has  resulted  in  carrying  one  rock  mass  over 
another  for  a  distance  of  as  much  as  70  miles.  The  most  stu- 
pendous examples  of  such  thrustings  are  seen  in  the  Alps,  where 
successive  sections  have  overridden  one  another,  producing  a 
structure  of  extreme  complexity.  In  the  great  thrust  along  the 
Front  Range  of  the  Rocky  Mountains  in  Montana,  the  observed 
distance  of  movement  is  fifteen  miles,  while  actually  it  was  probably 
much  greater. 

Duplication  and  Inversion  of  Order  of  Strata  by  Thrusting 

As  the  result  of  great  overthrusts,  the  strata  of  a  given  region 
may  be  repeated,  and  because  of  the  nearly  horizontal  nature  of 


626     Deformation  of  Rocks  of  the  Earth's  Crust 


the  thrust  the  repetition  may  appear  to  be  perfectly  conformable. 
Unless  the  thrust  is  recognized,  such  a  series  may  be  conceived 
as  representing  one  of  unbroken  succession.  By  such  thrusting, 
formations  of  older  may  come  to  rest  upon  those  of  younger  age, 
and  thus  an  inversion  of  the  strata  may  seem  to  have  taken  place. 
Several  examples  may  be  cited. 

The  Helderberg  Overthrusts.  —  In  the  Helderberg  range  of 
mountains,  from  Catskill  southward,  a  great  overthrusting  of  the 
strata  of  Silurian  and  Devonian  age  has  occurred  along  the  western 
border  of  the  Appalachian  folds,  probably  for  the  most  part  as  a 


FIG.  547.  —  Section  through  North  Hill,  Kingston,  N.  Y.,  showing  the 
overthrust  which  led  to  a  repetition  of  the  strata.  E,  Esopus  shale ;  0,  Ori£- 
kany  sandstone;  P,  Port  Ewen;  B,  Becraft;  N,  New  Scotland;  C,  Coey- 
mans  with  Manlius  below ;  W,  Cobleskill ;  Z,  Hudson  River  strata.  (After 
Van  Ingen ;  N.  Y.  State  Museum  Report.) 

single  overthrust  (with  perhaps  locally  a  number  of  parallel  slips). 
By  erosion  the  overthrust  mass  has  been  divided  into  a  series  of 
blocks  of  which  the  above  is  an  illustration  (Fig.  547).  In  all 
cases  the  thrusting  is  from  the  southeast,  and  the  entire  series  of 
strata  from  the  Upper  Silurian  to  the  Middle  Devonian  is  repeated. 
Before  the  thrust  was  recognized  it  was  thought  that  the  beds  ex- 
posed in  those  ridges  represented  a  single  conformable  series  in 
which  there  occurred  a  remarkable  repetition  6f  similar  sediments 
enclosing  similar  organic  remains. 

The  Hudson  Highlands  Thrust  (Fig.  548).  —The  Highlands  of 
the  Hudson  likewise  present  a  thrust  of  considerable  magnitude, 
with  the  result  that  the  older  crystalline  rocks  of  the  Highlands  rest 


Deformation  by  Faulting 


627 


upon  the  younger  ones,  which  are  exposed  farther  to  the  north. 
This  thrusting  appears  to  have  been  of  a  compound  nature,  the 


FIG.  548. — Diagrammatic  section  (generalized),  showing  the  overthrust 
at  the  northern  end  of  the  Hudson  Highlands.  Irregular  dashes,  granite, 
gneiss,  and  other  crystalline  rocks  of  the  Highland  region;  dotted,  Lower 
Cambrian  (Poughquag)  quartzite;  blocked,  Cambro-Ordovician  limestones 
(Wappinger,  etc.) ;  lined,  Hudson  River  shale  —  Ordovician.  The  main  over- 
thrust  on  the  right  brings  the  crystalline  rocks  against  the  Cambrian  quartzite, 
while  the  subsidiary  thrust  brings  the  crystallines  above  the  Hudson  River 
slates.  Thrust  movement  towards  the  northwest 


FIG.  549  a.  —  Chief  Mountain,  Montana.  An  erosion  remnant  of  a  mass  of 
Algonkian  limestone  carried  by  thrust  faulting  over  soft  sandstones  and  shales 
of  Mesozoic  age,  for  a  distance  of  at  least  fifteen  miles.  The  monolith  has  a 
height  of  1500  feet  above  the  thrust  plane,  which  is  indicated  in  the  photograph 
by  the  upper  line  of  the  forest.  (After  Campbell,  U.  S.  G.  S.)  (See  Fig.  515, 
page  599,  and  Fig.  549  6.) 

movement  occurring  along  several  parallel  planes  of  low  inclination. 
As  a  result  the  old  crystalline  rocks  have  been  repeatedly  brought  up 


628     Deformation  of  Rocks  of  the  Earth's  Crust 

to  the  same  level,  where  subsequent  erosion  has  uncovered  them. 
At  several  places  the  old  Highland  gneisses  of  Archaean  age  may 
be  seen  to  rest  upon  the  upturned  Hudson  River  shales  and  sand- 
stones of  Ordovician  age.  The  movement  in  this  case  also  was 
from  the  southeast  to  the  northwest. 

The  Chief  Mountain  Thrust  of  Montana  (Figs.  549  a,  b).  —  In 
northern  Montana  along  the  Front  Range  of  the  Rocky  Mountains, 


FIG.  549  b.  —  A  series  of  diagrams  showing  the  development  of  the  Chief 
Mountain  thrust.  The  older  rocks  from  which  the  mountains  are  cut  are  repre- 
sented by  cross-lines,  the  younger  rocks  of  the  plains  (Mesozoic)  in  white. 
(After  Campbell,  U.  S.  G.  S.) 

strata  of  Algonkian  age  are  seen  to  rest  with  apparent  conformity 
upon  nearly  horizontal  beds  of  Cretaceous  age.  These  Algonkian 
masses  are  erosion  remnants  of  a  formerly  continuous  series,  Chief 
Mountain  being  the  most  conspicuous  of  these  remnants.  This 
position  of  pre-Cambrian  upon  late  Mesozoic  rocks  at  once 
suggests  overthrusting,  and  an  examination  of  the  region  to  the 
west  shows  that  this  has  actually  occurred.  From  the  data  avail- 
able the  extent  of  the  movement  is  recognized  to  have  been  at 
least  fifteen  miles,  the  direction  of  movement  in  this  case  being 
eastward.  The  actual  movement  was  probably  much  greater,  and 
a  considerable  portion  of  the  overthrust  mass  has  apparently  been 


Deformation  by  Faulting 


629 


removed  by  erosion.  The  development  of  this  thrust 
shown  in  the  diagram  (Fig.  549  &),  and  the  position 
Mountain  with  reference  to  the  other 
rocks  is  shown  in  Fig.  515  on  p.  599. 

Thrusts  in  the  Northwest  High- 
lands of  Scotland.  —  The  northwest 
region  of  the  Highlands  of  Scotland- 
shows  a  wonderful  series  of  thrust- 
ings  by  which  the  older  rocks  are 
repeatedly  made  to  override  the 
younger  ones,  the  ancient  gneiss  in 
many  places  resting  upon  Cambrian 
or  Ordovician  strata.  The  adjoin- 
ing illustration  shows  some  of  the 
overthrusts  in  this  region  (Fig.  550). 

The  Salt  Range  District  of  India. 
-  In  northwestern  India  a  series  of 
mountains  forming  what  is  known 
as  the  Salt  Range,  because  of  the 
abundance  of  rock-salt  in  it,  illus- 
trates an  interesting  problem  which 
may  arise  from  great  thrustings. 
The  rocks  of  this  region  appear  to 
represent  a  conformable  series,  with 
red  sandstones,  shales,  and  extensive 
salt  beds  at  the  base,  overlain  by 
marine  strata  with  organic  remains 
of  Cambrian  age.  From  this  re- 
lationship it  was  originally  assumed 
that  the  great  salt  beds  of  the  Salt 
Range  were  of  early  Cambrian  or  of 
Pre-Cambrian  age,  constituting  thus 
the  oldest  salt  deposits  of  the  earth. 
More  recent  studies,  however,  have 
shown  that  we  are  dealing  here,  not 
with  a  conformable  series,  but  with 
an  enormous  overthrust,  which  has 
carried  the  fossiliferous  Cambrian 
sediments  over  the  much  younger 
salt  beds,  upon  which  they  have 


•plane   is 
of  Chief 


630     Deformation  of  Rocks  of  the  Earth's  Crust 

come  to  rest  in  apparently  conformable  position.     The  salt  beds 
themselves  are  probably  of  Tertiary  age. 

The  Alpine  Overthrusts.  —  By  far  the  most  complicated  series 
of  overthrusts  which  have  yet  been  worked  out  are  found  in  the 


Sentis 


FIG.  551.  —  Block  diagram  [A]  and  section  [B]  illustrating  overthrust  in  the 
Alps.     (After  Lugeon.) 

Alps,  where  the  magnitude  of  the  thrust  movement  seems  at  times 
to  be  almost  incredible.  In  the  preceding  illustration  some  of  these 
great  overthrusts  are  shown  (Fig.  551,  see  also  Fig.  524,  p.  607). 


TOPOGRAPHIC  FEATURES  DUE  TO  FAULTING 

Fault  Scarps.  —  We  have  already  seen  that  both  normal  and  re- 
verse faults  are  expressed  upon  the  surface  of  the  earth  by  fault 
scarps,  if  the  dislocation  is  more  rapid  than  the  erosion  which  tends 
to  cut  away  inequalities  (Fig.  543,  p.  620).  In  all  but  the  most  re- 
cent faults,  the  scarp  is  modified  by  erosion,  which  may  remove 
it  completely  or  to  a  position  distant  from  the  fault  line.  Scarps 
along  fault  lines  may  also  be  resurrected  by  erosion  so  that  the 
"cliff  is  not  the  original  fault  scarp,  but  an  erosion  cliff  due  to  the 
wearing  away  of  the  rocks  on  one  or  the  other  side  of  the  fault  line. 
(See  further,  Chapter  XXIII.) 

Rift  Valley  or  Graben.  —  By  the  downfaulting  of  a  long  series 
of  narrow  blocks  of  the  earth's  crust  or  a  series  of  parallel  blocks  a 
rift  valley  or  graben  is  produced.  This  is  well  illustrated  by  the 
Graben  of  the  Rhine  in  the  Basel-Strassburg  region  (Fig.  552), 
where  ancient  rocks  form  the  Vosges  Mountains  and  Black  Forest  on 
opposite  sides,  while  the  center  represents  a  series  of  much  younger 
strata  which  originally  were,  in  part  at  least,  continuous  with 


Topographic  Features  Due  to  Faulting         631 


\I 


those  now  lying  on  the  outer  flanks  of  these  mountains,  and 
probably  formed  an  arch  with  them,  the  center  of  which  was 
broken  down,  as 
illustrated  in  the 
following  diagram 

(Fig.  553)- 

Other  rift  valleys 
are  found  in  eastern 
Africa  (Fig.  554), 
and  of  one  of  these 
the  Dead  Sea  of 
Palestine  is  appar- 
ently the  northern 
continuation  or  a 
part  of  a  parallel 
rift  which  also  in- 
volves the  Vale  of 
Araba  and  the  Gulf 
of  Akaba  (see  map 
Fig.  649).  On  the 
African  Continent 
these  rift  valleys 
have  been  divided 
by  the  building  up 
of  volcanic  cones  at 


FIG.  552.  —  Map  of  rift  valley  of  the  Rhine, 
showing  the  main  faults  on  either  side,  and  the 
dying-out  folds  of  the  Jura  Mountain  system  on  the 
south  where  they  are  crowded  against  the  Vosges 
and  Black  Forest  massifs.  '  (After  Lake  and  Rastall.) 


various  points,  and  the  resulting  divisions  of  the  valleys  are  partly 
occupied  by  lakes,  of  which  Lake  Tanganyika  is  one  of  the  largest. 


VOSGES 


etACKfOREST 


FIG.  553.  —  Section  of  the  rift  valley  of  the  Rhine,  i,  granite;  2-7,  Meso- 
zoic  rocks ;  8,  9,  Tertiary  and  recent.  (After  Lake  and  Rastall.)  For  the 
development  of  this  graben  see  Fig.  650. 

The  rift  valley  in  which  this  lake  lies  is  more  complicated  in 
structure  than  that  of  the  Rhine,  consisting  of  a  number  of  tilted 


o    So  100       200       3oo        too        SooKilom 

Fault  block  depressions 


FIG.  554.  — Rift  valleys  of  East  Africa.     After  Suess  etc.  from  de  Martonne. 

AN,  Lake  Albert;  AE,  L.  Albert  Edward;  K,  L.  Kivu;  RK,  L.  Rikwa  or 
Rukwa;  £,  L.  Eyassi;  H,  L.  Hohenlohe;  M,  L.  Manyara;  GN,  L.  Guano 
Nyiro;  KJ,  Mt.  Kilimanjaro;  X^,  Mt.  Kenia;  N,  L.  Naivasha;  EG,  Mt. 
Elgon;  St.  L.  Stefanie;  AB,  L.  Abaya;  47,  L.  Afdjada. 

632 


Topographic  Features  Due  to  Faulting         633 

blocks  instead  of  a  normal  downfaulted  mass,  as  in  the  latter  case. 
This  is  shown  by  the  following  cross  section  (Fig.  555). 


FIG.  555.  —  Cross-section  of  Lake  Tanganyika  and  Lake  Rukwa,  Africa, 
showing  the  block  and  graben  faulting.  Vertical  scale  exaggerated  five  times. 
(After  Moore.) 


T    •^t;£--:  "--._        ^x?"V-____         A::Cp    '~'^^~^f^!^£^:'-- 


FIG.  556.  —  Sketch  of  Lake  Albert,  Oregon,  a  fault  basin  lake.     (After  Russell.) 


A  similar  valley,  with 
occupied  by  Albert  Lake 
other  examples  of  such 
asymmetric  rift  valleys, 
all  of  which  in  reality 
represent  block    fault- 
ing.    The  rift  valley  oc- 
cupied  by    the    Dead 
Sea  of  Palestine  is  of 
comparatively      recent 
origin,  for  on  opposite 
sides    are    still    found 
remnants  of  beds  which 
tended  across  this  region 


a  fault  scarp  on  one  side  only,  is  in  part 
in  Oregon  (Fig.  556),  and  there  are  many 


FIG.  557  a.  —  Diagram  illustrating  block 
faulting,  and  the  initial  stage  in  the  formation 
of  block  mountains.  See  further,  Figs.  643, 
pp.  647  and  648.  (After  W.  M.  Davis.) 

were  deposited  in  a  lake  which  once  ex- 
before  the  valley  was  formed. 


634     Deformation  of  Rocks  of  the  Earth's  Crust 

Block  Faulting  and  Mountains.  —  When  by  faulting  large  blocks 
of  the  earth's  crust  become  tilted,  they  will  present  a  steep  fault- 
scarp  on  one  side  and  a  slope,  that  of  the  original  surface,  on  the 
other.  When  of  sufficient  magnitude,  such  blocks  will  form  moun- 


FIG.  557  b.  —  Block  faulting  with  the  formation  of  elevated  horsts  and 
depressed  rifts.  Wasatch  Mountains,  Utah.  Men  are  standing  on  the  ends  of 
the  fault  blocks.  (Photo  F.  J.  Pack.) 

tains.  Of  this  type  the  Great  Basin  Ranges  of  the  western  United 
States  are  the  most  conspicuous  examples  (Figs.  557  o,  645).  An 
uplifted  block,  bounded  by  faults,  is  called  a  horst.  Faulting  of 
this  type,  but  without  surface  expression,  is  illustrated  in  the 
above  view  from  the  Wasatch  Mountains  (Fig.  557  b). 

MINOR  FEATURES  ACCOMPANYING  FAULTING 

Slickensides.  —  We  have  already  seen  that  along  the  sides  of 
the  fault  plane,  the  rocks  are  often  polished  and  grooved  by  the 
movement,  producing  the  characteristic  slickensided  surfaces. 
Such  surfaces  are  often  the  best  indications  of  the  position  of  the 
fault  planes,  but  they  are  also  developed  along  minor  planes  of 
movement  parallel  to  the  main  fault. 

Fault  Breccia.  —  Another  feature  often  produced  between  fault 
surfaces,  is  the  complete  fracturing  and  partial  powdering  of  the 
rocks  which  form  either  side  of  the  fault  plane.  This  broken  ma- 


Minor  Features  Accompanying  Faulting         635 


terial  may  accumulate  in  a  fissure  in  the  fault  and  there  produce  a 
fault  breccia  (Fig.  32,  p.  80).  Fault  breccias  are  most  conspicu- 
ous in  normal  faults,  but  they  are 
also  found  along  thrust  planes,  where 
they  may  be  so  much  modified  by 
the  movement  that  the  larger  frag- 
ments lose  their  angularity  and  come 
to  resemble  a  bed  of  pebbles.  The 
material  produced  by  great  ice 
masses  moving  over  a  rock  surface 
is  analogous  to  this,  as  already  out- 
lined in  a  previous  chapter.  When 
a  single  large  mass  of  rock  is  caught  between  two  faults  it  is  spoken 
of  as  a  "  horse  "  (Fig.  558). 

Collapse  Breccias.  —  Masses  of  angular  fragments  produced  by 
the  collapse  of  the  roof  of  a  cave  or  other  hollow,  may  have  a  very 
similar  appearance  to  a  fault  breccia,  but  would  occupy  a  more 
circumscribed  area,  while  at  the  same  time  they  are  apt  to  be  of 
greater  thickness. 

Crumplings  and  Drags.  —  Minor  crumplings  of  the  rocks  along 
the  fault  plane  may  also  result  from  the  movement,  and  the  angle 
of  the  beds  on  either  side  of  the  plane  may  be  modified  by  a  dragging 


FIG.  558.  —  A  "horse." 


FIG.  559.  —  Diagrams  illustrating  drag  of  strata  along  a  fault  plane  —  a, 
normal  fault;  b,  reverse  fault.  (The  reverse  bending  of  the  strata  near  the 
fault  plane  is  shown  in  inverted  faults  formed  upon  a  monoclinal  flexure  as 
shown  in  Fig.  518  b,  p.  602.) 

movement.  Thus  in  a  normal  fault  in  horizontal  strata,  the  beds 
of  the  hanging  wall  may  be  sharply  dragged  upward  along  the 
fault  plane  while  in  a  reverse  fault  they  may  be  dragged  downward 
(Fig.  559).  The  vertical  strata  from  which  the  Gateway  to  the 
Garden  of  the  Gods  and  the  "  Cathedral  Spires  "  are  cut  have  been 
considered  as  having  been  dragged  to  a  vertical  position  along  a 


636     Deformation  of  Rocks  of  the  Earth's  Surface 

fault  plane  (Figs.  560,  561).     Where  "  inverted  "  faults  are  formed 
upon  a  monoclinal  flexure,  the  strata  near  the  fault  plane  bend  in 


FIG.  560.  —  Gateway  to  the  Garden  of  the  Gods,  Colorado,  with  Pikes  Peak 
in  the  distance.  The  strata  which  form  the  "Gateway"  are  red  sandstones 
apparently  dragged  to  a  vertical  position  by  faulting. 

the  opposite  direction,  i.e.  down  on  the  downthrust  and  up  on  the 
upthrust  side.     (See  p.  600,  and  Fig.  518,  p.  602.) 

OTHER  STRUCTURES  PRODUCED  BY  DEFORMATION 

In  the  deformation  of  the  strata  of  the  earth's  crust,  structural 
features  of  a  less  conspicuous  character  than  the  folds  and  faults 
are  produced.  The  metamorphism  of  the  strata  due  to  pressure 
and  the  development  of  heat  thereby,  and  the  special  structural 
features  of  these,  will  be  discussed  in  the  next  chapter.  Two  types 
of  structure,  however,  may  be  considered  here ;  namely,  slaty  cleav- 
age and  fracture  planes  or  joints. 

Slaty  Cleavage 

When  the  finer-grained  rocks  are  subject  to  intense  squeezing, 
the  particles  of  such  a  rock  may  become  flattened  and  expand  at 
right  angles  to  the  compression.  This  results  in  a  squeezing  to- 
gether of  the  mass  in  the  direction  of  compression  and  a  proportional 
swelling  at  right  angles  thereto.  If  mica  scales  or  other  fragments 
which  already  have  a  scaly  form  are  present,  they  will  become  so 


Other  Structures  Produced  by  Deformation     637 

arranged  that  their  longer  axis  is  at  right  angles  to  the  direction  of 
compression.  In  this  manner,  a  secondary  parallel  structure  is 
produced  in  the  rock  thus  compressed,  this  parallel  structure  being  at 
right  angles  to  the  direction  of  compression,  and  having  no  reference 
to  the  original  structures,  such  as  bedding  planes,  etc.  On  weather- 


'f ' 

i 


, 


FIG.  561.  —  The  Cathedral  Spires,  Garden  of  the  Gods,  Colorado.  These 
spires  are  parts  of  vertical  beds  of  red  sandstones,  which  are  modeled  out  by 
erosions  of  the  softer  beds.  (Photo  by  Darton,  U.  S.  G.  S.) 

ing,  the  rock  thus  affected  will  split  into  thin  plates  along  these 
newly-produced  structure  planes,  or  the  rock  may  be  artificially 
split  along  them.  This  is  called  slaty  cleavage,  and  it  is  most  com- 
monly developed  on  clay  mud-rocks,  such  rocks  when  thus  af- 
fected producing  the  common  slates  used  for  roofing  purposes,  etc. 
(Fig.  562).  Slates  of  this  kind  may  appear  banded  transversely  to 


638     Deformation  of  Rocks  of  the  Earth's  Surface 


their  surfaces,  such  bands  commonly  representing  the  original  bed- 
ding-planes of  the  mass  before  it  was  affected  by  the  compression. 

When  the  original  rock  contains 
fossils,  these  will  be  compressed 
and  variously  distorted,  but  such 
rocks  are  not  as  a  rule  capable 
of  furnishing  good  roofing  or 
other  slates. 

Joints 

The  term  joints  and  jointing 
as  applied  to  rocks  covers  a  num- 
ber of  structures,  such  as  the 
columnar  jointing  of  basalts,  the 
horizontal  or  arched  jointing  of 
granites,  etc.,  and  the  vertical 
fissures  which  are  found  in  nearly 
all  clastic  rocks.  It  is  this  last 
type  alone  which  is  of  defor- 
mational  origin,  though  it  is 
not  necessarily  confined  to  the 
clastic  rocks,  where,  however,  it  is  most  perfectly  developed. 
As  seen  in  beds  of  shale,  limestones,  or  fine-grained  sandstones, 
such  joints  are  com- 
monly found  arranged 
in  systems,  the  joints  of 
each  system  being  par- 
allel, while  the  different 
systems  form  angles 
with  one  another.  In 
general,  there  are  two 
such  systems  present, 
crossing  each  other  at  a 
high  angle  and  dividing 
the  rock  mass  perpen- 
dicularly to  the  bedding 
planes  into  a  series  of 

quadrangular  blocks  or 

,  .  i               ,,  FIG.  563.  —  Master  joints  in  soft  shale  (Upper 

prisms,     which     greatly  Marcellus)  on  the  shore  of  Lake  Erie.     (Photo 

facilitate     quarrying  by  author.) 


FIG.  562. — A  piece  of  rock  in 
which  slaty  cleavage  has  been  de- 
veloped, as  shown  by  the  fine  lines 
(p) ;  the  coarser  bands  represent  the 
original  bedding  planes,  now  com- 
pressed and  contorted.  (After  Le 
Conte.) 


Other  Structures  Produced  by  Deformation     639 


operations  and  permit 
weathering  and  mechanical 
erosion  to  produce  a  va- 
riety of  forms.  In  figures 
563  to  566  some  illustra- 
tions of  such  jointing  are 
given ;  in  soft  shales  on  the 
shore  of  Lake  Erie  (Fig. 
563) ;  in  sandy  shales  and 
sandstones  on  Cayuga  Lake 
(Fig.  564),  and  in  limestone 
or  chalk  where  they  control 
the  erosion  on  the  coast  of 
France  (Figs.  565,  566). 
(See  also  Figs.  341  a,  p.  407, 
and  474,  p.  571.) 

Some  of  these  joints  ex- 
tend through  the  entire 
mass  of  the  formation, 
others  are  limited  to  a 
single  stratum.  The  first 
are  called  master  joints  and 
often  exert  an  important  influence  on  the  topography  (Figs.  564, 
565).  The  others,  the  minor  joints,  are  of  small  significance. 


FIG.  564.  —  Joint  planes  in  Sherburne 
sandstone,  shore  of  Cayuga  Lake,  N.  Y.  Two 
sets  of  joints  cut  the  horizontally  bedded 
fine-grained  sandstones,  and  erosion  has  re- 
moved the  portion  on  the  right,  leaving  the 
huge  square  prisms  of  rock  facing  the  lake. 
(E.  M.  Kindle,  photo;  from  U.  S.  G.  S.) 


FIG.  565.  —  Sea  cliff  100  meters  high  at  Treport,  France,  showing  the  effect 
on  the  coast  topography  of  joints  which  traverse  the  beds  of  chalk  in  two 
principal  directions.  (Copied  from  Crosby.) 


640     Deformation  of  Rocks  of  the  Earth's  Surface 

Such  joints  appear  to  be  the  result  of  the  fracturing  of  strata 
under  a  torsional  strain.  If  a  great  horizontally  lying  plate  of  glass 
were  raised  by  one  corner,  the  weight  of  the  plate  would  tend  to 
set  up  sagging  or  torsional  strains  and  these  might  be  so  powerful 


FIG.  566.  —  Plan  of  the  joints  in  the  chalk  cliffs  of  Tr£port,  showing  how 
they  influence  the  process  of  erosion.     (Copied  from  Crosby.) 

as  to  overcome  the  cohesion  of  the  mass  and  produce  fracture. 
This  can  be  illustrated  by  gently  twisting  a  strip  of  glass  held 
firmly  by  one  end,  as  illustrated  in  the  annexed  figures  (Figs. 
567  a,  b).  When  the  strain  becomes  too  great,  the  glass  will-  break, 


FIG.   567  a.  —  Apparatus  for  breaking  a  plate  of  glass  by  torsion,  with  an 
example  of  results  produced.      (After  Daubree.) 

with  the  formation  of  two  or  more  regular  systems  of  parallel 
cracks,  the  cracks  of  one  system  crossing  those  of  the  other 
at  a  high  angle.  Such  cracks  reproduce  in  all  essentials  the  joint 
cracks  formed  in  stratified  rocks.1  If  the  twisting  is  not  carried 
far  enough  to  produce  actual  shattering  of  the  glass,  this  can  be 

1  In  this  experiment  the  glass  should  be  reenforced  by  a  sheet  of  paper  glued  to  one 
side  which  will  keep  the  fragments  in  place  after  cracking. 


Other  Structures  Produced  by  Deformation     641 

produced  by  striking  a  blow  with  a  hammer  on  the  table  on  which 
the  experiment  is  made.  The  shock  thus  produced  will  com- 
plete the  shattering  not  accomplished  by  the  torsion  alone.  This 
illustrates  how  great  sheets  of  stratified  rocks  placed  under  a 


FIG.  567  b.  —  Arrangement  of  fractures  in  a  large  plate  of  glass  which  was 
broken  by  torsion.     (After  Daubree.) 

torsional  strain  by  unequal  elevation  or  warping  may  be  shattered 
by  the  passage  through  them  of  an  earthquake  shock,  and  this 
may  be  the  usual  way  in  which  such  jointing  is  produced,  as  sug- 
gested by  W.  O.  Crosby.  Joint-like  fissures,  but  of  lesser  regularity 
of  arrangement,  may  also  be  produced  in  a  variety  of  other  ways. 


CHAPTER  XX 
METAMORPHISM   AND    METAMORPHIC   ROCKS 

DEFINITION  AND  CLASSIFICATION  OF  METAMORPHISM 

Definition.  —  All  rocks  are  subject  to  alteration  in  nature. 
This  alteration  may  be  slight  or  intense ;  it  may  be  accompanied  by 
disturbances  in  the  earth's  crust,  or  it  may  be  the  direct  result  of 
such  disturbances.  Changes  in  the  character  of  the  rock  are  gen- 
erally recognized  by  changes  in  mineral  constitution,  in  texture,  or  in 
both,  as  well  as  in  other  characteristics.  Such  changes  are  termed 
metamorphic,  and  the  process  is  one  of  metamorphism  (Greek 
/xera,  dftioting  interchange  -f-  /xop</>rj,  form).  Some  geologists  con- 
sider all  changes  as  metamorphic  changes,  but  in  practice  the  term 
metamorphic  rocks  is  generally  restricted  to  those  that  have  been 
strongly  altered,  and  as  a  result  have  taken  on  a  crystalline 
character. 

Classification  of  Metamorphism  according  to  Forces.  —  The 
natural  forces  producing  rock  metamorphism  are  (a)  chemical 
energy,  (b)  heat,  and  (c)  pressure.  All  three  are  commonly  active, 
but  one  or  the  other  may  predominate  to  such  an  extent  as  to 
give  the  process  its  distinctive  character.  Accordingly  we  may 
in  a  general  way  classify  metamorphism  as  (i)  chemical  or  dia- 
genetic,  (2)  thermal  and  (3)  dynamic,  but  it  must  be  clearly  under- 
stood that  these  divisions  refer  only  to  the  dominant  force,  and 
that  there  can  be  no  complete  dissociation  of  any  one  of  them  from 
the  others.  Chemical  energy  is  indeed  active  in  all  processes 
of  metamorphism  and  may  be  regarded  as  the  chief  of  the  forces 
producing  changes  .in  rock,  but  this  may  be  set  in  motion,  or  ac- 
celerated, by  the  special  intervention  of  heat,  as  in  the  case  of  a 
contact  with  igneous  masses  or  by  the  influence  of  great  pressure 
in  mountain-making  disturbances.  It  may  also  act,  though 
slowly,  under  the  ordinary  conditions  of  aging  of  rocks  with  time. 

Classification  of  Metamorphism  according  to  Extent.  —  Metamor- 

642 


Activities  of  Agencies  Producing  Metarnorphism     643 

phism  may  be  local,  when  only  a  limited  area  is  affected,  as  in  the 
case  of  the  contact  of  a  rock  with  a  hot  igneous  mass  or  with  heated 
waters  or  gases,  or  it  may  be  regional,  when  extensive  areas  are 
affected,  as  in  the  case  of  great  deformation  of  the  earth's  crust. 
Because  of  the  association  of  regional  metamorphism  with  ex- 
tensive dynamic  disturbances,  the  two  terms,  regional  and  dynamic, 
are  often  used  interchangeably.  It  will  be  well,  however,  to  keep 
in  mind  the  fact  that  one  term  refers  to  the  areal  extent  of  the 
change  and  the  other  to  the  process  producing  it.  And  it  must 
be  further  understood  that  in  extensive  regional  metamorphism, 
deformational  changes  are  not  the  only  causes  of  alteration,  for 
all  other  causes,  such  as  invading  heat  from  igneous  masses  and 
the  passage  of  gases  and  liquids  which  produce  chemical  changes, 
may  be  equally  active. 

Local  metamorphism  is  most  readily  seen  where  heated  igneous 
masses  come  in  contact  with  other  rocks,  which  they  alter,  and  by 
which  they  are  altered  to  a  certain  extent.  This  is  therefore 
called  contact  metamorphism,  and  its  characteristics,  so  far  as 
igneous  contacts  are  concerned,  have  already  been  described 
(p.  207).  It  should,  however,  be  noted  that  contact  metamorphism 
is  also  produced  by  the  passage  of  heated  gases  and  waters,  which 
generally  carry  solutions  of  various  substances,  and  by  these  the 
rocks  in  the  neighborhood  are  affected.  Since  the  chief  cause  of 
contact  metamorphism  is  the  heat  of  the  igneous  body,  and  since 
such  bodies  in  contact  with  other  rocks  are  the  chief,  though  not 
the  only  source  of  heat,  the  terms  contact  and  thermal  metamor- 
phism are  often  used  synonymously.  In  the  following  table  some 
of  these  relations  are  shown : 

Force  Condition  Area 

Chemical  or  diagenetic.  Static.  Local  or  regional. 

Thermal.  Contactic.  Local. 

Dynamic.       .  Tectonic.  Regional,  more  rarely  local. 

ACTIVITIES  OF  THE  AGENCIES  PRODUCING  METAMORPHISM 

Pressure.  —  Simple  pressure  is  exerted  upon  rocks  by  the  weight 
of  other  rock  masses  which  overlie  them.  Such  pressure  tends 
to  bring  closely  together  the  particles  of  which  the  rock  is  composed 
and  may  weld  them  into  a  more  or  less  solid  mass.  Certain  more 
plastic  layers  may  also  give  way  at  some  points  and  undergo  a 


644       Metamorphism  and  Metamorphic  Rocks 


flowing  movement,  with  the  result  that  crowding  and  crumpling 
of  that  layer  (enterolithic  structure]  (Fig.   568)  is  produced  else- 


FIG.    568.  — •  Enterolithic    structure    in    fine-grained    or   compact   limestone ; 
Muschelkalk,  Neckar  Valley,  Wiirttemberg.     (After  Koken.) 

where.  Pressure  and  unequal  solution  along  irregular  bedding 
surfaces  in  massive  limestone  produces  the  remarkable  structure 
known  as  stylolite  (Figs.  569,  570).  Similar  deformations  are  pro- 
duced in  the  formation  of  the  great  domes  of  salt  which  character- 
ize certain  districts 
of  the  southern 
United  States  and 
of  the  Old  World 
as  well.  In  these 
contortion  of  the 
salt  mass  takes 
place  often  from  the 

F,G.  569.  -  Appearance  on  the  transverse  face  of  Pressure  of  growing 
a  limestone  layer,  showing  the  formation  of  stylolitic  salt  crystals,  aided 
structure.  The  interlocking  masses  have  been  pro-  m  some  cases  by 
duced  by  solution  along  a  minute  fracture  plane, 
producing  hollows  on  opposite  sides  into  which  the 
projecting  masses  fit.  The  residual  clay  left  on 
solution  is  found  in  both  the  under  and  upper  hol- 
lows and  indicated  in  black.  The  amount  of  solu- 
tion is  approximately  indicated  by  the  depths  of 
the  hollows.  (After  Wagner.)  Greatly  reduced. 


tectonic  forces  (Fig. 

571)- 

It  is  probably  also 
true  that  rocks  un- 
der enormous  pres- 


sure   of    overlying 

beds  are  subject  to  intense  thermal  ai\d  chemical  activities, 
but  of  this  there  is  as  yet  no  positive  evidence.  It  has  some- 
times been  assumed  that  the  older  rocks  of  the  earth's  crust 


Activities  of  Agencies  Producing  Metamorphism     645 


have  become  metamorphosed  in  part  because  of  the  great  weight 
of  overlying  sediments,  of  which  those  of  the  Palaeozoic  alone,  in 
the  Appalachian  region, 
aggregate  some  40,000 
feet.  But  it  appears 
that  the  older  meta- 
morphic  rocks,  wherever 
they  have  been  exposed 
by  erosion,  are  overlain 
by  sediments  altered 
but  slightly  or  not  at 
all,  and  that  those  rocks 
were  already  meta-  FIG.  570.  —  Two  of  the  interlocking  prisms  or 

morphosed  before  these  stylolites,  showing  the  striated  sides  produced 
, .  by  friction  during  the  process  of  interpenetra- 

sediments    were    de-      tion     About  one  half  natural   size.    (After 

posited     upon     them.      Wagner.) 

Thus  the  mere  pressure 

of  superincumbent  rocks  seems  a  minor  cause  of  metamorphism. 
Movement.  —  The  case  is  otherwise  where  movement  of 

rock  masses  has  taken  place,  especially  where  such  movement  has 

been  intense  and  resulted  in  the  production  of  greatly  disturbed 

structures.  Simply 
folded  strata,  such 
as  those  of  the  Jura 
and  Appalachian 
mountains,  have 
been  but  little  af- 
fected by  the  dis- 
turbances, and  as  a 
rule  the  amount  of 
metamorphism  in 
them  is  compara- 
tively slight,  though 
locally  it  may  be 

of    considerable    in- 
FIG.  571.  —  Contorted  salt  layers  in  the  roof  of  .  w, 

a  chamber  in  Myles  Salt  Mine,  Weeks  Island,  La.  1 W'        W  n 

(Photo  by  Veatch;  from  U.  S.  G.  S.)     These  def-       rocks    are    intensely 
ormations  are  believed  by  many  to  be  wholly  due 
to  the  pressure  of  growing  salt  crystals,  while  others 
hold  that  tectonic  forces  from  without  aid  in  their 
formation,  or  may  even  be  the  chief  cause. 


deformed,  metamor- 
phism is  generally 
pronounced. 


646        Metamorphism  and  Metamorphic  Rocks 

Intense  deformation  is  accompanied  not  only  by  a  change  in 
the  form  and  position  of  the  strata,  as  in  the  change  from  a  hori- 
zontal to  a  folded  structure,  but  also  by  interior  readjustment 
of  rock  substances,  a  rearrangement  of  the  rock  particles  com- 
monly in  a  parallel  manner,  together  with  chemical  and  mineral- 
ogical  changes.  This  is  called  rock  flowage,  as  distinct  from  rock 
fracture,  and  it  is  believed  to  be  a  process  which  in  most  rocks 
can  take  place  only  at  a  considerable  depth,  whereas  rock 
fracture  is  essentially  a  surface  phenomenon.  Consequently, 
the  earth's  crust  is  believed  to  be  divisible  beneath  the  belt  of 
weathering  into  an  upper  zone  of  rock  fracture  (katamorphism), 
and  a  lower  one  of  rock  flowage  (anamorphism).1  The  limit  be- 
tween the  two  is  at  the  level  where  pressure  is  so  great  as  to  close 
all  fractures  which  may  result  from  movements.  It  must,  how- 
ever, be  emphasized  that  the  depth  at  which  rocks  flow  rather 
than  fracture  depends  upon  many  things,  chief  among  them  being 
perhaps,  the  nature  of  the  rock.  Some  rocks,  like  shales,  undergo 
a  flowage  comparatively  close  to  the  surface,  while  rigid  rocks, 
like  quartzites,  may  still  fracture  at  a  considerable  depth  beneath 
it.  Therefore  if  we  speak  of  zones  of  flowage  and  fracture,  we 
refer  to  conditions  rather  than  to  actual  depth.  Nevertheless, 
at  great  depths  probably  no  fracturing  takes  place,  all  rocks  under- 
going flowage.  Therefore  it  may  be  better  to  speak  of  the  upper 
portion  of  the  earth's  crust  as  the  zone  of  combined  fracture  and 
flowage  (according  to  the  nature  of  the  rock),  and  the  lower  part 
as  the  zone  of  flowage  only. 

Heat.  —  Heat  not  only  affects  the  mineral  particles  of  the  rocks 
which  it  invades,  but  also  aids  greatly  in  making  the  rock  undergo 
flowage  rather  than  fracture.  Thus  where  a  rock  under  pressure 
is  invaded  by  heat  from  an  igneous  mass,  flowage  may  occur  much 
nearer  the  surface  than  is  the  case  for  the  same  rock  unaffected 
by  the  heat.  Heat  also  greatly  increases  the  activity  of 
liquids  and  gases  which  enter  the  rock  masses.  While  heat 
may  be  produced  in  various  ways,  as  by  pressure,  movements, 
etc.,  its  most  frequent  source  is  probably  found  in  intruded 
igneous  masses. 

Liquids  and  Gases.  —  These  are  important  agents  in  producing 
rock  metamorphism.  Water  is,  of  course,  the. most  important  of  the 

1  Leith  and  Mead  ("  Metamorphic  Geology  "),  use  katamorphic  for  destructive  altera- 
tions, including  weathering,  and  anamorphic  for  constructive  alterations. 


Metamorphic  Structures  and  Textures          647 

liquids.  Combined  with  heat  and  pressure  it  becomes  a  powerful 
solvent  agent  and  aids  in  the  crystallization  of  the  minerals.  Gases 
arising  from  an  intruded  igneous  magma  are  also  very  active  in 
altering  the  minerals  of  the  affected  rock  and  in  introducing  new 
ones.  These  agencies  appear  to  be  most  effective  in  contact 
metamorphism. 

METAMORPHIC  STRUCTURES  AND  TEXTURES 

The  more  important  structures  produced  by  metamorphism 
in  rocks  are:  (i)  cleavage,  (2)  schistosity,  (3)  gneissic  structure  and 
banding.  The  chief  texture  of  metamorphic  rocks  is  crystalline. 

Slaty  Cleavage.  —  This  has  already  been  discussed  in  connection 
with  structures  produced  by  deformation  (p.  636). 

Schistosity.  —  This  structure  represents  a  further  development 
of  cleavage  in  rocks  which  are  subject  to  intense  deformation,  with 
the  development  of  a  crystalline  texture.  The  crystals  in  this 
case  are  elongated  and  have  a  more  or  less  parallel  arrangement 
not  only  of  form  but  of  mineral  cleavage  as  well.  As  a  result, 
schistose  rocks  split  readily  along  the  planes  of  schistosity,  some- 
times separating  between  the  mineral  partings,  but  more  generally 
along  the  cleavage  planes  of  these  minerals.  This  is  especially 
the  case  where  mica  and  hornblende  abound,  whereas  if  they  are 
less  common  than  quartz  and  feld- 
sparj  the  splitting  is  mostly  between 
these  mineral  particles.  As  a  rule  the 
mica  and  hornblende  are  new  to  the 
schist,  having  been  developed  there 
during  the  process  of  metamorphism 
by  recrystallization  .  of  substances 
already  in  the  rock  mass. 

Gneissic  Structure.  —  "Gneissic 
structure  means  a  banding  of  con- 
stituents of  which  feldspar  is  impor- 
tant, with  or  without  the  parallel 
dimensional  arrangement  necessary  FIG.  572  <z.  — Banded  and 
r  ,  ,  ,,  /T  .,,v  /T^.  contorted  gneiss,  Fordham, 

for    rock    cleavage"    (Leith)    (Figs.      Ny     (B.  Hubbard,  photo.) 

572  a,  b).     Cleavage  is  not  necessarily, 

nor  commonly,  very  well  developed  in  gneissic  structure,  but  the 
parallel  banding  is  always  marked.  "  The  essential  mineralogical 
difference  between  gneisses  and  schist  is  the  possession  by  the 


648       Metamorphism  and  Metamorphic  Rocks 


gneisses  of  a  relatively  small  amount  of  the  platy  and  columnar 
minerals  so  necessary  for  a  good  rock  cleavage,  and  correspond- 
ingly more  feldspar  and 
quartz  "  (Leith). 

Crystalline  Texture.  — 
This  may  or  may  not 
be  accompanied  by  the 
formation  of  schistose  or 
gneissic  structure  in  rock 
flowage.  Marble  is  one 
of  the  best  examples  in 
which  recrystallization  of 
the  carbonate  of  lime  may 
take  place  without  the 
formation  of  a  flow  struc- 
ture or  cleavage.  Re- 
crystallization  tends 
toward  an  enlargement  of 
the  rock  particles,  and  as 
a  result,  the  original  tex- 
ture of  the  rock  is  de- 
stroyed (Fig.  573).  If  the  original  limestone  was  of  homogeneous 
character,  the  bedding  planes  also  are  obliterated,  but  if  there 
were  layers  differing  in  character  and  composition,  these  are 
affected  in  a  separate  manner,  and  the  bedding  structure  of  the 
marble  or  other  highly  crystalline  rock  will  be  retained. 


FIG.  572  b.  — Banded  and  folded  gneiss,  Utah. 
(Photo,  by  F.  J.  Pack.) 


OCCURRENCE  AND  AGE  OF  METAMORPHIC  ROCKS 

Metamorphic  rocks  are  widespread  over  the  earth's  surface. 
Vast  areas,  such  as  those  of  the  Canadian  Shield,  and  the  greater 
part  of  Scandinavia  and  Finland,  show  chiefly  metamorphic  rocks. 
These  are  among  the  oldest  rocks  of  the  earth's  crust,  and  it  will 
generally  be  found  that  the  later  rocks  (Palaeozoic  and  younger) 
rest  upon  them  with  a  very  pronounced  unconformity,  indicating 
a  long  time  interval  between  the  formation  of  the  two  series  and 
a  pronounced  erosion  of  the  metamorphic  rocks  before  the  de- 
position of  the  overlying  formations. 

Highly  metamorphosed  rocks  also  occur  in  the  axes  of  many 
mountain  ranges,  where  they  have  become  exposed  as  the  result 


Types  of  Metamorphic  Rocks  649 

of  erosion  of  the  younger  rocks.  They  in  some  cases  also  belong  to 
the  older  rock  series  of  the  earth,  but  in  others  are  metamorphosed 
sediments  of  quite  recent  date.  Thus  in  the  Alps  and  in  the  Coast 
Ranges  of  Western  America,  rocks  of  Mesozoic  age  have  been 


FIG.     573.  —  Granular    crystallized    limestone    (marble)    seen    under  crossed 
nicols;  enlarged  24  diameters.     Fichtelgebirge.     (After  Rosenbusch.) 

highly  metamorphosed,  and  rocks  of  Tertiary  age  have  suffered 
a  similar  change  in  some  parts  of  the  world.  In  general,  however, 
we  may  say  that  the  bulk  of  metamorphic  rocks  belongs  to  the  oldest 
geological  divisions,  being  for  the  most  part  of  pre-Palaeozoic  age. 

TYPES  OF  METAMORPHIC  ROCKS 

For  the  sake  of  convenience,  we  shall  group  the  more  important 
metamorphic  rocks  under  the  following  divisions,  which  in  a  general 
way  lead  from  the  less  metamorphosed,  i.e.,  those  whose  original 
character  can  still  be  determined,  to  the  strongly  metamorphosed 
types. 

1.  Quartzites,  slates  and  phyllites. 

2.  Crystalline  schists. 

3.  Gneisses. 

4.  Marbles,  crystalline  dolomites,  ophicalcites,  etc. 

5.  Serpentines,  magnetitic  and  graphitic  rocks,  etc. 

In  general,  it  is  easier  to  predict  the  types  of  metamorphic  rocks 
derivable  from  the  common  unaltered  rocks  than  to  state  positively 
what  was  the  original  condition  of  a  given  metamorphic  rock. 
In  the  following  table,  the  more  usual  metamorphic  derivatives 
of  the  common  rocks  are  given. 


650        Metamorphism  and  Metamorphic  Rocks 

METAMORPHIC  DERIVATIVES  OF  COMMON  ROCKS 


ORIGINAL  ROCK 


METAMORPHIC  DERIVATIVES 


1.  Igneous  or  Pyrogenic  Rocks 

a.  Coarse-grained  acid  feldspathic  types, 

such  as  granites,  syenites,  etc. 

b.  Fine-grained    acid    feldspathic    types, 

felsites,  etc. 

c.  Coarse-grained  basic  types 

d.  Finer-grained  basic  types  rich  in  ferro- 

magnesian  minerals  (basalts,  dolerites, 
etc.) 

2.  Aqueous  (Hydro genie)  and  Organic  (Bio- 

genie)  Rocks 

e.  Precipitated  and  organic  limestones 

3.  Clastic  Rocks 

f.  Conglomerates  and  breccias  (rudytes) 

g.  Sandstones  (arenytes) 

h.  Clay  and  quartz  flour,  i.e.,  mud-rocks 
(lutytes) 

i.  Clastic  limestones 

j.   Pyroclastic  rocks  (tuffs,  etc.) 


a.  Gneiss 

b.  Phyllites  and  schists 

c.  Basic  gneiss ;  serpentine 

d.  Hornblende      schists      and 

other  basic  schists 


e.   Marbles 

/.    Gneisses;  schists 

g.  Quartzite;  quartz  mica 
schist,  etc. 

h.  Porcelanites,  argillites  and 
hornfels ;  slates ;  phyllites ; 
mica  schists 

i.   Marbles ;   calcareous  schists 

j.  Slates;  mica  schists;  horn- 
blende schists 


Characters  of  the  More  Important  Metamorphic  Rocks 

Quartzites.  —  These  range  in  composition  from  nearly  pure  silica  to  a 
mixture  with  1 5  per  cent  or  more  of  aluminum  oxide,  iron  oxide,  etc.  Quartzites 
are  derived  from  quartz-sandstones  and  are  characterized  by  the  presence  of 
silica  as  the  cementing  agent  of  the  grains,  this  silica  being  commonly  deposited 
in  crystallographic  continuity  with  the  quartz  of  the  grains.  Quartzites  thus 
have  a  crystalline  character,  and  they  are  harder  than  ordinary  sandstones 
into  which  they  grade.  When  much  clay  was  present  in  the  original  sandstone, 
mica,  especially  muscovite,  is  developed,  producing  a  micaceous  quartzite, 
which  passes  by  degrees  into  a  quartz-mica  schist.  A  flexible  form  of  mi- 
caceous quartzite,  in  which  the  grains  have  a  slight  power  of  movement  on  one 
another,  is  called  itacolumite.  Quartz  conglomerate,  too,  may  be  changed  to 
quartzite  and  the  pebbles  may  be  flattened  by  dynamic  movements. 

Quartzites  are  most  abundant  in  the  older  (pre-Cambrian)  strata,  but  are 
not  confined  to  them.  They  may  indeed  be  of  any  age. 

Slates.  —  These  are  mud-rocks  or  lutytes  in  which  a  high  degree  of  slaty 
cleavage  has  been  developed  as  the  result  of  compression.  This  cleavage,  as 
already  outlined,  has  commonly  no  relationship  to  the  bedding  planes,  though 
the  name  slate  is  also  applied  to  carbonaceous  mud-rocks  which  split  smoothly 
along  the  stratification.  Mica,  especially  sericite,  and  hornblende  scales  may 
be  developed,  but  these  are  not  visible  except  under  the  microscope.  When 


Types  of  Metamorphic  Rocks  651 

they  become  large  and  dominant,  the  slate  passes  into  phyllites  and  mica 
schists.  (See  Fig.  562,  p.  638.) 

Slates  may  also  be  developed  from  pyroclastic  rocks  (tuffs)  which  have  been 
subject  to  compression  without  extensive  recrystallization.  In  color,  slates 
generally  range  from  drab  to  black,  but  green,  red,  and  purple  tints  may  occur. 

The  chief  uses  are  for  roofing  purposes,  blackboards,  and  (formerly)  for 
school  slates.  The  most  important  American  slates  are  altered  Cambrian 
and  Ordovician  rocks,  but  pre-Cambrian  and  also  younger  slates  occur,  the  latter 
chiefly  foreign. 

Porcelanite,  Baked  Clay,  Hornfels.  —  Fused  clays  and  shales  in  the  roofs 
and  floors  of  burned  coal  seams  are  changed  into  a  hard,  homogeneous  rock 
resembling  porcelain  and  designated  Porcelanite.  Similar  results  are  produced 
in  contact  metamorphism,  where  the  shales  are  baked  into  a  hard,  flinty  rock 
to  which  the  name  hornfels  is  applied.  It  breaks  in  irregular,  angular  masses, 
and  closely  resembles  dense  trap-rock,  for  which  it  may  be  mistaken.  Biotite 
is  commonly  an  important  constituent  of  this  rock,  though  visible  only  under 
the  microscope.  Various  minerals,  such  as  andalusite,  garnet,  cyanite,  stauro- 
lite,  tourmaline,  ottrelite,  rutile,  hornblende,  feldspar,  etc.,  may  be  developed, 
sometimes  in  crystals  of  considerable  size.  When  the  minerals  are  rather 
evenly  scattered,  a  knotty  slate  or  hornfels  is  produced. 

Phyllites.  —  These  rocks  are  intermediate  between  slates  and  mica  schists, 
partaking  of  the  structure  of  the  former,  but  with  the  development  of  much 
fine  mica.  They  may  be  derived  from  silicious  clay  rocks  like  ordinary  slates, 
but  they  may  also  represent  altered  tuffs  or  even  felsitic  rocks.  The  fine  mica 
scales  are  commonly  sericite,  etc.,  which  has  also  been  called  hydromica,  on 
which  account  the  name  hydromica  schist  is  frequently  used. 

Mica  Schists.  —  These  are  schistose  rocks  in  which  mica,  chiefly  muscovite, 
but  also  biotite  appears  in  prominent  scales,  while  quartz  is  the  other  important 
mineral.  When  quartz  predominates,  the  rock  passes  into  a  micaceous  quartzite. 
Sometimes  the  rock  is  high  in  lime,  when  it  passes  into  a  micaceous  marble  or 
calc  schist.  Feldspar  in  greater  or  less  quantity  is  also  present,  especially  if 
the  schist  has  been  derived  from  an  igneous  rock.  As  accessory  minerals, 
garnet,  staurolite,  cyanite,  sillimanite,  tourmaline,  apatite,  pyrite,  and  magnetite 
may  be  mentioned. 

Mica  schists  are  perhaps  most  commonly  derived  from  the  alteration  of 
argillaceous  sandstones  and  silicious  shales,  etc.,  but  they  may  also  originate 
from  igneous  rocks  (rhyolites,  trachites,  etc.).  When  derived  from  elastics, 
they  are  generally  lower  in  alkalies,  and  the  magnesia  is  in  excess  of  the  lime. 

Schists  with  graphite  disseminated  through  them  are  not  infrequent.  These 
are  probably  in  most  cases  developed  by  the  metamorphism  of  carbonaceous 
shales. 

Hornblende  Schists.  —  These  are  derived  from  the  basic  igneous  rocks  high 
in  ferromagnesian  silicates,  by  the  development  in  them  of  schistose  structure. 
More  rarely,  sediments  have  yielded  such  rocks  by  metamorphism,  when 
analysis  generally  shows  a  low  alumina  content  and  a  great  excess  of  magnesia 
over  lime. 

While  hornblende  forms  the  chief  mineral  of  this  rock,  biotite,  augite,  and 
plagioclase  may  also  be  present  in  varying  proportion.  Besides  these,  there 


652        Metamorphism  and  Metamorphic  Rocks 

are  accessory  minerals,  such  as  garnet,  magnetite,  pyrite,  and  pyrrhotite,  but 
quartz  is  normally  absent,  or  at  least  very  rare.  By  alteration,  the  hornblende 
passes  into  chlorite,  with  the  production  of  a  chlorite  schist.  The  plagioclase 
may  be  replaced  by  secondary  products,  such  as  epidote,  calcite,  scapolite,  etc. 
The  schistose  structure  is  due  to  the  parallel  arrangement  of  the  hornblende 
crystals.  When  this  structure  is  indistinct,  the  name  amphibolite  has  been 
used  for  the  rock.  Hornblende  schists  may  form  extensive  areas  by  them- 
selves or  they  may  represent  the  altered  basic  dikes  intruded  in  more  acid  rocks 
with  which  they  have  become  thoroughly  metamorphosed  by  dynamic  dis- 
turbances. Such  dikes,  changed  to  hornblende  schist,  abound  in  the  Man- 
hattan Island  rocks  and  are  also  found  elsewhere  in  the  older  metamorphic 
series  of  eastern  North  America. 

Chlorite  Schists.  —  These  are  further  alteration  products  of  hornblende 
schists,  but  may  also  be  derived  from  other  rocks  rich  in  anhydrous  iron-alumina 
silicates.  The  schistosity  is  well  developed  in  these  rocks  and  the  chief  mineral 
is  chlorite,  a  green,  micaceous,  and  rather  soft  mineral.  More  or  less  quartz  is 
also  present,  and  besides  this,  plagioclase,  talc,  epidote,  and  magnetite  may 
form  common  accessories.  Owing  to  the  pronounced  green  color,  certain  of 
these  rocks  are  sometimes  called  "  green  schists."  They  are  not  uncommonly 
members  of  the  metamorphic  series  of  the  Appalachian,  New  England,  and 
Lake  Superior  regions. 

Talc  Schists.  —  These  are  the  metamorphic  products  of  rocks  high  in  an- 
hydrous magnesian  silicates  but  low  in  iron.  The  chief  mineral  is  talc,  but 
quartz  is  also  often  quite  abundant.  Feldspars  and  some  micaceous  minerals 
also  occur  in  the  latter,  and  are  often  difficult  to  distinguish  from  the  talc  scales. 
The  MgO  content  of  these  rocks  may  be  30  per  cent  or  more.  It  is  possible  that 
silicious  dolomites  in  a  sedimentary  series  may  in  some  cases  be  the  original 
rock  from  which  talc  schists  are  derived.  The  distribution  of  talc  schists  is 
similar  to  that  of  chlorite  schists. 

Epidote  Schists.  —  These  are  rarer  schists,  characterized  by  the  predomi- 
nance of  the  mineral  epidote.  They  have  a  light  greenish  appearance  from  the 
color  of  the  ferromagnesian  silicate,  epidote,  and  they  are  generally  the  product 
of  metamorphism  of  pyroxenic  and  hornblendic  rocks.  Their  CaO  and  MgO 
content  is  often  very  similar  (between  7  and  8  per  cent).  These  schists  have  a 
distribution  similar  to  the  preceding  but  are  less  common. 

Graphite  Schists.  —  These  are  the  common  product  of  the  metamorphism 
of  carbonaceous  sedimentary  (clastic)  rocks,  and  they  are  not  uncommon  in  the 
metamorphosed  Palaeozoic  series  of  eastern  North  America,  but  also  occur  in 
the  older  rocks.  Graphite  is  present  as  a  rule  in  only  moderate  amounts, 
occurring  in  scaly  flakes  along  the  planes  of  schistosity.  Mica,  quartz  and 
feldspar  are  the  common  associates  in  these  schists,  and  when  the  graphite  is 
present  only  in  small  quantities,  the  rock  may  be  a  graphitic  mica  schist. 

Gneiss.  —  The  rocks  classed  under  this  term  (pronounced  nice)  include  a 
variety  of  types  in  which  the  characteristic  gneissic  structure  is  developed. 
The  structure  is  more  coarsely  laminated  than  that  of  schists,  and  the  rocks 
split  less  readily  along  these  planes  of  lamination.  Feldspar  is  always  a  char- 
acteristic constituent  of  typical  gneisses  and,  besides  this,  there  occur,  as  a 
rule,  quartz,  mica  and  hornblende.  Many  accessory  minerals  may  also  occur. 


Types  of  Metamorphic  Rocks  653 

There  are,  however,  gneisses  produced  from  the  alteration  of  basic  igneous  rocks 
in  which  quartz  is  absent,  while  the  ferromagnesian  minerals  abound. 

Gneisses  may  be  produced  from  the  alteration  of  igneous  as  well  as  clastic 
rocks.  In  many  cases  it  is  possible  to  refer  the  rock  back  to  its  original  type, 
but  in  other  cases  this  cannot  be  done.  According  to  the  rocks  from  which 
gneisses  are  derived,  the  following  types  have  been  recognized : 

Corresponding  Gneisses 
Original  Rock  „     ,       ,  ,     , ,  .          . ,  . 

Produced  by  Metamorphtsm 

Granite  Granite-gneiss 

Syenite  Syenite-gneiss 

Diorite  Diorite-gneiss 

Gabbro  Gabbro-gneiss 

Pyroxenite  Pyroxenite-gneiss 

Peridotite  Peridotitic-gneiss 

Conglomerate,  etc.  Conglomerate-gneiss 

Sandstone  Quartzite-gneiss 

The  conglomerate  gneiss  of  Munson,  Mass.,  a  well-known  building  stone, 
is  so  thoroughly  crystallized  that  it  is  commercially  referred  to  as  a  granite. 
The  conglomerate  was  probably  originally  derived  from  granites  and  diorites, 
and  its  age  appears  to  have  been  basal  Palaeozoic. 

Gneiss  forms  the  dominant  rock  of  the  old  crystalline  areas  around  and 
upon  which  the  younger  sediments  were  deposited.  In  America  the  largest 
area  of  such  ancient  gneisses  forms  the  so-called  Canadian  Shield.  Gneisses  also 
occur  in  the  older  Appalachians  and  in  the  Cordilleran  region.  In  Europe  a 
similar  area  is  found  in  the  Scottish  Highlands,  Scandinavia,  and  Finland. 
Younger  gneisses  are  also  abundant,  especially  in  New  England,  in  parts  of 
Scotland,  and  in  the  Alps,  where  even  Mesozoic  rocks  have  been  metamor- 
phosed into  gneisses  by  the  great  dynamic  disturbances. 

Marbles.  —  These  comprise  both  crystalline  limestones  and  dolomites, 
but  not  all  of  these  have  the  qualities  demanded  of  commercial  marble. 
Some  very  slightly  altered  limestones  occasionally  furnish  this  product.  All 
kinds  of  limestones,  of  whatever  origin,  may  be  changed  to  marbles,  and  in 
this  process  the  original  bedding-structure  and  the  fossils  are  commonly  ob- 
literated. When  the  original  limestone  was  impure,  as  is  usually  the  case  in 
clastic  limestones,  silicate  minerals,  such  as  tremolite,  light  colored  pyroxenes, 
various  micas,  especially  phlogopite,  etc.,  are  developed  as  the  result  of  meta- 
morphism.  When  the  original  limestone  was  carbonaceous,  the  product  of  meta- 
morphism  is  a  graphitic  marble.  Sometimes  the  rock  becomes  very  coarsely 
crystalline,  as  in  the  case  of  the  Westchester  dolomite  marble,  used  in  the  con- 
struction of  St.  Patrick's  Cathedral  in  New  York  City.  (See  also  Fig.  573, 
p.  649.) 

Marbles  abound  in  the  metamorphic  districts  of  the  Appalachian  belt,  es- 
pecially in  Pennsylvania,  Vermont,  Massachusetts,  New  York,  and  Georgia. 
They  are  also  extensively  developed  in  western  Colorado  and  in  the  Sierras 
of  California,  etc.  In  the  Pyrenees,  Alps,  Carpathians,  and  Himalayas  many 
marble  quarries  exist.  The  famous  Carrara  marble  is  a  metamorphosed  upper 
Triassic  limestone  of  the  Apennines  in  North  Italy.  It  is  associated  with  sericite, 


654        Metamorphism  and  Metamorphic  Rocks 

chlorite,  ottrelite,  and  mica  schists,  and  is  underlain  by  conglomerate  gneisses 
believed  to  be  of  Permian  age.  The  main  zone  of  this  marble  has  a  thickness 
ranging  up  to  1000  meters.  Both  white  and  dark  common  marble  (ordinario) 
make  up  the  main  deposit.  "  This,  however,  incloses  locally  the  famous  fine- 
grained snow-white  statuary  marble  (statuario),  which  comprises  about  5  per 
cent  of  the  entire  deposit.  The  "  antique  marble  "  of  eastern  Greece  is  in- 
cluded in  crystalline  schists  and  is  partly  metamorphosed  Cretaceous  limestone 
and  in  part  belongs  to  the  Archaean.  Among  the  several  varieties  are  the  snow- 
white  to  yellowish  Pentelic  marble  found  northeast  of  Athens,  the  translucent 
Parian  marble  from  the  Island  of  Paros  in  the  ^Egean,  of  snow-white  color 
with  frequently  a  bluish  tinge,  uniformly  grained,  and  prized  as  the  finest  of 
•  all  marbles,  and  others.  Great  marble  deposits  also  occur  in  the  metamor- 
phic  regions  of  Norway,  and  good  statuary  marble  has  been  obtained  here. 
Excellent  statuary  marble  also  occurs  in  the  Tyrol  (Laas  region).  Fine 
marbles  for  decorative  purposes  are  obtained  from  various  parts  of  Africa. 

Ophicalcites.  —  These  are  crystalline  magnesian  limestones  or  dolomites, 
mottled  with  inclusions  of  the  mineral  serpentine  in  varying  amounts,  and 
they  mark  a  transition  from  marbles  to  serpentines.  In  color,  the  stone  is  a 
beautiful  mottled  green  and  white.  The  serpentine  is  probably  derived  from 
crystals  of  pyroxene  which  originally  were  included  in  the  rock,  and  this  may 
have  been  a  silicious  magnesian  limestone  of  sedimentary  (clastic)  origin, 
altered  by  regional  metamorphism.  The  rock  is  also  called  Verd-antique. 

Serpentine.  —  This  is  a  rock  composed  of  green  or  red  scales,  fibers,  and 
massive  aggregates  of  the  mineral  serpentine,  a  hydrous  silicate  of  magnesium 
and  iron  and  generally  formed  by  the  static  alteration  of  basic  igneous  rocks, 
such  as  pyroxenites  and  peridotites.  Serpentines  vary  greatly  in  texture,  and 
they  may  contain  small  quantities  of  secondary  minerals  such  as  chromite, 
magnetite,  garnet,  etc.  Veins  of  calcite  or  magnesium  carbonates  often  intersect 
the  serpentine  in  all  directions  and  produce  a  striking  appearance.  The  rock 
is  much  used  as  a  building  and  ornamental  stone. 

Soapstone  or  Steatite.  —  This  is  a  massive  talc  rock,  differing  from  talc 
schist  mainly  in  the  absence  of  schistosity.  Quartz  veins  and  scattered  quartz 
grains  are  not  uncommon,  and  magnesium  carbonate  is  also  present.  The  rock 
often  occurs  in  association  with  crystalline  limestones,  as  in  the  Adirondacks, 
and  may  be  the  alteration  product  of  a  silicious  dolomite  or  a  non-ferruginous 
basic  intrusive  igneous  rock.  Soapstones  are  of  great,  economic  importance, 
and  their  distribution  is  generally  similar  to  that  of  the  crystalline  limestones  and 
the  serpentines. 

Magnetite  Rock.  —  This  is  a  metamorphic  product  of  iron  ores,  and  where 
found  in  abundance  constitutes  a  valuable  source  of  iron. 

Anthracite,  Graphite  Rock.  —  Anthracite  is  sometimes  considered  the 
product  of  slightly  metamorphosed  coal  beds,  but  may  also  originate  as  an 
original  deposit.  The  extreme  metamorphism  of  carbonaceous  deposits  produces 
graphite,  which  is  pure  carbon. 


CHAPTER  XXI 

MOVEMENTS    OF   THE   EARTH'S    SURFACE   AND    THEIR 
GEOLOGICAL  EFFECTS 

THE  crust  of  our  earth  is  subject  to  a  variety  of  movements, 
some  of  which  are  so  pronounced  and  sudden  that  they  produce  the 
tremblings  known  as  earthquakes,  while  others  are  slow  and  gradual, 
and  are  only  recognized  by  careful  measurements,  or  by  a  com- 
parison of  the  characters  of  a  region  at  widely  separated  time 
intervals,  or  by  special  features  which  indicate  them  to  the  trained 
observer.  Among  these  are  the  great  earth-movements  by  which 
areas  once  land  became  submerged  beneath  the  sea,  while  sea- 
bottoms  are  upraised  into  land  areas  or  even  into  mountain  masses 
of  great  height  and  extent. 

From  a  purely  geological  point  of  view  earthquakes  are  of  less 
significance  than  the  other  movements  referred  to,  but  they  have  a 
very  real  interest  for  us,  because  they  occur  practically  everywhere 
and  at  all  times  and  because  their  immediate  effect  upon  human 
welfare  is  more  pronounced  than  that  of  any  other  geological  phe- 
nomenon. On  this  account  we  shall  devote  more  space  to  them 
than  their  geological  importance  would  demand. 

SUDDEN  CRUSTAL  MOVEMENTS  —  EARTHQUAKES 
General  Consideration 

Earthquakes  are  the  tremors  or  vibrations  set  in  motion  in  the 
outer  layers  of  the  earth's  surface  by  sudden  disturbances  in  the 
crust.  These  disturbances  may  be  due  to  volcanic  or  other  ex- 
plosions, or  to  local  deformation  of  the  crust.  They  are  not  in- 
frequently indicated  upon  the  surface  by  changes  in  topography, 
while  the  tremors  resulting  from  and  accompanying  the  disturb- 
ances are  often  the  cause  of  enormous  destruction  of  life  and 
property,  as  well  as  modification  of  the  surface  features  of  the 
earth.  In  how  far  the  great  disturbances  in  the  earth's  crust,  of 

655 


656  Movements  of  the  Earth's  Surface 

which  we  have  a  record  in  the  rock  structures,  affected  the  surface 
in  the  past,  can  only  be  conjectured,  but  it  is  safe  to  say  that  most, 
if  not  all,  of  them  were  accompanied  by  earthquake  tremors, 
and  many  of  these  may  have  been  more  violent  than  any  recorded 
in  historic  time.  It  is,  however,  conceivable  that  great  disturbances 
may  go  on  slowly,  deep  down  in  the  earth's  crust,  and  that  they 
are  manifested  upon  the  surface  only  by  occasional  or  intermittent 
earthquakes,  which  are  due  to  secondary  disturbances,  set  in 
motion  near  the  surface  of  the  earth  during  the  progress  of  readjust- 
ment of  the  rocks  at  greater  depths. 

Types  of  Seismic  Disturbances 

The  name  seismic  disturbances  is  applied  to  shocks  produced  by 
sudden  changes  in  the  earth's  crust,  and  they  may  be  manifested 
in  earthquakes,  seaquakes  and  atmospheric  disturbances  (air- 
quakes).  Seaquakes  and  great  air- waves  may  be  originated  by 
violent  explosions,  such  as  those  of  the  volcano  Krakatoa  in  Java 
in  1883,  where  the  air-waves  passed  around  the  earth  several  times, 
while  the  great  sea-disturbances,  the  so-called  tsunamis,1  were 
noticeable  more  than  five  hundred  miles  away. 

The  quaking  of  the  lands  may  also  be  brought  about  by  vol- 
canic explosions,  but  the  larger  earthquakes  are  probably  all  pro- 
duced by  dislocations  within  the  earth's  crust,  and  the  readjust- 
ment of  sections  under  strain.  Such  disturbances,  if  near  the 
coast  or  upon  the  sea-bottom,  are  communicated  to  the  water, 
and  thus  sea-disturbances  or  tsunamis  are  also  induced.  At- 
mospheric disturbances  are,  however,  as  a  rule,  of  slight  pr  negli- 
gible importance  as  effects  due  to  readjustment  in  the  crust. 

According  to  their  mode  of  origin,  seismic  disturbances,  of  which 
earthquakes  are  the  typical  expressions,  have  been  divided  into 
the  two  following  groups  : 

Volcanic  or  Explosive  Earthquakes.  —  These  affect  the  earth, 
sea  and  air.  As  a  subordinate  type,  the  disturbances  due  to  ex- 
plosions of  gases  in  coal  mines,  gas  reservoirs,  etc.,  and  the  con- 
cussions due  to  the  detonations  of  artificial  explosives  either  on 
land  or  in  the  sea  or  air  may  be  included.  Such  explosions  affect 
chiefly  the  air  and  the  water,  though  minor  tremors  may  be  induced 
by  them  in  the  rocky  crust  of  the  earth. 

1  A  word  of  Japanese  origin  for  the  great  seismic  waves  often  wrongly  called  "tidal 
waves." 


Sudden  Crustal  Movements  —  Earthquakes      657 

Dislocation,  Fault  or  Fracture  Earthquakes.  —  These  produce 
the  larger  seismic  disturbances  of  the  land  and  are  communicated 
to  the  sea  as  well,  but  affect  the  air  only  to  a  minor  degree.  The 
actual  dislocation  is  probably  the  accompaniment  rather  than 
the  cause  of  the  shock,  this  being  produced  chiefly  by  the  sudden 
fracturing  of  the  rock  along  a  line  where  strains  have  accumulated 
until  they  exceed  what  the  strength  of  the  rock  can  withstand.  As 
a  subordinate  type,  the  tremors  due  to  the  collapse  of  the  roofs  of 
caves.,  such  as  frequently  characterize  the  "  Karst  "  region  of 
the  former  Austrian  Coast-land,  may  be  cited.  They  affect  chiefly 
the  land,  but  only  to  a  minor  degree.  Tremors  due  to  the  collapse 
of  buildings  form  another  subordinate  type. 

While  even  these  subordinate  tremors  may  become  of  great 
human  or  economic  importance,  they  are  insignificant  in  their 
geological  effects,  and  we  may  confine  our  attention  primarily 
to  the  disturbances  due  to  volcanic  explosions  and  those  due  to 
dislocations  of  the  earth's  crust,  and  chiefly  to  the  latter. 

Centers  and  Areal  Extent  of  Disturbances 

In  the  case  of  disturbances  due  to  volcanic  explosions,  we  may 
consider  that  they  center  about  a  circumscribed  spot  or  point, 
which  is  termed  the  focus  or  the  hypocenter  of  the  earthquake 
(also  called  seismic  center,  centrum  or  origin).  In  the  case  of  a 
dislocation,  the  disturbance  extends  along  a  line,  though  it  may  be 
most  violent  at  one  or  several  points  along  that  line.  In  general, 
the  shock  appears  to  originate  comparatively  near  the  surface,  so 
that  the  focus  or  center  is  probably  never  deeper  than  30  geo- 
graphical miles,  and  probably  does  not,  as  a  rule,  exceed  5  to  15 
miles.  The  point  or  locus  upon  the  earth's  surface  immediately 
above  this  focal  point  or  line  is  called  the  epicenter  or  the  epicentral 
or  epifocal  point  or  line. 

Earthquake  Waves.  —  The  disturbance  at  the  focus  starts  a 
series  of  vibrations  or  earthquake  waves  which  rapidly  spread  from 
this  point  through  the  earth  in  ever  widening  spheres  (Fig.  574). 

The  rate  at  which  such  vibrations  travel  is  enormous ;  they  will  pass  through 
the  8000  miles  of  the  diameter  of  the  earth  in  20  to  22  minutes,  or  at  the  rate 
of  about  375  miles  per  minute.  The  rate  of  transmission,  however,  varies  with 
the  depth.  From  data  afforded  by  the  California  earthquake  of  1906,  Reid 
has  calculated  that  while  the  transmission  at  the  surface  was  4.5  miles  per 
second,  at  272  miles  below  the  surface  it  was  6  miles  per  second;  at  612  miles 


658 


Movements  of  the  Earth's  Surface 


depth,  6.9  miles  per  second;    at  1225  miles,  7.8  miles  per  second;   and  at  1968 
miles  depth,  it  was  7.9  miles  per  second.     This  relatively  lessening  rate  of  in- 


FIG.  574.  —  Diagrammatic  representation  of  the  propagation  of  the  earth- 
quake waves  and  their  record  by  the  seismograph  in  different  parts  of  the 
earth.  (After  Sieberg,  from  Keilhack's  Praktische  Geologic.) 

Explanation  and  translation  of  terms :  H,  hypocentrum ;  Ortsbeben,  local 
vibration  (at  epicentrurn) ;  Nahbeben,  vibrations  not  far  distant ;  Fernbeben, 
distant  vibrations;  Stoss-strahl,  the  "wave  normal/'  represented  by  the  heavy 
lines  which  extend  outwards  from  the  hypocentrum  (H)  in  all  directions  at 
right  angles  to  the  successive  wave  lines  (shown  by  the  fine  lines  in  the  figure) 
and  along  which  a  vertical  movement  takes  place.  FI,  V^  first  and  second 
preliminary  vibrations  (Vorlaufer) ;  TFi,  surface  waves  sent  from  the  epicen- 
trum;  W^  surface  waves  returned  from  the  antiepicentrum ;  Wa,  third  set  of 
waves  (rarely  recorded)  sent  out  again  from  epicentrurn ;  B,  principal  or  great 
vibrations ;  N,  af tervibrations  (Nachlaufer) ;  Oberflachenwellen,  surface  waves ; 
Erd-Rinde,  earth's  crust;  Erd-Kern,  earth's  center. 

crease  with  depth  indicates  that  the  density  and  elasticity  of  the  earth's  crust 
increases  with  depth,  down  to  a  certain  point. 

Earthquake  waves  may  be  resolved  into  their  longitudinal  or  compressive 
components  which  vibrate  in  the  direction  of  the  propagation  of  the  shock, 


Sudden  Crustal  Movements  —  Earthquakes      659 

and  into  the  transverse  components  which  vibrate  at  right  angles  to  the  di- 
rection of  propagation.  To  record  these  waves,  special  instruments  called 
seismographs  are  devised  (Fig.  575  a),  in  which  three  heavy  pendulums  are  sus- 
pended in  such  a  way  that  they  will  record  the  three  movements  at  right  angles, 
N.  and  S.,  E.  and  W.,  and  up  and  down,  though  in  many  seismographs  the  latter 
is  omitted  (Fig.  576).  The  weight  of  the  pendulums  keeps  them  stationary  for  a 


FIG.  575  a. — The  Ewing  seismograph,  constructed  to  record  all  three 
movements  (University  of  California,  after  Le  Conte).  Three  pendulums  are 
arranged  to  swing  in  the  manner  of  a  bracket  or  a  gate,  in  three  planes,  at  right 
angles  to  one  another.  Two  of  them  are  placed  vertically  and  oscillate  in  a 
horizontal  manner,  a,  position  north  and  south  records  east- west  movement; 
b,  position  east  and  west  records  north-south  movement ;  the  third,  c,  oscillates 
in  a  vertical  manner,  being  placed  in  a  horizontal  position  and  being  retained 
by  sensitive  spiral  springs.  Styles  from  these  pendulums  make  the  record  on 
a  circular  smoked-glass  plate  rotating  in  a  horizontal  plane.  (Fig.  575  &.) 
d,  driving  clock ;  e,  time-recording  clock. 

•> 

long  tune,  while  the  earth  vibrates  beneath  them.  The  point  of  a  pencil  inserted 
in  the  pendulum  makes  the  record  upon  sheets  of  paper  which  are  moved  by 
clockwork  at  a  regular  rate.  These  records  are  called  seismograms,  and  two  or 
three  are  formed  for  each  earthquake,  according  to  the  construction.  They 
consist  of  a  series  of  wave-like  or  zigzag  lines  crossing  the  central  line  of  the 
paper,  and  their  amplitude  records  the  varying  magnitude  of  the  vibration. 


66o 


Movements  of  the  Earth's  Surface 


Thus  a  record  made  on  the  side  of  the  earth  opposite  that  of  the  epicentrum 
(Fig.  577  b)  will  record  first  a  series  of  minor  or  preliminary  tremblings  (V\,  V2), 


FIG.  575  b. — •  Part  of  record 
made  by  Ewing  seismograph. 
a,  east  and  west  motion ;  b,  north 
and  south  motion;  c,  up  and 
down  motion.  (After  Sek'iya 
from  Le  Conte.) 

FIG.  576  a.  —  Seismograph    recording 
two    horizontal    movements     (Spindler 

and  Hoyer,  Manuf.  Gottingen).  A  cylindrical  mass  or  pendulum  weighing 
from  80  to  200  kilograms  is  supported  on  a  vertical  pillar  about  i  meter  long, 
which  is  hinged  at  the  base  to  one  side  of  the  framework  so  as  to  permit  motion 
in  two  directions  at  right  angles.  From  the  center  of  gravity  of  the  pen- 
dulum mass  extend  two  horizontal  shanks,  oriented  respectively  in  a  north- 
south  and  east-west  position,  and  which  through  the  medium  of  two  levers 
operate  two  styles  or  markers.  These  rest  upon  a  cylinder,  covered  with  smoked 
paper  (see  Fig.  576  6),  which  revolves  on  its  horizontal  axis  under  the  influence  of 
a  weight.  During  an  earthquake  the  suspended  mass,  owing  to  its  inertia, 
remains  immobile  with  reference  to  the  vertical,  and  with  it  the  shanks  which 
bear  the  levers  and  styles,  while  the  framework  with  the  recording  cylinder 
moves  with  the  oscillations  of  the  earth.  The  levers  amplify  the  movements, 
which  are  recorded  upon  the  cylinder  in  two  sets  of  lines,  one  representing  the 
east-west  and  the  other  the  north-south  movement.  From  a  practical  view- 
point the  framework  and  recording  cylinder  remain  stationary,  while  the  ar- 
ticulated pendulum  with  the  shanks,  levers,  and  styles  sways  with  the  earth 
movements.  (After  Stanislas  Meunier.  Courtesy  of  Popular  Scie nee  Monthly.) 


which  appear  at  the  antiepicentrum  20  to  22  minutes  after  the  shock.     These 
are  followed  by  the  strong  vibrations  of  the  principal  shock  (B),  and  these,  in 


Sudden  Crustal  Movements — -Earthquakes      661 


turn,  are  followed  by  the  feeble  vibrations  of  the  dying  shock  (N).  The  pre- 
liminary tremors  are  recorded  only  at  points  more  than  1000  kilometers  from 
the  seat  of  disturbance,  and 
they  are  believed  to  have 
come  by  the  shortest  path 
through  the  earth,  or  in  gen- 
eral along  the  direction  of  the 
chord  between  the  seat  of 
disturbance  and  the  recording 
station  (Sloss-strahl,  Fig. 
574),  while  the  larger  waves 
travel  by  a  longer  route  over 
the  surface  (Oberflachenwellen, 
Fig.  574)- 


These  earthquake 
waves  represent  actual 
vibratory  movements  of 
the  rock  particles,  such 
movements  being  ex- 


FIG.  576  b.  —  Diagrammatic  view,  from 
above,  of  the  seismograph  shown  in  Fig.  5  76  a. 
(After  Stanislas  Meunier.  By  courtesy  of 


tremely  complex  and  in    Popular  Science  Monthly.} 

all  directions  (Fig.  578). 

The  distance  to  which  the  particles  vibrate  from  their  original 

point  of  rest,  or,  as  it  is  called,  the  amplitude  of  the  vibration, 


FIG.  577  a.  —  One  of  the  seismograms  of  the  Messina  earthquake  of  De- 
cember 28,  1908,  as  registered  at  Gottingen,  Germany,  by  the  seismograph 
illustrated  in  Figs.  576  a,  b.  (After  Stanislas  Meunier.  By  courtesy  of  Popular 
Science  Monthly.) 


662 


Movements  of  the  Earth's  Surface 


tt 


o     <U     «o 

I"  831- 


Great  Earthquakes  of  Modern  Times          663 

is  seldom  very  great,  though  the  surface  waves  produced  in  water 
or  unconsolidated  rock  material  or  soil  are  often  pronounced. 
Water  waves  40  feet  in  height  and  piling  up  on  the  coast  to  60 
feet  were  produced  by  the  Lisbon  earthquake  of  1755,  and  the 
loose  alluvial  material  of  the  Mississippi  Valley  bottom  was  thrown 
into  such  wave-like  commotions  during  the  New  Madrid  earth- 


FIG.  578.  —  Model  of  a  part  of  the  path  traveled  by  a  particle  on  the  earth's 
surface  during  the  earthquake  of  Tokyo,  January  15,  1887.  (After  Seikei 
Sekiya.)  This  model  is  constructed  by  the  combination  of  the  three  movements 
recorded  by  the  Ewing  type  of  seismograph. 

quake  of  1811  that  the  trees  bent  over  and  interlocked  with  their 
branches.  Here  the  actual  movement  of  the  particles  of  the  solid 
underlying  rock,  or  the  amplitude  of  vibration,  was  probably  not 
over  a  few  centimeters.  It  has  been  ascertained  that  the  ampli- 
tude of  the  vibration  of  rock  particles  in  earthquakes  of  sufficient 
violence  to  destroy  an  entire  city  is  often  not  greater  than  20  mil- 
limeters, or  about  three  fourths  of  an  inch,  while  amplitudes  of 
half  or  even  one  quarter  that  amount  are  productive  of  destruc- 
tive effects.  The  pronounced  effects  of  earthquakes  are  produced 
by  the  suddenness  of  the  shock  rather  than  by  the  amount  of 
motion  of  the  rock  particles. 

GREAT  EARTHQUAKES  OF  MODERN  TIMES 

In  order  that  the  student  may  gain  some  concrete  understanding 
of  earthquakes  and  the  phenomena  accompanying  them,  we  will 
briefly  review  some  of  the  more  violent  earthquakes  of  modern 
times,  after  which  the  main  characteristics  may  be  summarized. 


664  Movements  of  the  Earth's  Surface 

• 

The  Lisbon  Earthquake  of  1755 

One  of  the  most  destructive  earthquakes  which  has  visited 
Europe  in  historic  times  was  that  which,  on  November  i,  1755, 
devastated  the  city  of  Lisbon,  Portugal.  Without  previous  warn- 
ing, except  for  a  thunderous  underground  noise  immediately 
before  the  shock,  the  greater  part  of  the  city  was  laid  in  ruins,  and 
60,000  persons  perished  within  the  next  six  minutes.  The  sea 
retired  abruptly,  laying  bare  the  bar  off  the  coast,  and  then  rolled 
back  over  the  land  as  a  huge  series  of  waves,  rising  50  feet  or  more 
above  its  ordinary  level.  The  shock  was  marked  in  the  mountains 
of  Portugal,  which  seemed  to  rock,  while  fissures  opened  at  the 
summits  of  some  of  them,  rending  them  in  a  most  intricate  manner, 
huge  masses  being  precipitated  into  the  valleys  below.  Flames 
appeared  to  issue  from  these  clefts,  apparently  the  play  of  electrical 
phenomena,  and  clouds  of  dust  gave  the  appearance  of  smoke. 

A  new  quay  .built  entirely  of  marble,  upon  which  many  persons 
had  congregated  for  safety,  sank  suddenly,  carrying  the  hapless 
mortals  with  it,  and  of  their  bodies  not  one  is  said  to  have  floated 
again  to  the  surface.  Many  boats  and  small  vessels  anchored 
near  by,  and  filled  with  people,  were  swallowed  by  the  waters,  the 
depth  of  which  in  the  region  of  the  quay  became  100  fathoms  or 
more.  The  lower  part  of  the  city  and  the  quay  were  built  upon 
the  blue  clay  and  other  Tertiary  and  younger  strata  at  the  mouth 
of  the  Tagus  River,  and  it  was  the  structures  thus  located,  which 
suffered  chiefly,  not  a  building  upon  the  Mesozoic  limestones  or 
the  basalts  being  injured. 

The  shock  of  this  earthquake  affected  a  portion  of  the  earth's 
surface  estimated  to  have  been  four  times  greater  than  the  extent 
of  Europe.  It  was  felt  in  the- Alps,  on  the  coast  of  Sweden  and 
elsewhere  on  the  Baltic,  over  much  of  north  Germany,  in  Thuringia 
and  in  Great  Britain,  where  the  water  of  Loch  Lomond  in  Scotland 
rose  suddenly  over  two  feet  against  the  banks  and  then  subsided 
below  its  usual  level.  At  Kinsale  in  Ireland  a  body  of  water 
rushed  into  the  harbor,  whirling  around  vessels  there  stationed, 
and  pouring  into  the  market  place.  In  the  West  Indies,  where  the 
tide  is  usually  only  two  feet  high,  the  water  rose  suddenly  by  more 
than  20  feet,  appearing  discolored  and  of  an  inky  blackness.  Even 
in  the  region  of  the  Great  North  American  Lakes  the  shock  was 
felt.  It  was,  of  course,  most  violent  in  the  Mediterranean  region, 


Great  Earthquakes  of  Modern  Times          665 

the  agitation  being  as  marked  in  Algiers,  north  Africa,  and  in 
Morocco  as  in  Portugal  and  Spain.  Many  persons  (8000  to  10,000 
in  one  district)  were  said  to  have  been  swallowed  up  by  fissures 
which  opened  and  closed  again.  Even  at  a  distance  out  at  sea, 
the  shock  was  felt  on  the  decks  of  vessels,  being  so  violent  at  a 
point  40  leagues  west  of  St.  Vincent  that  the  men  were  said  to 
have  been  thrown  a  foot  and  a  half  perpendicularly  up  from  the 
deck.  A  great  sea  wave  or  tsunami,  originating  on  the  ocean  floor 
50  or  more  miles  off  the  coast  of  Lisbon,  and  estimated  to  have 
been  60  feet  high  at  Cadiz,  swept  the  coast  of  Spain.  It  was 
followed  by  others  of  decreasing  heights.  At  Tangier,  Africa,  the 
water  rose  and  fell  18  times ;  on  the  Madeira  coast  (at  Funchal) 
it  rose  fully  15  feet  above  high-water  mark,  the  tide  at  the  time 
being  at  half  ebb.  Several  of  the  coast  cities  were  flooded.  It 
took  the  sea-wave  2-|  hours  to  reach  the  Madeira  coast,  although 
the  shock  was  transmitted  through  the  earth  in  25  minutes.  This 
shock  caused  no  retreat  of  the  sea  there  on  account  of  the  steep- 
ness of  the  coast.  The  effects  of  those  waves  were  felt  even  on 
the  shores  of  the  West  Indian  Islands  across  the  whole  expanse 
of  the  Atlantic. 


The  Calabrian  Earthquake  of  1783-86,  and  the  Messina  Earthquake 

of  1908 

The  Calabrian  peninsula  of  Italy  (Calabria  Ultra)  (Fig.  579  a) 
was  the  scene  of  a  violent  series  of  earthquake  shocks,  which  began 
in  February,  1783,  and  lasted  for  nearly  four  years  —  to  the  end 
of  1786.  This  is  the  first  of  the  great  earthquakes  the  effects  of 
which  were  carefully  noted  by  men  of  scientific  training.  The 
convulsions  extended  not  only  over  the  whole  of  Calabria  Ultra, 
but  also  over  the  southeastern  part  of  Calabria  Citra  and  across 
the  sea  to  Messina,  affecting  an  area  of  about  500  square  miles. 
The  concussions  were  noted  over  a  great  part  of  Sicily  and  as  far 
north  as  Naples. 

The  formations  covering  the  area  chiefly  affected  consist  of 
thick  argillaceous  beds  with  marine  fossils  and  some  sands  and 
limestone,  all  of  Tertiary  or  younger  age  and  abutting  against  the 
central  Apennine  chain  of  granite  and  other  rock.  The  Calabrian 
plain  formed  by  these  Tertiary  and  younger  rocks  is  flat  and  level 
except  where  dissected  by  streams,  which  have  cut  gorges,  in  places 


666 


Movements  of  the  Earth's  Surface 


600  feet  deep,  and  with  steep,  sometimes  almost  perpendicular 
sides,  due  to  the  binding  together  of  the  upper  beds  by  roots  of 
trees,  etc. 

The  greatest  destruction  occurred  within  a  radius  of  22  miles  of 
the  city  of  Oppido  in  Calabria  Ultra.  The  first  shock,  which  oc- 
curred on  February  5,  1783,  "  .  .  .  threw  down  in  two  minutes 

the  greater  part  of  the 
houses  in  all  the  cities, 
towns,  and  villages,  from 
the  western  flanks  of  the 
Apennines  in  Calabria 
Ultra  to  ~  Messina  in 
Sicily,  and  convulsed  the 
whole  surface  of  the 
country "  (Lyell).  A 
second  shock  of  almost 
equal  violence  occurred 
on  March  28,  and  this 
rudely  shook  the  granite 
chain  which  passes 
through  Calabria  from 
north  to  south,  and  which 


FIG.  579  a.  —  Map  of  Calabria,  showing  the 
regions  chiefly  affected  by  the  earthquakes. 
(After  Lyell.) 


was  only  slightly  shaken 
by  the  first  shock.  Along 
the  flanks  of  this  chain 

the  soil  slid  downwards,  producing  a  chasm]  from  9  to  10  miles 
in  length  between  the  solid  granite  and  the  sandy  soil.  These 
landslides  were  sometimes  carried  for  considerable  distances  over 
the  lower  ground,  thus  causing  the  properties  of  different  indi- 
viduals to  become  superimposed  and  leading  to  subsequent  dis- 
putes of  ownership.  The  chasm  along  the  mountain  base  has 
been  explained  as  caused  by  the  reflection  and  refraction  of  the 
earthquake  wave  in  passing  from  a  body  of  low  elasticity,  such 
as  the  clay  and  gravel,  to  one  of  high  elasticity  like  the  granite,  a 
shock  being  thus  produced  in  the  opposite  direction. 

The  surface  of  the  plain  was  thrown  into  undulating  movements 
by  each  shock  to  such  an  extent  that  rooted  trees  are  said  to  have 
touched  the  ground  with  their  branches,  a  fact  attested  by  com- 
petent authority.  A  vorticose  movement  was  indicated  by  the 
behavior  of  the  large  stones  of  two  obelisks  at  the  convent  of 


Great  Earthquakes  of  Modern  Times          667 


St.  Bruno,  the  pedestals  of  which  remained  in  position  while  the 
separate  stones  were  partly  rotated  horizontally  (Fig.  579  b). 
Pavement  stones  in 
many  towns  were 
thrown  up  and  over- 
turned, and  a  round 
tower  at  Terranuova 
was  faulted  through 
the  center  (Fig. 
579  c).  Along  this 
fault-line  houses  on 
one  side  were  lifted 
above  those  of  the 

t>»  r          VrTi      cant  ^IG<  579^' —  Shifts  in  the  two  obelisks  in  the 

er>  convent  of  St.  Bruno,  Calabrian  earthquake  of 

with  the  ground,  and      x 7s3.    (After  Lyell.) 

walls  crossing  it  were 

faulted,  yet  with  sides  so  firmly  adhering  that  the  fault  was  only 
"marked  by  the  displacement  of  the  tiers  of  stone  on  opposite 
sides.  In  this  town,  too,  a  stone  well  was  apparently  driven  from 


FIG.  579  c.  —  Fault  in  the  round  tower  of  Terranuova  in  Calabria  occasioned 
by  the  earthquake  of  1783.     (After  Lyell.) 

the  ground,  resembling  a  small  tower  eight  or  nine  feet  in  height, 
and  a  little  inclined.  This  was  probably  effected  by  the  settling 
of  the  soil  around  the  stone  well.  In  Monteleone,  some  streets 


668  Movements  of  the  Earth's  Surface 

had  all  their  houses  but  one  thrown  down,  others  all  but  two, 
these  excepted  buildings  being  often  scarcely  injured.  In  many 
Calabrian  cities  all  the  more  solid  buildings  were  destroyed,  the 
lighter  ones  escaping,  but  the  reverse  was  true  at  Messina  and 
elsewhere. 

Rents  and  chasms  were  opened  and  closed  again  along  the  path 
of  the  earthquake,  engulfing  houses,  cattle,  and  human  beings, 
but  in  a  few  cases,  it  was  stated,  individuals  thus  engulfed  were 
thrown  out  again,  sometimes  still  alive,  by  an  immediately  follow- 
ing shock.  Radiating  fissures  were  formed  in  many  places  and 
the  surface  of  the  country  was  broken  by  cracks  resembling  those  of 
a  shattered  pane  of  glass. 

In  the  central  district  around  Oppido,  many  houses  were  com- 
pletely engulfed  in  the  opening  and  closing  fissures,  while  at  Canna- 
maria  ".  .  .  four  farm  houses,  several  oil-stores,  and  some  spacious 
dwelling  houses  were  so  completely  engulfed  in  one  chasm,  that 
not  a  vestige  of  them  was  afterwards  discernible  "  (Lyell).  Similar 
phenemona  occurred  elsewhere,  and  later  excavations  showed  that 
detached  parts  of  the  buildings  and  their  contents  were  so  firmly 
jammed  together  by  the  closing  of  the  fissure  that  they  formed 
one  compact  mass.  Many  fissures  closed  more  gradually,  several 
near  Mileto,  which  had  engulfed  an  ox  and  nearly  one  hundred  goats, 
being,  when  later  visited,  still  nearly  a  foot  in  width.  One  fissure, 
however,  on  a  hillside  near  Oppido,  which  had  swallowed  part 
of  a  vineyard  and  a  considerable  number  of  olive  trees  with  much 
soil,  remained  open  for  a  length  of  500  feet  and  a  depth  of  200  feet. 
Many  fissures  formed  by  the  shock  of  February  5  were  greatly 
widened,  lengthened,  and  deepened  by  the  shock  of  March  28, 
some  of  them  becoming  nearly  a  mile  in  length  by  from  150  to 
200  feet  .in  depth.  They  were  usually  straight,  but  sometimes 
crescent  form. 

But  the  most  remarkable  features  formed  were  numerous  small 
craterlets  or  funnel-shaped  sinks,  of  about  the  size  of  a  carriage 
wheel  or  larger,  which  covered  parts  of  the  plains  (Fig.  579  d).  These 
were  in  some  cases  filled  witji  dry  sand,  in  others  with  water  which 
arose  through  a  neck  or  tube  at  the  base.  Innumerable  cones  of 
sand  were  thrown  up  in  marshy  places  by  the  spouting  upward 
of  the  water  in  jets.  Rivers  were  dried  up  by  the  violent  shocks, 
but  immediately  afterward  overflowed  their  banks.  River  courses 
were  deranged  by  extensive  landslides,  and  215  lakes  and  small 


Great  Earthquakes  of  Modern  Times          669 

ponds  were  formed.  In  one  case,  a  river  valley  was  transformed 
by  a  landslide  into  a  lake  two  miles  long  and  one  mile  broad/  Oaks, 
olive  trees,  vineyards,  and  corn  slid  with  the  land  into  the  river- 
valley  at  Terranuova,  where  they  continued  to  grow  as  did  those  of 
the  portion  from  which  they  were  detached  at  least  500  feet  higher 
and  about  three  quarters  of  a  mile  distant.  Near  Seminara  an 
extensive  olive  orchard  was  hurled  to  a  distance  of  200  feet  into  a 
valley  60  feet  in  depth,  the  region  from  which  it  was  detached 
opening  in  a  deep  chasm  which  was  appropriated  by  the  river, 


FIG.   579  d.  —  Craterlets  formed  during  the  Calabrian   earthquake  of   1783. 

(From  C.  Vogt.) 

leaving  its  former  channel  completely  dry.  On  this  mass  of  earth 
stood  a  small  house,  which  was  carried  down  with  it  entire  and 
without  injury  to  its  inhabitants.  The  olive  trees  thus  transported 
bore  an  abundant  crop  of  fruit  the  same  year.  The  greater  part 
of  the  town  of  Polistena,  "  consisting  of  some  hundreds  of  houses, 
travelled  into  a  contiguous  ravine  and  nearly  across  it,  about  half 
a  mile  from  their  original  site,  "  and  several  of  the  inhabitants 
were  dug  out  of  the  ruins  alive  and  unhurt.  Near  Mileto,  two 
tenements,  "  occupying  an  extent  of  ground  about  one  mile  long 
and  half  a  mile  broad,  were  carried  for  a  mile  down  a  valley." 
Here  the  ground  had  been  long  undermined  by  rivulets.  A 
thatched  cottage  and  large  olive  and  mulberry  trees  were  carried 
uninjured  for  this  same  distance,  most  of  the  trees  remaining 
erect. 

In  many  places  mud-streams  were  formed  which  buried  houses 
and  trees  in  their  paths.  In  one  case  two  such  mud-streams, 
rolling  forward  like  streams  of  lava,  united  in  a  valley,  forming  a 


670  Movements  of  the  Earth's  Surface 

stream  225  feet  wide  and  15  feet  deep,  and  before  it  ceased  to  move 
it  covered  a  surface  one  Italian  mile  in  length.  "  In  its  progress 
it  overwhelmed  a  flock  of  30  goats  and  tore  up  by  the  roots  many 
olive  and  mulberry  trees,  which  floated  like  ships  upon  its  surface" 
(Lyell).  This  mud  was  highly  calcareous,  and  it  gradually  dried 
and  hardened,  the  mass  decreasing  7^  feet  in  thickness. 

Along  the  straits  of  Messina,  near  the  famous  rock  of  Scylla, 
huge  masses  of  rock  were  detached  from  the  lofty  cliff  and  over- 
whelmed many  gardens  and  villas.  "  At  Gian  Greco,  a  continuous 
line  of  cliffs,  for  a  mile  in  length,  was  thrown  down.  The  sea  was 
violently  agitated,  and  rising  more  than  20  feet  rushed  back  and 
forth  over  the  low  coast,  causing  great  destruction  of  life.  The 
aged  prince  of  Scylla,  and  1430  of  his  people,  who  had  taken  to  the 
fishing  vessels  for  safety,  perished,  all  the  boats  being  destroyed." 

The  total  number  of  persons  that  perished  by  this  earthquake 
in  the  two  Calabrias  and  Sicily  was  estimated  at  40,000,  and  about 
20,000  more  died  from  epidemics  caused  by  insufficient  nourishment 
and  exposure,  and  by  malaria  arising  from  the  new  stagnant  pools. 
Many  people  were  burned  to  death,  as  numerous  fires  resulted, 
and  many  were  swallowed  alive  by  the  fissures  which  opened  and 
closed  and  might  have  been  saved  if  help  had  been  at  hand.  But 
by  far  the  greater  number  perished  in  the  ruins  of  their  houses, 
while  on  the  coast  drowning  was  the  chief  cause  of  death. 

This  same  region,  along  the  straits  which  separates  Sicily  from 
the  mainland  of  Italy,  was  visited  by  the  most  destructive  earth- 
quake of  modern  times  on  December  28,  1908.  The  cities  of  Mes- 
sina and  Reggio  were  completely  destroyed,  and  so  were  many 
smaller  towns  and  villages.  The  whole  of  Calabria  and  of  eastern 
Sicily  was  affected  by  the  shock.  The  catastrophe  has  been  called 
the  most  appalling  of  its  kind  that  has  visited  any  country.  The 
number  of  persons  killed  was  approximately  78,000,  while  the 
number  of  injured  was  beyond  calculation. 

The  New  Madrid  (Missouri)  Earthquake  of  1811-12 

On  March  26,  1812,  violent  earthquake  shocks  destroyed  the 
city  of  Caracas,  Venezuela,  and  previously  to  this  and  continuing 
for  some  time  after,  earthquake  shocks  were  felt  in  South  Carolina 
and  in  the  valley  of  the  Mississippi  from  New  Madrid  to  the 
mouth  of  the  Ohio  in  one  direction,  and  to  St.  Francis,  Arkansas, 


Great  Earthquakes  of  Modern  Times          671 

in  the  other.  At  New  Madrid,  subterranean  rumblings  had 
been  heard  frequently  for  many  years  before  and  up  to  within  a 
year  of  the  earthquake.  The  first  great  shock  came  about  2 
o'clock  in  the  night  of  December  16,  1811,  accompanied  by  a  noise 
like  thunder,  and  in  a  few  minutes  the  air  was  saturated  with  sul- 
phurous vapors.  Between  December  16,  1811,  and  March  16, 
1812,  a  total  of  1874  shocks  was  recorded,  eight  of  them  being  of 
the  first  order  of  intensity.  The  most  violent  one  occurred  on 
February  7  and  was  accompanied  by  sulphurous  vapors  and 
unusual  darkness. 

The  region  affected  is  a  part  of  the  flood  plain  of  the  Mississippi 
and  is  underlain  by  unconsolidated  sands  and  muds.  The  quaking 
of  the  ground  continued  for  several  successive  months,  and  great 
changes  in  the  surface  were  produced.  Large  lakes  were  formed, 
sometimes  in  the  course  of  an  hour ;  others  already  in  existence 
were  drained.  The  most  noted  of  these  newly  formed  water  bodies 
is  Reelfoot  Lake,  in  Obion  County,  northwest  Tennessee,  which 
has  a  length  of  more  than  20  miles  and  a  width  of  7  miles,  while 
the  water  in  places  covers  ihe  tops  of  submerged  cypress  trees 
which  grew  on  the  ground  before  it  settled.  Near  Little  Prairie, 
a  lake  many  miles  in  length  but  only  from  3  to  4  feet  in  depth  came 
into  existence.  Later  it  disappeared,  leaving  behind  a  stratum  of 
sand.  Lake  Eulalie,  near  New  Madrid,  300  yards  long  and  100 
yards  wide,  was  suddenly  drained  through  parallel  fissures  which 
opened  in  the  bottom  and  which  were  not  yet  closed  when  Lyell 
visited  the  region  34  years  later.  The  ancient  bed  of  the  lake  is 
now  largely  overgrown  with  forest  trees. 

During  the  first  shock,  the  current  of  the  Mississippi  River 
was  reversed  in  direction  north  of  New  Madrid,  continuing  so  for 
several  minutes.  During  later  shocks,  mountainous  waves  were 
generated  in  the  river,  which  receded  from  its  banks,  leaving  boats 
high  upon  the  sand,  and  then  moving  forward  as  a  wall  of  water 
15  to  20  feet  high,  tore  them  from  their  moorings  and  swept  them 
into  a  creek  as  a  close-packed  mass,  a  quarter  of  a  mile  long.  The 
graveyard  at  New  Madrid  was  precipitated  into  the  bed  of  the 
Mississippi ;  and  the  ground  on  which  the  town  is  built,  and  the 
river  bank  for  15  miles  above  it,  are  said  to  have  sunk  eight  feet 
below  their  former  level.  The  trees  of  the  region  suffered  break- 
ing, and  many  were  inclined  in  all  directions,  continuing  to  grow 
thus.  Many  fissures  were  opened,  usually  in  a  northeast-southwest 


672  Movements  of  the  Earth's  Surface 

direction,  and  much  water,  sand,  and  lignite  was  discharged  often 
with  great  force.  Hundreds  of  these  chasms  were  still  visfble  in  the 
aUuvial  so,  seven  years  after  the  event,  and  Lyell  noted  many 
of  them  only  partly  filled  35  years  after  their  formation 

Numerous  circular  craterlets  or  sinkholes,  from  10  to  30  yards 
mde  and  20  feet  in  depth,  appeared.    These  were  located  on  the 
border  of  the     Sunk  Country,"  west  of  New  Madrid,  which  extends 
along  the  course  of  the  White  Water  and  its  tributaries  for  a  dis- 
ance  of  between  70  and  80  miles  north  and  south  and  30  miles 
east  and  west.     In   this   area   innumerable   dead  trees  are  sub- 
merged, some  erect,  but  many  prostrate.     The  borders  of  the 
submerged  area  are  gradually  filling  up  by  growth  of  swamp  vege- 
tation and  sediments  washed  into  it.     East  of  this  area  a  low 
ae,  20  miles  in  diameter  and  rising  20  or  25  feet  above  the 
alluvial  plain,  was  formed. 

In  this  case  also  the  trees  are  said  to  have  bent  down  during  the 
shocks   many  of  them  becoming  interlocked  with  the  branches  of 
trees  similarly  bent,  and  being  thus  prevented  from  righting 
themselves  again.     One  result  of  the  disturbance  due  to  this  earth- 
quake was  the  great  confusion  into   which   boundary  lines  were 
rown,  so  that  the  government  found  it  necessary  to  resurvey 
an  area  of  1,000,000  acres. 

The  Chilean  Earthquakes  of  1822  and  Later  Periods 
Chile  has  been  visited  by  many  earthquakes,  of  which  those 
22   1835,  and  1906  are  especially  noteworthy.     The  coastal 
ion  (Fig   580)  was  visited  by  a  particularly  destructive  earth- 
quake on  November  19,  l8«,  which  was  felt  simultaneously  for 
Distance  of  I2oo  miles  north  and  south.     Much  damage  was 
done  m  Valparaiso,  Santiago,  and  other  places,  and  the  coast  near 
Valparaiso  was  raised  from  three  to  four  feet  above  its  former  level 
exposing  beds  of  oysters,  while  mussels  and  other  Mollusca  and 
saweeds  adhering  to  the  rocks  were  left  high  and  dry     Vast 
quantities  of  fish  were  killed  and  their  bodies  left  to  decay  over  - 
the  raised  ground.     The  slopes  of  streams  back  of  the  coast  were 
increased,  one  of  them  gaining  a  fall  of  14  inches  in  little  more  than 
This  indicates  a  greater  inland  rise,  which  was  esti- 
ated  to  be  from  five  to  six  or  even  seven  feet  at  a  distance  of  a 
mile  from  the  coast.     Parallel  fissures  opened  in  the  granitic  rocks 
thus  elevated,  some  of  these  being  traceable  for  a  mile  and  a  half 


Great  Earthquakes  of  Modern  Times          673 


inland.     As  in  the  Calabrian  earthquake,  cones  of  earth  up  to  four 

feet   in  height  were  built  up  on  alluvial  soil  by  water,  mixed 

with  sand,  being  forced 

through      funnel-shaped 

openings.      The    houses 

built  on  alluvial  soil  were 

more  damaged  than 

those    built    upon     the 

granite. 

The  total  area  elevated 
during  this  earthquake 
is  estimated  at  100,000 
square  miles,  an  area 
equal  to  half  that  of 
France.  Accepting  this 
estimate,  Lyell  figured 
that  the  entire  mass  of 
land  raised  above  sea- 
level  was  57  cubic  miles 
in  bulk,  and  of  a  weight 
100,000  times  that  of 
the  great  Pyramid ;  and 
if  a  moderate  estimate 
of  the  weight  of  the  entire 
mass  displaced  is  made, 
assuming  the  depth  af- 
fected to  be  two  miles, 
it  would  be  3630  times 
that  much.  Lyell  has 
further  estimated  that 
the  amount  of  solid  mat- 
ter thus  raised  above 
the  sea-level  by  this 
single  earthquake  is  equal  to  that  which  the  Ganges  River  would 
carry  into  the  sea  during  a  period  of  four  centuries. 

The  shocks  continued  at  intervals  of  24  to  48  hours  for  nearly 
a  year,  or  until  the  end  of  September,  1823.  Twelve  years  later, 
on  February  20,  1835,  another  great  earthquake  visited  this  coast, 
being  felt  for  nearly  a  thousand  miles  from  north  to  south  between 
Copiapo,  400  miles  north  of  Valparaiso,  and  Chiloe,  and  for  about 


FIG.  580.  —  Map  of  a  part  of  Chile,  showing 
the  regions  principally  affected  by  earth- 
quakes. (After  Lyell.) 


674  Movements  of  the  Earth's  Surface 

500  miles  east  and  west  from  Mendoza  to  the  island  of  Juan 
Fernandez,  365  miles  from  Chile.  Its  effects  were  particularly 
noted  in  Concepcion  Bay,  where  the  sea  retreated,  stranding  vessels 
which  lay  in  seven  fathoms  of  water,  and  then  rushed  in  again  with 
several  repetitions,  the  waves  being  thrown  1 6  to  20  feet  in  height 
and  rushing  far  up  on  the  shelving  beach.  Large  numbers  of 
cattle  were  washed  into  the  sea  by  these  waves,  and  others  stand- 
ing on  a  steep  slope  near  the  shore  were  rolled  into  the  sea  by  the 
shock. 

Upward  of  a  hundred  villages  were  destroyed  on  the  coast  and 
the  islands  of  the  bay,  and  rocks  broken  off  beneath  the  sea  were 
cast  high  upon  the  shore.  Darwin  found  one  of  them  to  have  a 
length  of  six  feet,  a  breadth  of  three,  and  a  thickness  of  two  feet. 
On  Quiriquina  Island,  Darwin  found  many  fissures  extending  in 
a  north-south  direction,  some  of  them  a  yard  wide.  The  hard, 
slaty  rock  of  the  island  was  shivered  superficially  as  if  blasted  by 
gunpowder,  and  huge  blocks  were  precipitated  to  the  beach, 
others  being  loosened  and  left  in  a  position  where  the  heavy  rains 
would  tend  to  their  further  displacement.  Darwin  believed  that 
this  single  convulsion  "  has  been  more  effectual  in  lessening  the 
size  of  the  island  of  Quiriquina  than  the  ordinary  wear  and  tear  of 
the  sea  and  weather  during  the  course  of  a  whole  century." 

The  island  of  Santa  Maria,  about  25  miles  southwest  of  Con- 
cepcion, and  about  seven  miles  long  by  two  broad,  was  raised  ten 
feet  at  the  northern  and  eight  feet  at  the  southern  end,  as  shown  by 
the  mussels  which  were  found  clinging  to  the  steep  faces  of  the  rock. 
A  large  flat  at  the  northern  end  of  the  island,  formerly  submerged, 
became  permanently  exposed,  causing  the  extermination  of  great 
beds  of  shellfish,  while  the  water  all  around  the  island  was  dimin- 
ished in  depth  by  a  fathom  and  a  half. 

The  great  sea  waves  or  tsunamis  which  originated  from  this 
earthquake  "  traversed  the  ocean  to  the  Society  and  Navigator 
Islands,  3,000  and  4,000  miles  distant,  and  to  the  Hawaiian  Islands, 
6,000  miles  away  "  (Dana).  The  velocity  of  such  waves  is  very 
great,  one  originating  in  the  earthquake  of  1868  running  to  the 
Hawaiian  Islands  at  a  rate  of  465  miles  per  hour. 

During  the  earthquake  of  1835  and  for  some  time  preceding  and 
following  it,  the  whole  volcanic  chain  of  the  Chilean  Andes,  1300 
miles  in  length,  was  in  a  state  of  unusual  activity.  Lava  flowed 
from  the  crater  of  Osorno  at  the  southern  end,  and  a  submarine 


Great  Earthquakes  of  Modern  Times          675 

volcano  broke  out  about  a  mile  from  the  island  of  Juan  Fernandez 
(365  geographical  miles  from  Chile),  this  island  being  violently 
shaken  and  devastated  by  a  great  wave. 

Another  earthquake  shock  occurred  at  Concepcion  in  November 
of  the  same  year,  and  at  the  same  time  the  volcano  Osorno,  400 
miles  distant,  renewed  its  activity. 

Two  years  later,  on  November  7,  1837,  Valdivia,  situated  near 
the  coast  about  300  miles  south  of  Concepcion,  was  destroyed 
by  a  violent  earthquake,  and  the  sea-bottom  near  the  island  of 
Lemus  in  the  Chonos  Archipelago  was  raised  more  than  eight  feet. 

In  August,  1906,  the  coast  of  Chile  was  subjected  to  another 
severe  earthquake  which  did  much  damage  in  Valparaiso  and 
other  places,  killing  several  thousand  persons.  After-shocks 
continued  for  a  long  time,  while  the  readjustment  along  the  fault 
lines  took  place. 

The  New  Zealand  Earthquake  of  1855 

New  Zealand  has  been  visited  by  many  earthquakes,  which 
have  caused  profound  alteration  of  its  surface  and  coast  line.  One 
of  the  most  marked  of  them  occurred  on  the  night  of  January  23, 
1855,  being  most  violent  in  the  narrowest  parts  of  Cook  Strait, 
between  the  two  main  islands  (Fig.  581),  and  affecting  an  area  of 
land  and  water  estimated  at  860,000  square  miles  —  three  times  the 
size  of  the  British  Islands.  Near  Wellington,  in  the  North  Island, 
a  tract  of  land  comprising  4600  square  miles  was  permanently 
upraised  from  one  to  nine  feet.  The  uplift  was  especially  marked 
along  the  eastern  flank  of  the  Rimutaka  Mountains,  a  range  running 
northeast  from  Cook  Strait,  and  rising  to  heights  of  4000  feet  above 
the  sea.  A  fault  scarp  came  into  existence  along  the  eastern  face 
of  the  range,  and  where  shown  at  the  Muka  M.uka  cliff,  on  the 
coast,  12  miles  southeast  of  Wellington,  it  was  found  that  the 
older  rocks  of  the  mountains  had  experienced  an  elevation  of  nine 
feet,  while  the  Tertiary  rocks  on  the  east  remained  undisturbed 
(Fig.  582).  The  elevation  of  the  older  rock  on  the  west  of  the 
fault  was  clearly  marked  by  a  line  of  nullipores  or  calcareous 
seaweeds,  originally  at  sea-level,  but  immediately  after  the  shock 
nine  feet  above  it.  A  beach  100  feet  wide  also  came  into  existence 
at  the  foot  of  this  cliff,  where  formerly  the  water  had  washed  it 
closely  at  high  tide. 


Great  Earthquakes  of  Modern  Times          677 

The  fault  scarp  extends  continuously  into  the  interior  of  the 
country  along  the  base  of  the  Rimutaka  Mountains,  being  marked 
by  nearly  perpendicular  fresh  cliffs  nine  feet  in  height,  and  traceable 
for  a  distance  of  about  90  miles.  In  many  places  the  fault  line 
was  marked  by  an  open 
fissure  from  six  to  nine  feet 
broad  and  filled  locally  with 
soft  mud  and  loose  earth. 

The  effects  of  the  eleva- 
tion were  also  seen  at  Port 
Nicholson,  about  12  miles  FIG.  582. —  Section  of  Muka  Muka 

west  of  Muka  Muka  cliff,  the     cliff>  Cook  Strait>  New  Zealand,  showing 

,,         r    •       r      c  recent  fault.     A,  argillite:    B.  Tertiary 

elevation  there  being  five  feet     beds .  c^  Hne  of  fissure  and  fauk     (Affa£ 

on  the  eastern  and  four  feet     Lyell.) 
on   the  western  side  of  the 

harbor.  We  have  here  an  interesting  example  of  block  faulting 
or  the  tilting  of  a  great  block  of  the  earth's  crust  by  differential 
elevation.  It  is  also  significant  that  although  the  fault  plane 
was  apparently  vertical,  the  strata  on  the  east  remained  undis- 
turbed and  without  change  in  position,  while  the  block  was 
actually  raised  along  the  fault  plane.  Thus  the  fault  is  really 
an  upward  thrust  although  it  has  the  appearance  of  a  gravity 
fault. 

Owen's  Valley,  California,  Earthquake  of  1872 

Owen's  Valley  lies  in  eastern  California  near  the  eastern  base  of 
the  Sierra  Nevada  Mountains.  The  earthquake  occurred  on  March 
26,  1872,  and  the  ground  sank  in  strips  producing  several  fault 
scarps,  the  principal  one  of  which  followed  the  base  of  the 
mountains,  and  in  places  rose  to  20  feet  and  extended  for  about 
40  miles.  Opposite  the  highest  point  a  second  scarp  appeared  10 
feet  high  and  facing  in  the  opposite  direction.  Other  parallel 
faults  were  formed  and  an  area  of  several  thousand  acres  of  land 
was  not  only  lowered  bodily,  but  also  shifted  northward  for  about 
15  feet.  At  Big  Pine,  many  extensive  fissures  were  opened,  trace- 
able for  several  miles,  while  an  area  of  ground  200  to  300  feet  wide 
sank  in  places  to  a  depth  of  20  feet  or  more,  leaving  vertical  fault 
scarps  on  opposite  sides.  This  depression  was  filled  with  water, 
forming  a  pond  one  third  of  a  mile  in  length.  A  road  crossed  by 


678 


Movements  of  the  Earth's  Surface 


a  north-south  fissure  had  the  part  to  the  west  of  the  fissure  shifted 
1 8  feet  to  the  south. 

The  earthquake  consisted  of  only  one  violent  shock,  and  all 
the  changes  were  produced  within  a  few  seconds.  After-shocks 
and  slight  tremors,  however,  continued  for  two  months  longer. 


The  Charleston  Earthquake  of  1886 

On  August  31,  1886,  a  violent  shock  visited  the  city  of  Charleston, 
S.  C.,  at  9:51  P.M.,  there  having  been  two  light  premonitory 
shocks  on  the  28th  and  2;th  preceding.  About  14,000  chimneys 

were  thrown  down,  and 
in  some  streets  walls  and 
roofs  of  buildings  col- 
lapsed. The  shocks 
lasted  a  little  more 
than  half  a  minute,  the 
greatest  destruction  be- 
ing accomplished  in  the 
first  twenty  seconds. 

Numerous  craterlets 
of  the  type  formed  in 
Calabria  and  the  New 
Madrid  region  were 
opened  on  the  flat 
country,  some  of  them, 


FIG.  583.  —  A  small  craterlet  or  funnel- 
shaped  depression  formed  during  the  Charleston 
earthquake.  The  umbrella  indicates  the  size. 


which  were  aligned  along 
a  fissure,  being  twenty 

feet  in  diameter.     Water,  mud,  and  sand  gushed  from  these,  in 
some  cases  to  a  height  of  twenty  feet  (Fig.  583). 

Three  railroads  entering  Charleston  from  different  directions 
had  their  rails  twisted,  bent,  and  wrenched,  especially  where  the 
fissures  crossed  the  tracks.  The  number  of  killed  and  wounded 
was  slight  compared  with  other  earthquakes.  The  shock  of  this 
disturbance  was  distinctly  felt  in  Chicago,  800  miles  away 
(Fig.  584). 

The  Sonora  Earthquake  of  1887 

On  May  3,  1887,  a  violent  earthquake  affected  more  than  one 
half  of  Mexico  and  two  thirds  of  New  Mexico  and  Arizona,  but 


Great  Earthquakes  of  Modern  Times          679 


owing  to  the  sparsely  settled  condition  of  the  district,  little  damage 
was  done  to  life  and  property.  The  region  most  affected  was  the 
old  province  of  Sonora  on  the  northern  border  of  old  Mexico. 
"  Here  a  range  of  mountains,  the  Sierra  Teras,  was  uplifted  between 
faults  which  opened  on 
either  side"  (Hobbs). 
The  displacement  varied 
from  zero  to  20  feet.  In 
places  along  the  western 
fault,  the  displacement 
was  in  the  opposite  di- 
rection. 

"  Millions  of  cubic  feet 
of  rock  were  thrown  down 
from  the  slopes  into  the 
canyons  and  water 
courses,  and  cliffs  of  com- 
pact rock  were  shattered 
and  split  as  though  by  a 
charge  of  giant  powder  " 
(Hobbs).  Hundreds  of 
small  fissures,  accom- 
panied frequently  by  ver- 
tical displacements  of 
•one  or  two  feet,  opened 
in  the  flat  country  (Fig. 
585),  some  of  these  dis- 
charging water,  and  nu- 
merous craterlets  two 
feet  or  more  in  diameter 
opened  along  the  fissure 
lines  and  gushed  forth 
water  and  sand.  All  the 

water  courses  of  the  San  Bernardino  Valley  experienced  a  change 
in  level  of  from  six  inches  to  two  feet.  While  much  water 
gushed  up  and  the  streams  became  swollen  during  the  shocks, 
the  water  dried  away  immediately  after,  the  springs  also  going 
dry. 

Numerous  forest  fires  were  started,  probably  by  friction  or  by 
sparks  struck  from  flint  rocks  during  the  land-slips. 


FIG.  584.  —  Map  of  the  Charleston  earth- 
quake, showing  the  isoseismic  curves  formed 
by  connecting  the  points  in  which  the  shocks 
were  of  equal  magnitude.  The  order  of 
shocks  is:  i,  microseismic ;  2,  extremely 
feeble;  3,  very  feeble;  4,  feeble;  5,  of 
moderate  intensity ;  6,  fairly  strong ;  7,  strong; 
8,  very  strong ;  9,  extremely  strong ;  10,  shock 
of  extreme  intensity. 


68o 


Movements  of  the  Earth's  Surface 


Japanese  Earthquakes  of  1891  and  1896 

Japan  is  probably  one  of  the  most  unstable  countries  of  the 
earth.  During  a  period  of  nearly  1500  years,  225  destructive 
earthquakes  have  been  recorded,  and  the  careful  records  kept 
since  the  beginning  of  the  seventeenth  century  show  that  a  destruc- 
tive earthquake 
has  occurred  some- 
where in  Japan 
about  once  in  every 
two  and  a  half 
years. 

On  October  28, 
1891,  occurred  the 
greatest  shock  so 
far  recorded  in 
Japan.  It  affected 
an  area  of  243,000 
square  miles,  or 
more  than  three 
fifths  of  the  whole 


FIG.  585.  —  Small  fissure  and  fault  in  the  Arizona 
desert,  formed  during  the  Sonora  earthquake  of 
1887.  (After  Branner.) 


of  Japan.  The  most  violent  manifestations  were,  however,  largely 
confined  to  the  provinces  of  Mino  and  Owari,  which  form  a  kettle- 
shaped  basin  covered  by  broad  rice  fields  and  surrounded  by  high 
mountains  except  on  the  south. 

This  region  had  been  undisturbed  for  a  long  time,  and  the  great 
shock  came  with  unforeseen  suddenness.  Within  a  few  moments 
20,000  buildings  were  destroyed,  7000  persons  were  killed,  and  1 7,000 
more  injured,  many  perishing  by  the  fires  which  blazed  up  every- 
where. The  center  of  the  district  was  fissured  in  an  extraordinary 
way,  and  an  open  fissure  a  mile  in  length  appeared  on  the  banks  of 
the  Shonai  River,  where  a  bamboo  grove  "  with  pines  and  thatched 
houses  was  shifted  en  bloc  60  feet  back  from  the  river  embankment, 
the  trees  remaining  upright  and  the  thatched  roofs  falling  to  the 
ground  without  fracture  "  (Hobbs).  Small  mud- volcanoes  and 
sand  craters  arose  in  numbers  all  over  the  plain. 

This  earthquake  was  the  accompaniment  of  a  great  fault  which 
transected  the  valley  in  a  general  north-northwest  and  south- 
southeast  direction.  The  part  on  the  east  of  this  line  was  lowered 
with  reference  to  the  other  side  and  also  was  shifted  along  the 


Great  Earthquakes  of  Modern  Times  68 1 

fault  to  the  northwestward  from  three  to  six  feet.  In  general,  the 
vertical  displacement  was  less  than  two  feet,  and  there  was  no 
fault  scarp,  but  a  ridging  of  the  ground  resembling  a  gigantic  mole 
track.  In  some  places,  however,  as  at  Midori,  there  was  a  vertical 
displacement  of  not  less  than  18  feet,  but  in  the  opposite  direction, 
with  the  formation  of  a  great  fault  scarp.  The  lateral  displace- 
ment in  this  case  was  in  the  same  direction  as  elsewhere,  and 
amounted  to  about  12  feet  (Fig.  586). 


FIG.  586.  —  View  of  the  great  fault  produced  by  the  earthquake  of  1891  at 
Midori,  in  the  Neo  Valley,  Japan.     (After  B.  Koto.) 

Many  smaller  displacements  occurred  along  the  rifts  opened 
around  Omori,  some  blocks  rising,  others  sinking,  as  in  the  cases 
of  the  Owen's  Valley  earthquake  of  1872.  At  one  place  a  reservoir 
was  cut  in  two  by  the  fault,  the  northern  half  being  depressed  and 
shifted  and  the  southern  half  drained.  Two  village  sites  were 
transformed  by  depression  into  a  deep  swamp  two  square  kilo- 
meters in  area. 

During  the  five  months  succeeding  this  shock,  no  less  than 
2588  after-shocks  were  recorded,  as  many  as  318  occurring  on  the 
day  after  (102  on  the  first  day),  gradually  diminishing  in  number 
and  intensity  thereafter. 


682 


Movements  of  the  Earth's  Surface 


On  August  31,  1896,  a  heavy  earthquake  affected  the  province 
of  Northern  Honshiu,  killing,  however,  only  about  1000  persons, 
since  preliminary  shocks  from  six  to  eight  hours  before  had  given 

the  warning.     A  great  mag- 
**"  3°*°      netic    disturbance    occurred 

33  hours  before  the  great 
shock.  A  mountain  range 
forms  the  backbone  of  Japan 
in  this  province  (Fig.  587), 
and  on  opposite  sides  of  this 
two  great  clefts,  the  Senya 
and  Kawafune  clefts,  opened 
with  the  great  shock.  The 
latter  cleft  was  accompanied 
by  a  vertical  displacement 
of  six  feet.  Where  it  crossed 
a  crooked  road  near  the  vil- 
lage of  Kawafune,  it  cut  out, 
raised,  and  laterally  displaced 
a  section  of  it.  A  small 
house  on  the  fault  line  was 

stood  upon  its  head  without  further  injury.     The  fault  was  traced 
for  15  kilometers,  though  not  always  showing  displacement. 

The  Senya  cleft  or  fault  on  the  opposite  side  of  the  mountain 
range  was  traced  with  some  interruption  for  60  kilometers,  and  it 
had  a  maximum  vertical  displacement  of  10  feet.  It  appears  to 
be  the  continuation  of  an  older  cleft  farther  to  the  southwest  at 
Sakata,  formed  by  the  earthquake  of  1894. 

Icelandic  Earthquake  of  1896 

A  series  of  destructive  earthquake  shocks  affected  southwestern 
Iceland  on  August  26  and  27  and  September  5,  6,  and  10,  1896. 
The  region  affected  is  a  triangular  plateau  bordered  by  lofty  moun- 
tains, which  include  Mount  Hecla  and  other  volcanoes.  These, 
however,  were  quiescent  before,  during,  and  after  the  earthquake. 

Each  of  the  five  great  shocks  affected  a  different  area  or  block 
of  the  region,  though  these  were  all  contiguous.  The  blocks  thus 
affected  were  outlined  by  great  fissures  upon  the  surface,  and  the 
succession  of  movements  passed,  in  general,  from  the  east  west- 
ward. 


FIG.  587.  —  Map  of  part  of  the  prov- 
ince of  Northern  Honshiu,  Japan,  show- 
ing the  area  affected  by  the  earthquake 
of  August,  1896.  (After  Yamasaki.) 


Great  Earthquakes  of  Modern  Times  683 

The  surfaces  of  the  blocks  were  agitated  so  violently  that  neither 
men  nor  cattle  could  stand.  Persons  lying  near  a  cliff  were  thrown 
over  it,  and  in  one  village  a  heavy  stove,  six  feet  in  height,  was 
thrown  a  distance  of  twenty-five  feet.  Many  fissures  were  opened, 
one  of  them  extending  for  seven  and  another  for  nine  miles  in  length. 
The  mountains  around  the  plain  were  fissured,  and  land-slips 
occurred  on  the  steep  bordering  slopes.  Many  funnel-shaped 
openings  were  formed,  and  swamps  and  ponds  in  some  cases  were 
drained  by  them.  A  new  hot  spring  arose  after  the  shock  of  Sep- 
tember 5,  throwing  water  streams  and  volcanic  rock  to  an  esti- 
mated height  of  600  feet.  After  a  few  hours,  however,  it  rose  only 
10  or  12  feet  in  the  air,  and  ten  days  later  had  entirely  ceased  to 
flow.  The  geysers  of  the  region  were  also  affected,  one  of  them, 
the  Strokr,  which  had  come  into  existence  during  an  earlier  earth- 
quake (1789),  becoming  suddenly  extinct  and  remaining  inactive 
since.  (See  p.  186.) 

Assam  (India)    Earthquake  of  1897 

On  June  12,  1897,  a  violent  earthquake  shook  the  district  about 
Assam,  India,  laying  in  ruins  a  region  150,000  square  miles  in  area 
within  the  period  of  fifteen  seconds.  The  heavy  shocks  were  all 
over  in  less  than  two  and  one  half  minutes,  having  shaken  an  area 
of  1,750,000  square  miles. 

A  rumbling  sound  like  thunder  preceded  the  shocks  by  a  second 
of  time,  increasing  to  intensity  so  as  to  drown  the  noise  of  falling 
heavy  masonry  near  by.  The  ground  rocked  in  waves  "  as  though 
composed  of  soft  jelly."  Many  monuments  were  twisted,  and 
posts  and  houses  were  driven  into  the  sandy  ground,  only  the  roofs 
of  the  houses  remaining  visible. 

Numerous  fissures,  generally  parallel  to  the  mountain  ranges, 
were  opened,  and  some  closed  again  as  if  under  great  pressure. 
Craterlets  six  or  more  feet  in  diameter  appeared,  and  sand  and 
water  were  thrown  from  these  to  heights  of  seven  or  eight  feet  or 
more.  With  the  sand  were  fragments  of  peat,  lignite,  resin,  half 
petrified  pieces  of  timber,  and  a  black  earth  not  previously  known 
from  that  district.  Similar  material  issued  from  the  cracks  and 
was  spread  by  the  water  over  the  surrounding  country.  The 
streams  were  temporarily  swollen,  the  Brahmaputra  advancing  as 
a  wall  of  water  10  feet  in  height. 

The  largest  of  the  many  displacements  formed  at  this  time  was 


684  Movements  of  the  Earth's  Surface 

the  great  Chedrang  fault,  which  crossed  and  recrossed  a  meandering 
stream,  closely  following  its  general  course  for  a  distance  of  about 
12  miles.  The  maximum  displacement  was  33  feet,  but  generally 
less.  In  some  cases  the  fault  was  distributed  over  a  series  of  parallel 
rifts,  with  shiftings  of  as  much  as  several  inches  along  these.  In 
cutting  the  stream  channel,  the  displacement  caused  the  produc- 
tion of  many  ponds,  lakes,  and  waterfalls.  Readjustment  in  the 
hills  resulted  in  many  local  changes  in  level,  in  amounts  up  to 
twelve  feet,  and  changes  in  location  of  similar  amounts  as  well. 

After-shocks  of  lesser  intensity  followed  the  initial  destructive 
shocks  and  continued  for  more  than  a  week,  but  gradually  faded 
away. 

The    Yakutat  Bay  (Alaska)  Earthquake  of  1899 

A  heavy  earthquake  occurred  in  southern  Alaska  in  September, 
•1899,  but  little  was  known  of  it  until  the  region  was  explored  in 
detail  in  1905.  The  country  was  broken  into  blocks,  some  being 
elevated,  others  depressed,  the  extent  of  such  changes  being  fre- 
quently measurable  on  the  fjords  and  other  parts  of  the  coast. 
In  general,  the  changes  of  level  ranged  from  5  to  12  feet,  but  ex- 
treme changes  up  to  30  and  even  47  feet  were  noted.  Large 
blocks  were  broken  into  smaller  ones,  and  these  underwent  indi- 
vidual adjustments. 

New  reefs  and  islands  appeared  as  the  result  of  this  block  adjust- 
ment, one  of  these  being  450  feet  long,  about  75  feet  broad,  and 
apparently  rising  from  deep  water.  In  some  places  the  sea  has 
encroached  on  sunken  forest  lands,  killing  the  trees;  in  others, 
raised  beaches  indicate  elevation,  several  periods  of  elevation 
sometimes  being  shown. 

The  great  wave  or  tsunami  which  accompanied  this  earthquake 
devastated  a  forest  40  feet  above  the  level  of  the  bay,  the  twisted 
and  fallen  trunks  now  lying  in  utter  confusion.  Great  changes 
were  also  produced  in  the  glaciers  of  the  region. 

California  Earthquake  of  1906 

The  great  earthquake  which  partly  destroyed  San  Francisco 
on  April  18,  1906,  is  still  fresh  in  the  minds  of  the  present  genera- 
tion. The  heavy  shocks  came  without  warning  at  5.12  A.M. 
They  continued  for  about  a  minute  and  then  graded  off  into  lighter 
shocks,  which  were  repeated  during  a  period  of  several  days.  The 


Great  Earthquakes  of  Modern  Times          685 


FIG.  588.  —  Map  of  the  fault  trace  of  the  San  Francisco  earthquake  of  April 
18,  1906.  Broken  lines  indicate  alternative  hypothesis  as  to  its  extension 
north  of  Point  Arena.  (U.  S.  G.  S.) 


686 


Movements  of  the  Earth's  Surface 


shocks  accompanied  movement  along  the  Great  Fault  Line  which 
runs,  in  general,  parallel  to  the  coast  and  is  traced  from  Punta 
Arena  on  the  north  to  the  vicinity  of  Mount  Pirnos  in  Ventura 
county  on  the  south,  a  distance  of  about  400  miles,  and  with  a 
general  direction  of  N.  35°  W.  (Fig.  588).  This  is  an  old  fault  line, 
and  repeated  movements  have  occurred  along  it,  all  probably 
accompanied  by  earthquakes.  In  the  latest  earthquake,  move- 

ments  took  place  along  this 

line  for  at  least  185  miles, 
these  movements  being  partly 
vertical  and  partly  lateral 
displacements.  On  the  south- 
west side  of  the  fault,  the 
shifting  was  generally  to  the 
northward,  ranging  from  a 
few  inches  to  20  feet.  A 
reverse  displacement,  how- 
ever, occurred  at  Tomales 
Bay,  north  of  San  Francisco, 
where  the  offset  in  the  op- 
posite direction  was  about 
20  feet.  The  southwestern 
side  was  usually  also  the  up- 
lift side,  the  amount  being 
not  over  four  feet.  In  some 
cases,  however,  an  uplift  of 
as  much  as  two  feet  occurred 
on  the  eastern  side. 

As  no  large  cities  are  lo- 
cated upon  the  fault  line,  the 
amount  of  destruction  along  it  was  limited.  The  chief  destruction 
was  done  along  a  line  nearly  parallel  to  the  fault  and  northeast  of  it, 
where  it  is  marked  by  the  straight  eastern  shore  of  San  Francisco 
Bay.  On  it  are  located  the  towns  of  Ukiah,  Cloverdale,  Healdsburg, 
and  Santa  Rosa  in  the  north,  and  San  Jose,  south  of  Oakland, 
through  which  it  passes.  All  of  these  towns  suffered  severely, 
especially  the  parts  built  on  unconsolidated  material,  which  was 
thrown  into  vibrations  of  greater  amplitude  than  was  the  case  with 
the  solid  rock.  San  Francisco  lies  between  these  two  lines  of 
faulting  and  over  a  subordinate  fault  line.  The  main  destruction 


FIG.  589.  —  Earthquake  fissure,  as- 
sociated with  faulting.  California  earth- 
quake. The  main  fault  between  Point 
Keys  Station  and  Olima  looking  south- 
east. The  ground  at  the  right  of  the 
fault  has  moved  toward  the  observer ;  at 
the  left,  from  the  observer.  (U.  S.  G.  S.) 


Great  Earthquakes  of  Modern  Times  687 

wrought  there  by  the  earthquake  itself  extended  roughly  in  a  north- 
east-southwest line  (along  the  direction  of  Market  Street).  When 
this  line  is  extended,  it  roughly  forms  the  straight  northwestern 
coast  of  Suisun  Bay,  and  there  the  railroad  track  sank  in  the  marsh 
at  the  time  of  the  earthquake. 

This  earthquake  was  accompanied  by  the  usual  phenomona  of 
fissuring  of   the  ground    (Fig.    589),   of   lateral    displacement  of 


FIG.  590.  —  Fence  displaced  during  the  California  earthquake  of  1906.     (Photo 
by  G.  K.  Gilbert,  from  U.  S.  G.  S.) 

fences,  roads,  etc.,  to  the  extent  of  10  feet  (Fig.  590),  the  lateral 
shifting  of  the  foundations  underneath  the  buildings,  the  twisting 
of  statues  on  their  pedestals  or  their  overthrow,  etc.1 

The  loss  of  life  was  comparatively  small,  though  perhaps  1000 
persons  perished.  Much  of  the  destruction  in  San  Francisco  was 
due  to  the-  fires,  which  could  not  be  extinguished  because  the 
water  pipes  laid  across  the  fault  line  were  cut  in  two. 

1  For  details  see  Bulletin  324,  U.  S.  G.  S.,  1907 ;  170  pp.,  57  plates. 


688  Movements  of  the  Earth's  Surface 

The  Jamaica  Earthquake  of  1907 

The  island  of  Jamaica  in  the  Greater  Antilles  lies  along  lines 
of  deformation  which  go  back  at  least  to  Tertiary  time,  and  has 
been  repeatedly  subjected  to  seismic  disturbances.  The  city  of 
Kingston  seems  to  be  a  focal  point  from  which  the  lines  of  chief 
disturbance  radiate.  The  old  town  of  Port  Royal,  situated 
across  the  harbor,  at  the  end  of  a  seven  mile  sand-spit,  was 
destroyed  by  an  earthquake  in  1692,  the  ground  settling  and 
causing  the  submergence  of  a  large  part  of  the  old  city.  This 
settling  has  recurred  lately.  A  heavy  shock,  preceded  by  subterra- 
nean rumblings,  occurred  on  January  14,  1907,  about  3.30  P.M., 
and  was  followed  about  20  seconds  later  by  a  second  one.  Within 
a  period  of  35  seconds  the  main  destruction  was  completed,  although 
eight  after-shocks  followed,  between  February  5  and  June  14. 
The  east-west  walls  of  the  buildings  within  the  city  suffered  most 
damage.  Destruction  almost  equal  to  that  at  Kingston  was 
wrought  at  Bluff  Bay  on  the  north  coast,  and  it  appears  that  a 
line  of  disturbance  (seismotectonic  line)  crosses  eastern  Jamaica 
between  these  points.  Extended  northward,  this  line  passes  through 
Santiago  de  Cuba,  where  the  shocks  were  strongly  felt.  Other 
lines  of  this  type  intersect  Jamaica  westward  from  Kingston,  where 
they  converge.  Along  one  of  these  lines  the  cable  to  the  city  of 
Colon  was  fractured,  four  miles  out  from  Bull  Bay. 

The  faulting  produced  at  this  time  seems  to  have  been  confined 
to  a  zone  around  the  river  harbor.  A  series  of  parallel  step 
faults  was  produced  on  the  inner  side  of  the  sand-spit,  descending 
progressively  toward  the  shore,  and  making  an  aggregate  displace- 
ment of  not  less  than  24  feet.  At  the  intersection  of  one  of  these 
faults  with  a  railroad  track  the  latter  was  deformed  by  a  short, 
sharp  kink.  The  fissures  repeatedly  opened  and  closed,  ejecting 
water  and  sand,  and  a  series  of  craterlets  was  formed  on  the 
bottom  of  some  of  them. 

In  one  instance  the  harbor  was  increased  in  depth  to  the  extent 
of  27  feet,  while  the  western  end  of  the  peninsula  on  which  Port 
Royal  stands  was  submerged  from  8  to  25  feet,  so  that  only  the 
tops  of  the  palm  trees  and  roofs  of  buildings  project  above  the  water 
level.  The  usual  twisting  of  statues  and  other  phenemona  were 
manifested  during  this  earthquake. 


Phenomena  Accompanying  Earthquakes        689 


SUMMARY  or  PHENOMENA  DUE  TO  AND  ACCOMPANYING 
EARTHQUAKES 

Having  now  reviewed  some  of  the  more  important  earthquakes 
of  historic  time,  we  may  summarize  their  characteristics  and  the 
accompanying  phenomena.  In  the  first  place,  it  must  be  empha- 
sized that  although  volcanic  eruptions  are  commonly  accompanied 
by  earth  tremors,  the  great  earthquake  disturbances  are  independ- 
ent of  such  volcanisms  and  are  due  to  the  giving  way  of  the  earth's 
crust  in  places  under  heavy  strains.  Such  giving  way  produces 

fissures,   or   opens    pre-  _ 

viously  formed  fissures,  '~^~ ]—  — j—  |  '  y  '  /  ^^ 
and  along  these  both  ver- 
tical and  horizontal  dis- 
placements suddenly 
take  place.  Vertical 
movements  are  both  up 
and  downward,  some- 
times only  one  occurring, 
as  in  the  case  of  the  up- 
ward tilting  of  the  block 
mountain  in  New  Zea- 
land; in  others  both 
movements  occur.  Along 


the  same  fault  line  dif- 
ferential vertical  move- 
ments may  be  in  opposite 
directions,  so  that  in  dif- 
ferent places  the  same 
side  of  the  fault  may 
form  the  relatively  raised 
or  depressed  portions. 
Again,  block  faulting,  the 
dropping  down  or  raising 
of  circumscribed  blocks, 
is  not  uncommon.  Upon 
the  surface,  the  topo- 


FIG.  591.  —  Diagram  of  an  ancient  fissure  in 
fine  Upper  Silurian  limestone,  etc.,  filled  with 
rounded  grains  of  sand,  secondarily  enlarged, 
and  including  fragments  of  the  wall-rock.  The 
character  of  the  fissure  shows  that  it  was 
formed  suddenly  and  that  the  sand,  which 
originally  covered  the  surface,  was  violently 
injected,  and  sometimes  driven  into  horizontal 
fissures  between  the  strata.  It  is  interpreted 
as  an  earthquake  fissure,  and  the  date  of  the 
earthquake  is  seen  to  have  been  sometime  in 
Lower  Devonian  time,  as  the  fissure  is  in 
Upper  Silurian  rocks,  and  the  covering  rock, 
which  is  unaffected,  is  of  Middle  Devonian 
age.  Cement  quarry,  Buffalo,  N.  Y.  For  detail 
of  included  fragment,  see  Fig.  592,  and  for  en- 
larged sand  grains  from  the  injected  mass,  see 
Fig.  472,  p.  565. 


graphical  effect  may  be 

a  single,  or  a  series  of  step-like  fault  scarps,  or  a  ridge  resembling 

a  gigantic  mole  track.     In  general,  the  height  of  the  scarp  is  only 


690  Movements  of  the  Earth's  Surface 

a  few  feet,  though  displacements  of  as  much  as  47  feet  have  been 
recorded.  Beneath  the  sea,  however,  much  greater  fault  scarps 
have  come  into  existence.  Thus  the  earthquake  of  October  26, 
1873,  caused  the  cable  to  break  seven  miles  from  the  cable  office 
at  Zante,  Greece,  by  the  formation  of  a  submarine  fault  scarp  600 
feet  in  height,  the  change  in  depth  being  from  1400  to  2000  feet. 
Other  fault  scarps  1000  feet  in  height  have  been  formed  in  this 
region,  and  in  one  case  a  difference  in  depth  of  2000  feet  has  been 

found    between    the    bow   and 
stern  of  the  cable  repair  ship. 

By  the  repeated  opening  and 
closing  of  fissures,   sands  from 
the  surface,  together  with  other 
objects,  are  injected  into  them, 
and  thus    sandstone    dikes  are 
FIG.  592.  -  Fragment  of  the  wall     formed,  penetrating  the  rocks  of 
rock  (limestone)  included  in  an  an-      the    region.      There  are  many 
cient    sandstone    dike;    the   rock    is       ancient  examples  of  such  sand- 
shattered  and  the  sand  (shaded)   in-  ...  /T,.  >. 
jected  into  the  fissures.     Somewhat       stone     dlkes     (Flf-    591,    592). 
reduced.     Sandstone  dike.     Buffalo,       Another    superficial    effect    pro- 
N.Y.    (For  character  of  sand  grains,       duced    upon    the    rocks   js   their 
Fig.  472,  P.  565-)                            shattering  and  the  dislodgment 
of  huge  masses  which  are  often  precipitated  to  some  distance. 

When  the  surface  consists  of  unconsolidated  material,  great 
earth  waves  are  produced,  which  cause  a  rocking  of  structures, 
a  bending  back  and  forth  of  trees,  and  a  rifting  of  the  surface, 
with  the  spouting  forth  of  sand  and  water  and  the  formation  of 
craterlets  and  mud-volcanoes.  Huge  landslides  are  also  produced, 
and  great  masses  of  soil  with  trees  and  buildings  may  be  shifted 
bodily.  Subsidences  also  occur  by  a  settling  of  the  soil,  and  thus 
lakes,  ponds,  and  submerged  areas  along  the  coast  result.  The 
damage  to  buildings  on  such  unconsolidated  bottoms  is  often  much 
greater  than  to  those  built  on  rock. 

Earthquakes  originating  near  or  under  the  sea  produce  great 
water-waves  or  tsunamis.  These  may  rise  40  feet  or  more,  and 
travel  with  enormous  velocities  and  over  distances  of  many  thou- 
sands of  miles.  Recorded  velocities  range  from  370  to  465  miles 
per  hour,  or  from  six  to  seven  and  three  fourths  miles  per 
minute. 


Slow  Changes  in  Levels,  Bradyseisms          691 

SLOW  CHANGES  IN  LEVELS,  BRADYSEISMS 

In  addition  to  the  rapid  modification  of  the  earth's  surface,  de- 
scribed as  accompanied  by  earthquakes,  there  are  gradual  changes 
in  elevation  which  are  not  associated  with  detectable  earthquake 
movements.  These  are  often  of  great  geological  importance, 
especially  when  they  produce  depressions  into  which  the  sea  may 
enter.  We  have  already  discussed  the  formation  of  geosynclines 
of  deposition,  and  have  seen  that  these  imply  slow  sinking  with 
simultaneous  deposition  of  sediment,  the  aggregate  of  the  slow 
subsidence  amounting  in  the  end  to  many  thousands  of  feet.  The 
actuality  of  such  subsidences  can  be  deduced  only  indirectly  from 
the  nature  and  extent  of  the  sediment  deposited  in  the  subsiding 
area,  and  indeed  actual  observation  of  all  slow  earth  movements, 
or  bradyseisms ,  as  they  are  called,  is  out  of  the  question.  There 
are,  however,  other  evidences  than  the  sediments  to  indicate  slow 
changes  in  level,  both  upward  and  downward  movements,  and  these 
may  be  found  not  only  upon  the  sea-coast,  but  in  certain  cases  in- 
land as  well.  We  will  take  three  illustrations  of  such  changes,  one 
from  the  Temple  of  Jupiter  Serapis  at  Pozzuoli,  Italy,  another  from 
the  raised  beaches  of  the  Atlantic  coast,  and  the  third  from  the 
changes  of  level  observed  in  the  region  of  the  Great  North  Amer- 
ican Lakes. 

The  Temple  of  Jupiter  Serapis  at  Pozzuoli 

The  coastal  district  of  Naples  is  of  interest,  not  only  because  of 
the  varied  manifestations  of  volcanic  activities,  but  also  because 
of  the  unequivocal  evidence  of  elevation  and  subsidence  since  the 
beginning  of  the  Christian  Era  which  is  afforded  by  such  monu- 
ments of  antiquity  as  the  ruined  temple  of  Jupiter  Serapis  at  Poz- 
zuoli. (See  map,  Fig.  51,  p.  no.)  It  is  true  that  there  is  other 
evidence  of  change  of  level  in  this  region,  but  none  that  is  quite 
so  convincing  as  that  furnished  by  these  ruins. 

This  temple  (Fig.  593),  built  some  centuries  before  the  opening 
of  the  Christian  Era,  was  of  circular  form,  70  feet  in  diameter, 
surrounded  by  a  quadrangular  court,  and  the  roof  was  supported 
by  46  noble  columns,  of  which  24  were  of  granite  and  the  rest  of 
marble.  Only  three  of  the  marble  pillars  remain  erect,  each  carved 
out  of  a  single  block,  their  height  being  40  feet  3^  inches.  One 
is  nearly  bisected  by  a  horizontal  fissure,  the  others  are  entire, 


692 


Movements  of  the  Earth's  Surface 


and  they  are  all  out  of  the  perpendicular,  inclining  slightly  to  the 
sea.  For  a  height  of  about  12  feet  above  their  pedestals,  the  sur- 
face of  the  pillars  is  smooth  and  uninjured.  Above  this  is  a  zone 
about  nine  feet  in  height,  where  the  marble  has  been  pierced  by  the 

boring  mollusk,  Lithodo- 
mus,  many  shells  of  which 
still  exist  in  the  pear- 
shaped  inward-enlarging 
"  hollows.  In  some  of  these 
borings,  shells  of  another 
marine  mollusk  (Area) 
also  occur.  These  holes 
clearly  indicate  that  at  the 
time  of  their  formation  the 
parts  of  the  columns  thus 
affected  were  beneath  sea- 
level,  and  their  depth  and 
size  is  such  as  to  suggest 
that  this  state  of  sub- 
mergence lasted  for  a  con- 
siderable time.  The  fact 
that  the  lower  part  of  the 
pillars  is  unaffected,  indi- 
cates that  at  that  time 
this  part  was  protected 
by  sediments  or  otherwise. 
The  amount  of  submer- 
gence is  marked  by  the 
height  of  the  bored  zone, 
the  upper  parts  of  the 
pillars  projecting  above  water-level.  Marble  columns  lying  on  the 
pavement  of  the  temple  were  also  attacked  by  borers,  while 
worm-tubes  (Serpulce)  became  attached  to  them. 

At  the  time  of  Lyell's  visit  in  1828,  the  platform  was  about  a 
foot  under  water,  —  the  sea,  only  100  feet  distant;  soaking  through 
the  intervening  soil.  The  top  of  the  bored  zone  was,  therefore, 
at  least  23  ieet  above  high- water  mark,  the  tides  in  the  Bay  of 
Naples  being  small.  An  older,  costly  mosaic  pavement  was  found 
by  excavation  about  5  feet  below  the  upper  one,  indicating  sub- 
sidence before  the  building  of  the  later  temple  floor.  That  the 


FIG.  593.  —  Ruins  of  the  temple  of  Jupiter 
Serapis  at  Pozzuoli  as  they  appeared  in  1840, 
with  the  floor  partly  submerged.  (After 
Lyell.) 


Slow  Changes  in  Levels,  Bradyseisms          693 

temple  was  intact,  and  stood  above  water-level  between  the  years 
222  and  235  A.D.  is  clearly  attested  by  historical  evidence,  and, 
therefore,  the  great  subsidence  occurred  much  more  recently.  That 
it  antedated  the  close  of  the  fifteenth  century  is  shown  by  a 
description  of  an  Italian  writer  dated  1580,  in  which  he  said  that 
50  years  before,  or  in  1530,  the  sea  washed  the  base  of  the  hills  which 
rise  behind  the  flat  land  on  which  the  remains  of  the  temple  stand, 
and  "  a  person  might  then  have  fished  from  the  site  of  those  ruins 
which  are  now  called  the  stadium,"  and  which  stand  upon  these 
hills. 

A  series  of  deposits  around  the  lower  part  of  the  pillars  and  the 
outer  walls  of  the  temple,  record  the  changes  in  history  and  show 
that  the  subsidence  was  gradual.  "  The  sea  first  entered  the 
court  or  atrium  and  mingled  its  waters  partially  with  those  of  the 
hot  springs  [which  still  exist].  From  this  brackish  medium  a  dark 
calcareous  precipitate  was  thrown  down,  which  became,  in  the 
course  of  time,  more  than  two  feet  thick,  including  some  serpulae 
in  it.  The  presence  of  these  annelids  teaches  us  that  the  water  was 
salt  or  brackish.  After  this  period  the  temple  was  filled  up  with 
an  irregular  mass  of  volcanic  tuff,  probably  derived  from  an  erup- 
tion of  the  neighboring  crater  of  the  Solfatara,  to  the  height  of  from 
five  to  nine  feet  above  the  pavement.  Over  this  again  a  purely 
fresh-water  deposit  of  carbonate  of  lime  accumulated  with  an  irreg- 
ular bottom  (because  of  the  uneven  surface  of  the  tuff)."  The  sur- 
face of  this  fresh-water  deposit  was  level,  and  upon  it  followed 
another  deposit  of  volcanic  ashes  and  rubbish.  Then  came  the 
subsidence,  which  permitted  the  boring  of  the  submerged  part 
of  the  pillars  not  protected  by  these  deposits.  Later  showers  of 
ashes  buried  the  pillars  in  places  to  a  height  of  35  feet  above  the 
pavement,  from  which  they  were  exhumed  in  recent  times. 

Certain  historic  evidence  shows  that  the  upward  movement 
had  begun  before  1530,  but  the  main  elevation  appears  to  have 
occurred  at  the  time  of  the  great  eruption  of  Monte  Nuovo  in  1538, 
when,  as  we  have  seen  (p.  112),  the  region  thereabout  was  elevated. 

Since  the  rediscovery  of  the  temple  in  1 749,  when  it  stood  higher 
than  now,  it  has  undergone  a  renewed  subsidence.  At  the  begin- 
ning of  the  nineteenth  century  the  floor  was  above  sea-level,  but  in 
1838  fish  were  caught  every  day  over  parts  of  the  pavement,  the 
conditions  being  as  shown  in  the  illustration  published  in  1840 
(Fig.  593).  Between  1822  and  1838  the  rate  of  subsidence  was 


694  Movements  of  the  Earth's  Surface 

about  one  inch  in  four  years,  in  1847  it  was  about  one  inch  a  year, 
and  in  1852  it  had  practically  ceased'.  In  1857-58  the  floor  of 
the  temple  was  covered  by  about  two  feet  of  water  at- high  tide,  on 
calm  days.  At  the  present  time  the  surface  of  the  pavement  is 
slightly  below  sea-level. 

Evidences  are  not  wanting  that  these  changes  of  level  affected 
the  whole  of  the  Bay  of  Naples,  and  represent  warpings  of  con- 
siderable areal  extent  instead  of  being  a  local  phenomenon  con- 
nected with  volcanism,  as  has  been  supposed  by  some. 

Raised  Beaches 

Scandinavia.  —  On  the  coast  of  Norway  raised  sea-beaches 
are  found,  which  are  held  to  indicate  a  movement  in  recent  times 
to  a  height  of  600  feet  or  more,  although  this  interpretation  has 
been  questioned.  It  is  a  noteworthy  fact,  however,  that  these 
terraces,  or  beaches,  are  not  only  found  in  fjords,  where  they  might 
have  been  formed  on  lakes  dammed  by  ice,  but  that  they  also  face 
the  open  sea,  and  occur  even  upon  some  of  the  islands.  These 
raised  beaches  are  both  of  the  wave-built  type  of  shingle  beach 
and  the  wave-cut  terrace  type.  On  the  Swedish  coast,  measure- 
ments of  the  rate  of  elevation  have  been  made  on  monuments 
especially  planted  for  this  purpose.  In  one  place,  the  rate  of 
elevation  was  found  to  be  about  two  feet  per  century,  but  it  is  not 
everywhere  uniform,  and  varies  from  period  to  period. 

Scotland.  —  Around  the  Scottish  coast,  such  beaches  occur  at 
levels  of  25,  50,  and  100  feet  above  the  sea,  indicating  step-like 
elevation.  The  higher  beaches  date  back  to  the  glacial  period, 
as  is  shown  by  their  organic  contents  and  their  association  with 
boulder  clay,  but  the  lowest  one  is  very  recent,  containing  in  many 
cases  the  shells  of  the  Mollusca  still  living  upon  that  coast  (espe- 
cially limpets)  and  incorporated  in  the  modern  beach.  The  lower 
two  beaches  can  be  traced  nearly  around  Scotland,  but  the  upper 
one  occurs  only  in  certain  places. 

Eastern  North  America.  —  In  the  coastal  region  of  Maine,  New 
Brunswick,  and  other  districts,  beds  of  clay  are  associated  with 
beaches  and  sand-spits,  and  cliffs  are  cut  into  the  glacial  deposits, 
these  cliffs  being  at  present  found  at  an  elevation  of  about  300  to 
600  feet  above  the  sea-level.  In  these  sands  and  clays  are  found 
the  shells  of  cold-water  mollusks  (Saxicava  arctica,  Fig.  594, 
Leda,  etc.),  which  are  still  living  upon  the  coast.  As  the  beaches 


Slow  Changes  in  Levels,  Bradyseisms          695 

are  partly  cut  into  the  glacial  deposits  at  those  elevations,  it  is  clear 
that  the  region  has  been  relatively  raised  some  time  after  the  forma- 
tion of  these  deposits  or,  in  other  words,  in  comparatively  modern 
times. 

There  are  many  other  examples  known  which  demonstrate  re- 
cent changes  in  level  along  the  coast,  but  those  cited  are  so  clear 
that  they  admit  of  no  other  explanation.  Evidences  of  subsidence, 
such  as  the  drowned  Valley  of  the  Hudson  and  other  rivers,  the 


FIG.  594.  —  Shells  of  a  pelecypod  (Saxlcava  arctka)  found  in  the  elevated  sand 
beds  of  northeastern  New  England,  New  Brunswick,  etc.  This  species  still 
lives  in  the  modern  ocean. 

submergence  of  fresh- water  peat  deposits,  of  former  forests,  etc., 
also  occur,  but  they  are  in  some  cases  explicable  in  other  ways. 
Enough  has  been  said,  however,  to  show  that  changes  of  level  have 
occurred  in  recent  times.  Some  of  these,  no  doubt,  were  due  to  a 
change  in  the  position  of  the  sea-level,  caused  in  part  by  the  attrac- 
tion of  the  great  ice  mass  which  covered  the  northern  Atlantic 
regions,  as  already  outlined  (p.  297).  Others,  however,  can  only 
be  regarded  as  representative  of  a  change  in  the  altitude  of  the 
land  itself.1 

Change  of  Level  in  the  Great  Lakes  Region 

The  evidence  of  change  of  level  in  the  region  of  the  Great  Lakes 
of  North  America  is  of  an  interesting  and  peculiar  type.  On  the 
northeastern  side  of  the  Lakes,  the  old  beaches  are  raised  above 
the  present  water-level,  in  some  cases  to  a  height  of  several  hun- 
dred feet.  Followed  southward  and  westward,  these  beaches 
descend  toward  the  lake  level  and  finally  pass  under  it.  This  in- 
dicates a  tilting  of  the  lake-basins  toward  the  southwest,  and  in 
support  of  this  hypothesis,  it  is  found  that  the  water  of  Lake  Michi- 
gan at  Chicago  rises  at  the  rate  of  about  9  inches  a  century.  At 

1  For  further  discussion  of  this  problem  see  D.  W.  Johnson,  "  Shore  Processes  and 
Shore  Line  Development."  John  Wiley  and  Sons. 


696  Movements  of  the  Earth's  Surface 

this  rate  it  is  estimated  that  the  permanent  discharge  of  the  Great 
Lakes  will  be  by  this  route  to  the  Mississippi  'in  3000  years  from 
now. 

The  tilting  of  the  lake  basin  is  explained  by  the  rising  of  the  old 
Ontario  dome  on  the  northeast,  a  dome  which  has  repeatedly  risen 
since  Palaeozoic  time,  with  intervening  periods  of  rest.  The  present 
rate  of  rise  is  estimated  to  be  several  inches  in  a  hundred  miles  per 
century. 


CHAPTER  XXII 
THE  SCULPTURING   OF  THE  EARTH'S   SURFACE 

INITIAL  CHARACTER  OF  THE  LAND  SURFACE 

Plains.  —  The  simplest  form  of  land  surface  upon  which  the 
agents  of  erosion  perform  their  sculpturing  process  is  the  plain. 
By  this  name  is  designated  the  surface  of  a  country  which  over 
extensive  areas  shows  very  little  relief.  Indeed,  some  plains  ap- 
pear to  the  eye  almost  as  level  as  the  surface  of  the  ocean.  The 
rivers  of  the  plain  have  as  a  rule  shallow  valleys  and  are  for  the 
most  part  sluggish,  frequently  with  swampy  or  boggy  borders. 

Young  Plains.  —  The  most  typical  plains  are  plains  of  construc- 
tion and  are  formed  of  horizontal  or  nearly  horizontal  strata  upon 
which  little  erosion  has  been  performed.  Such  plains  are  called 
young  plains,  and  they  may  be  divided  into  coastal  plains  and  in- 
land plains.  A  typical  coastal  plain  borders  the  sea-coast,  and  rep- 
resents portions  of  the  sea-bottom  which  have  been  elevated  but 
recently,  so  that  erosion  has  made  little  progress,  or  elevated  to 
such  a  slight  extent  that  the  work  of  the  rivers  and  other  agents 
of  erosion  has  been  unable  to  accomplish  much  dissection.  Such 
a  coastal  plain  borders  the  sea  in  southern  New  Jersey,  and  ex- 
tends inland  for  many  miles.  It  has  a  deep,  sandy  soil,  and  is 
generally  overgrown  with  pine  forests;  its  roads  and -railroads 
run  in  straight  lines  for  long  distances,  and  its  population  is 
scanty  because  of  the  infertility  of  the  soil  and  the  sluggishness  of 
the  streams  and  the  consequent  lack  of  water-power. 

Inland  plains  may  be  formed  by  the  deposition  of  strata  of  sand 
and  clay  either  in  old  lake  basins  or  by  the  confluence  of  river  flood 
plains,  as  in  the  case  of  the  Indo-Gangetic  plain  of  northern  India, 
(p.  468) ;  the  great  alluvial  plain  of  the  Hoang-Ho  of  China  (p.  467), 
and  others  of  this  type ;  or  they  may  represent  the  flat  till-covered 
surfaces  left  during  former  glaciation  by  the  continental  ice-sheet 
which  once  occupied  that  region. 

697 


698        The  Sculpturing  of  the  Earth's  Surface 

Extensive  plains  may  also  be  formed  by  wide-spreading  lava 
flows,  but  these  are  seldom  absolutely  level.     Plains  always  stand 


FIG.  595.  —  Winter  scene  on  the  plains  of  North  Asia,  showing  the  broad  and 
level  expanse  of  a  young  plain. 

at  a  moderate  altitude  above  the  sea  or  other  level  which  controls 
erosion,  so  that  the  rivers  do  not  cut  down  deeply  beneath  the 
surface.  The  great  plain  of  western  Siberia,  in  latitude  50°  to 


FIG.  596.  —  Relief  map  of  Asia. 


Initial  Character  of  the  Land  Surface          699 

60°  N.,  is  not  greatly  elevated  above  sea-level  and  preserves  an 
even  surface  over  hundreds  of  miles.  "  Vast  areas,  stretching 
further  than  the  eye  can  reach,  are  monotonous  in  the  extreme, 
almost  as  uniform  in  soil  as  in  surface.  The  flat  areas  between 
the  streams,  having  no  distinct  lines  of  water  parting  and  no  dis- 
tinct channels  of  water  discharge,  are  as  yet  practically  undivided 
among  the  rivers.  Marshes,  alternately  wet  and  dry  in  winter  and 
summer,  and  many  shallow  lakes  lie  in  faint  depressions,  as  if  slight 


FIG.  597.  —  Badlands  of  the  Great  Plains,  showing  horizontal  plains  structure. 
South  side  of  White  River,  three  miles  below  Porcupine  Creek.  (U.  S.  G.  S. ; 
courtesy  of  D.  W.  Johnson.) 

inequalities  in  the  original  surface  of  the  plain  had  not  yet  been 
drained  by  river  action.  The  valleys  are  few  and  far  between ; 
they  can  never  be  cut  deep  while  the  region  stands  low.  They  are 
narrow ;  hence  the  rivers  have  as  yet  worked  only  for  a  compara- 
tively short  time  in  the  earth's  history.  The  plains  are  still  young." 
(Davis.)  (Fig.  595.  See  also  relief  map  of  Asia,  Fig.  596.) 

Mature  Plains.  —  When  a  plain  has  been  thoroughly  dissected 
by  streams  and  by  the  work  of  the  atmosphere,  especially  the 
wind,  so  that  its  topography  presents  the  maximum  of  ruggedness 
which  it  can  acquire  under  the  existing  conditions,  it  is  said  to  be 
mature.  Such  thoroughly  dissected  plains  are  also  spoken  of  as 


700        The  Sculpturing  of  the  Earth's  Surface 

"Bad  Lands,"  because  of  the  difficulty  of  traversing  them 
(Fig.  597).  The  same  plane  may  be  maturely  dissected  in  one 
part,  while  in  another  the  original  evenness  of  surface  may  show 
but  little  modification. 

Old  Plains  (Erosion  plains).  —  A  plain  which  has  been  produced 
as  the  result  of  prolonged  erosion  of  a  region,  until  that  has  been 
worn  down  again  to  a  level  surface,  —  that  is,  a  plain  of  denuda- 
tion, —  is  spoken  of  as  an  old  plain.  The  great  plain  of  central 
Russia  is  one  of  the  largest  examples  of  such  a  plain  of  erosion,  and 


ATLANTIC 
. OCEAN 


\ 


EUROPE 


5&.I5     BLACK    SEA 


FIG.  598.  —  Relief  map  of  Europe, 

it  might  at  first  sight  be  mistaken  for  a  young  plain,  it  is  so  level 
and  monotonous  over  vast  areas.  The  rocks  underlying  it  do  not, 
however,  correspond  with  the  surface,  although  they  are  nearly 
horizontal.  They  are  mostly  firm  and  consolidated,  instead  of 
loose  sands,  etc.,  as  in  young  plains,  and  the  surface  is  not  formed 
by  the  same  stratum  as  is  the  case  in  young  plains,  but  bevels 
across  the  layers  at  a  very  gentle  angle.  Thus  from  place  to 
place  different  layers  are  exposed,  with  the  result  that  slight 
changes  in  the  form  of  the  surface  and  the  soil,  resulting  from  the 
decomposition  of  the  strata,  are  produced.  The  rivers  flow  in 


Initial  Character  of  the  Land  Surface 


701 


narrow  channels  of  moderate  depth,  indicating  that  little  erosion 
has  been  accomplished  since  the  formation  of  the  plain.  It  is 
customary  to  designate  such  an  erosion  surface  by  the  term  plane 

(Fig.  598). 

The  Peneplane.1  —  An  old  plane  of  erosion  is  also  designated  a 
peneplane,  because  it  is  seldom  a  perfect  plane,  but  generally  "  al- 
most a  plane,"  as  a  peninsula  is  almost  an  island.  The  great 
Russian  peneplane  above  referred  to  is  an  example  of  such  a  sur- 


FIG.  599.  —  Jail  Rock,  Cheyenne  County,  Nebraska.  A  butte  of  sandstone 
capping  clay,  rising  as  a  monadnock  above  the  peneplaned  adjacent  portions  of 
the  Great  Plains.  (Photo  by  Barton,  U.  S.  G.  S. ;  courtesy  of  D.  W.  Johnson.) 

face  of  moderate  elevation;  but  most  peneplanes  have  been  up- 
lifted again,  and  are  undergoing  renewed  dissection.  The  Great 
Plains  of  western  North  America  represent  such  an  erosion  surface 
on  nearly  horizontal  strata,  and  they  appear  to  the  eye  as  absolutely 
level  or  gently  rolling  surfaces,  as  shown  in  the  preceding  illus- 
tration, where  only  a  small  remnant  of  erosion,  "  the  Jail  Rock,  " 
rises  above  the  very  level  plane,  representing  one  of  a  number  of 
remnants  of  the  former  higher  surface  (Fig.  599).  This  surface  is, 
however,  undergoing  renewed  dissection  in  other  regions.  The 
Prairie  Plains  of  the  central  United  States  represent  a  broadly 

1  It  has  become  customary  to  use  the  terms  plane  and  peneplane  for  surfaces  of 
erosion,  restricting  the  term  plain  to  surfaces  of  construction  of  low  relief  and  hori- 
zontal strata. 


702         The  Sculpturing  of  the  Earth's  Surface 

peneplaned  surface  which  still  stands  at  a  moderate  altitude  above 
the  sea  (from  500  to  1000  ft.)  and  is  therefore  subject  only  to 
slight  dissection.  Much  of  this  area  is  covered  by  the  till  or 
ground  moraine  left  by  the  great  ice  sheets  which  buried  this 
region  in  Pleistocene  time,  while  other  parts  are  covered  by  the 
river  and  loess  deposits,  derived  from  the  ice-borne  material. 
These  newest  sediments  mantle  over  the  peneplane  surface,  and 
thus  in  reality  convert  it  into  a  constructional  or  true  plain.  It 


FIG.  600.  —  The  untamed  prairie.  The  broad  monotonous  expanse  of  these 
plains  is  due  to  the  peneplanation  of  the  underlying  rock  and  the  veneering  of 
the  surface  with  glacial,  alluvial,  eolian  and  loess  deposits. 

is  this  mantling  by  glacial  debris  which  gives  the  surface  of  the 
prairie  its  level,  monotonous  character  (Fig.  600). 

While  many  peneplanes  are  cut  on  nearly  horizontal  strata,  there 
are  others  which  have  been  cut  on  complexly  folded  rocks.  The 
New  England  peneplane  is  of  this  type,  and  so  is  that  which  cuts 
the  old,  much-folded  slate  mountains  of  western  Germany,  in  which 
the  Rhine  has  sunk  its  beautiful  gorge  (Fig.  601).  It  is  evident 
that  such  a  peneplane,  cutting  across  strata  of  varying  hardness, 
must  represent  the  lowest  level  to  which  streams  could  erode  at 
the  time  this  erosion  surface  was  formed,  this"  in  the  examples 
cited  being  approximately  sea-level.  From  the  fact  that  the 


Initial  Character  of  the  Land  Surface 


703 


modern  streams  have  again  cut  gorges  and  valleys  below  this 
level  we  must  conclude  that  the  peneplane  has  been  elevated,  with 
reference  to  the  sea-level,  since  its  formation. 

Plateaus.  —  A  typical  plateau,  like  a  typical  plain,  is  composed 
of  horizontal  strata,  but  its  surface  stands  at  a  much  greater  eleva- 
tion above  the  floors  of  the  valleys  which  transect  it,  than  does 
that  of  the  plain.  A  young  plateau  presents  a  wide,  level  surface, 
with  few  but  deep  and  narrow  canons  cut  in  it,  which  form  its 
most  pronounced  feature,  since  the  upland  surface  itself,  because 
of  its  even,  monotonous  character,  attracts  little  attention. 


I 


FIG.  601 .  —  Gorge  of  the  Rhine  at  St.  Goar,  Germany.  The  gorge  is  seen  to 
be  incised  in  an  older  valley,  remnants  of  the  floor  of  which  are  seen  somewhat 
below  the  remarkably  even  skyline  of  the  peneplane.  (Photo  by  D.  W.  John- 
son.) 

Many  so-called  plateaus  are  really  uplifted  peneplanes  cut  on 
horizontal  strata,  as  in  the  case  of  the  Alleghany  Plateau  of  the 
eastern  United  States.  Some  of  the  little  dissected  plateaus  of 
the  western  states  are,  however,  regarded  as  young  plateaus  in 
which  erosion  has  progressed  only  to  a  slight 'extent.  In  the  rivers 
dissecting  young  plateaus,  as  well  as  in  those  which  dissect  uplifted 
peneplanes,  waterfalls  often  form  a  characteristic  feature.  An- 
other marked  feature  of  many  recently  uplifted  plateaus  is  the 
broken  and  flexed  character  of  their  strata,  so  that  recent  fault 
scarps  and  monoclinal  flexures  diversify  the  surface.  Such  modi- 
fications of  the  surface  character  are  clearly  shown  in  the  Colorado 


704        The  Sculpturing  of  the  Earth's  Surface 

Plateau  of  Arizona  and  southern  Utah  represented  in  the  diagram 
(Fig.  6196). 

Deformed  Surfaces.  —  Many  initial  surfaces  are  far  from  being 
level'  but  are  more  or  less  strongly  deformed.  Such  deformed  sur- 
faces are  found  on  fault  blocks,  dome  mountains  and  anticlines. 
Besides  these  there  are  minor  constructional  surfaces,  such  as 
volcanic  cones,  drumlins,  etc.,  which  present  inclined  surfaces  and 
offer  special  opportunities  for  the  sculpturing  processes. 

THE  EROSION  CYCLE  IN  THE  SCULPTURING  OF  THE  LAND 
SURFACE 

The  surface  features  of  the  earth  are  largely  the  result  of  the 
sculpturing  processes  which  are  constantly  and  everywhere  at  work, 
and  the  details  of  operation  of  which  we  have  studied  in  previous 
chapters.  But  it  must  be  clearly  understood  that  the  nature  of 
the  rock  and  its  attitude,  —  that  is,  the  structural  features  of  the 
earth's  crust,  —  exercise  an  important  controlling  influence  upon 
the  sculpturing  processes,  and  that  it  is  only  by  taking  those  fea- 
tures into  account  that  we  can  understand  the  bewildering  series 
of  land  forms  which  have  been  produced  by  the  agents  of  erosion 
or  sculpture  upon  a  surface  of  diverse  structure  and  history.  Hence 
this  subject  has  been  reserved  until  the  discussion  of  such  features 
had  been  completed. 

The  Cycle.  —  We  shall  first  discuss  the  general  progress  of  sur- 
face sculpture  from  the  beginning  to  the  end,  that  is,  to  the  oblit- 
eration of  the  surface  features  produced  during  the  process.  For, 
unlike  a  human  sculptor,  nature  is  not  content  to  produce  finished 
land  forms  from  the  earth-block  she  is  working  on,  but  must  needs 
continue,  after  the  sculpture  is  finished,  to  reduce  its  salient  fea- 
tures, and  keep  up  the  process  until  the  originally  produced  com- 
plex form  has  been  again  destroyed  and  the  block  brought  back 
nearly  to  the  relative  simplicity  of  form  which  it  had  at  the  be- 
ginning of  the  process.  It  is  as  if  the  sculptor,  having  produced 
his  bas-relief  upon  a  stone  front,  were  to  continue  his  carving  and 
cutting,  gradually  removing  the  salient  features  until  all  resem- 
blance to  the  work  produced  originally  is  lost,  and  the  surface  of 
the  stone  again  becomes  plain  and  without  relief  features. 

This  process  of  earth  sculpture  from  formless  surfaces  to  com- 
plex bas-relief,  and  on  to  formless  surface  again,  constitutes  the 


The  Erosion  Cycle  705 

cycle  of  erosion,  and  the  agents  which  accomplish  this  cycle  are  the 
familiar  ones  of  the  weather,  that  is,  the  gases  and  vapors  of  the 
atmosphere,  the  wind,  rain,  frost,  and  diurnal  and  seasonal  tem- 
perature changes ;  and  also  the  streams  and  the  glaciers,  and  the 
waves  and  currents  upon  the  coast. 

Stages  of  Relief.  —  In  the  beginning,  when  the  diversity  of  re- 
lief is  slight,  the  surface  is  said  to  be  in  the  stage  of  youth.  As  dis- 
section progresses,  the  bas-relief  reaches  its  maximum  complexity 
and  diversity  of  form,  when  the  condition  of  maturity  is  approached, 
beyond  which  it  passes  on  to  old  age,  which  is  reached  when  the 
surface  is  again  reduced  to  a  condition  of  little  or  no  relief.  The 
last  stage,  in  which  the  surface  would  be  a  perfect  plane,  is  seldom 
if  ever  reached,  for  as  it  is  approached  the  agents  of  erosion  work 
more  and  more  slowly,  and  before  its  final  accomplishment  the 
unstable  nature  of  the  earth's  crust  may  effect  a  change  of  condi- 
tions and  a  new  cycle  of  sculpture  or  erosion  become  inaugurated. 
Such  an  unfinished  surface,  the  end  stage  of  an  incomplete  cycle, 
is  presented  by  the  peneplane. 

Monadnocks.  —  The  unreduced  relief  features  which  rise  above 
the  peneplane,  and  which  are  the  residual  remnants  of  the  former 
diverse  relief,  are  called  monadnocks,  from  the  mountain  of  that 
name  in  New  Hampshire,  which  was  first  recognized  as  representing 
such  a  residual  relief  feature  upon  a  peneplane  to  which  we  shall 
refer  again  later  on  (Fig.  635,  p.  742).  Jail  Rock,  shown  in  Fig. 
599,  is  such  a  monadnock,  rising  above  the  peneplane  surface  of 
the  Great  Plains. 

Relation  of  Peneplane  to  Base-level  of  Erosion.  —  Since  the 
materials  removed  by  the  natural  agencies  of  land-sculpture  are 
carried  away  by  agents  acting  under  the  influence  of  gravitation, 
it  is  evident  that  the  sculpturing  processes  may  continue  until  the 
surface  of  the  country  subjected  to  them  is  lowered  to  the  level, 
which  for  the  time  being,  limits  the  force  of  gravitational  control. 
This  downward  limiting  surface  is  called  the  base-level  of  erosion, 
and  in  most  regions  this  is  the  level  of  the  sea,  for  beneath  this 
level,  few  of  the  agents  of  erosion  may  cut  to  any  great  extent. 
Therefore,  we  may  assume  that  sea-level  is  the  ultimate  base- 
level  of  erosion,  and  that  in  a  prolonged  cycle  the  surface  of  the 
land  as  it  approaches  the  peneplane  condition  also  approaches  the 
level  of  the  sea.  This  level  will  of  course  not  be  reached  by  the 
peneplane  the  rivers  of  which  are  tributary  to  the  sea,  except  at  its 


706        The  Sculpturing  of  the  Earth's  Surface 


margin,  for  the  surface  which  is  undergoing  subaerial  erosion  must 
always  have  a  certain  amount  of  slope,  in  order  that  the  material 
removed  from  it  may  be  carried  away.  A  peneplane  which  faces 

the  sea  may,  however, 
be  planed  down  by 
the  waves  from  the 
margins  inward,  thus 
producing  a  plane  of 
marine  erosion,  or  ma- 
rine peneplane. 

Local  Base-levels. 
-There  are,  how- 
ever, local  base-levels 
of  erosion  which  are 
independent  of  the 
level  of  the  sea.  The 
floor  of  the  Sahara 
Desert  lies  in  part  at 
least  beneath  the  level 
of  the  sea,  and  so 
does  the  surface  of  the 
Caspian  Sea  (Lake). 
Both  of  these  deeper 
levels  control  the  ex- 
tent to  which  erosion 
of  the  bordering  lands 
can  be  carried,  and 
erosion  surfaces 
formed  about  their 
margins  will  be  low- 
ered until  they  ap- 
proach these  deeper 
levels.  In  the  great 
interior  deserts,  such 
as  those  of  Tibet  and 
other  Asiatic  regions, 
the  base-level  of 
erosion,  which  is  the 
desert  floor,  lies  thou- 
sands  of  feet  above 


The  Erosion  Cycle  707 

the  sea-level.  Nevertheless,  for  the  time  being  it  controls  the  level 
to  which  erosion  of  the  surrounding  country  may  progress.  Where 
the  local  base-level  of  erosion  in  an  arid  region  is  an  enclosed  dry 
basin  which  receives  the  sediment  from  the  surrounding  regions, 
this  base-level  will  gradually  rise  as  the  basin  is  filled  with  the 
wastage  of  the  surrounding  hills,  and  so  the  ruggedness  of  the  dis- 
trict will  be  partly  reduced  by  filljnj^or  aggrading,  and  partly  by 
down-cuttingor^i^^g^g.  In  this  manner  a  complex  high-level 
plane,  partly  of  erosioo? and  partly  of  local  filling  by  sediments, 
will  come  into  existence.  Sucha  plain  is,  of  course,  produced  only 
where  the  hollows  formed  by  wind  erosion  are  filled  by  sediments 
washed  into  them,  and  this  can  take  place  only  in  a  region  which 
is  not  absolutely  rainless. 

The  Inauguration  of  the  New  Cycle.  —  When  the  peneplane  is 
raised  with  reference  to  the  sea-level,  or  where  by  a  change  in  con- 
ditions the  base-level  of  erosion  controlling  an  interior  peneplane  is 
lowered,  a  second  cycle  of  erosion  begins.  The  streams  once  more 
will  incise  their  courses  into  the  surface,  until  they  again  approach 
a  condition  of  grade,  that  is,  have  such  a  relation  to  the  new  base- 
level  of  erosion  that  their  cutting  power  has  practically  ceased. 
As  the  streams  upon  a  peneplane  generally  assume  a  meandering 
course,  winding  about  over  the  surface  to  follow  the  easiest  path, 
they  will  upon  elevation  of  the  peneplane  maintain  this  course,  and 
so  large  rivers  of  a  region  in  the  second  or  later  cycles  of  erosion 
will  present  the  phenomenon  of  intrenched  meanders  (Fig.  602). 
Such  intrenched  meanders  may  also  present  the  phenomenon  of 
a  cut-off,  leaving  an  intrenched  oxbow,  as  at  Lauffen  on  the 
Neckar  River  in  Wiirttemburg  (Fig.  603).  Lateral  tributary 
streams  will  gradually  dissect  the  elevated  upland.  Finally,  as 
the  second  cycle  approaches  completion,  the  region  will  again 
assume  the  character  of  a  peneplane. 

Relative  Ages  of  the  Rivers  and  of  the  Land 

There  is  no  relation  between  the  topographic  age  of  the  land  sur- 
face and  that  of  the  river  dissecting  it.  It  is  true  that  in  newly 
emerged  coastal  plains,  young  rivers  are  a  characteristic  feature, 
but  previously  existing  rivers  will  also  flow  across  such  a  coastal 
plain  from  the  old  land  bordering  it,  and  while  the  part  entering 
the  coastal  plain  will  at  first  have  the  characters  of  a  youthful 
stream,  its  greater  volume  of  water  will  soon  produce  a  valley  that 


708        The  Sculpturing  of  the  Earth's  Surface 


has  all  the  characteristics  of  maturity.  In  like  manner,  young 
rivers  may  come  into  existence  on  an  old  land  surface,  such  as  a 
peneplane,  by  uplift  of  this  surface,  while  the  older  streams  upon 
such  a  surface  will  become  rejuvenated  and  assume  the  charac- 
teristics of  a  youthful  stream. 


FIG.  603.  —  Intrenched  meander  cut-off  of  the  Neckar  River  at  Lauffen  and 
at  Kirchheim.  The  cut-off  portions  are  occupied  by  small  streams.  The  cut- 
off at  Lauffen  is  the  most  recent,  the  form  of  the  old  valley  still  being  clearly 
marked.  At  Kirchheim  the  river  has  cut  below  the  level  of  the  old  cut-off. 
(From  de  Martonne.) 

A  young  stream  is  characterized  by  a  narrow  valley  or  gorge 
with  steep  sides,  and  with  little  or  no  flood-plain  surface  on  the 
valley  floor,  which  is  occupied  for  the  most  part  by  the  stream. 
The  gorge  of  Niagara  and  that  of  the  Genesee  at  Portage  present 
such  youthful  conditions,  and  these  are  also  seen  in  the  Colorado 
River,  and  in  fact  in  all  rivers  which  run  in  narrow  canons  which 
they  are  still  actively  eroding;  such  rivers  are  frequently  char- 
acterized by  waterfalls.  A  mature  river,  on  the  other  hand,  has 


The  Erosion-Cycle  on  a  Coastal  Plain         709 

widened  its  valley  by  lateral  cutting,  and  built  a  flood-plain  upon 
its  floor,  which  it  no  longer  completely  fills.  On  this  flood-plain 
it  will  meander,  cutting  away  portions  at  one  place  (the  concave 
bank),  and  building  at  another  (the  convex  bank)  (Fig.  604). 
Such  continued  erosion  results  frequently  in  the  cutting  off  of  the 
larger  meanders,  leaving  them  as  "  oxbows  "  still  occupied  by 
water,  as  in  the  case  of  Lakes  Chicot  and  Lee  and  others  in  the 
Mississippi  Valley  (Fig.  605),  or  as  dry  oxbow  valleys.  Such 


FIG.  604.  —  Crooked  Creek,  near  Long  Valley  dam  site,  California.  Meanders 
of  stream  on  flat  floor  of  a  mature  valley  whose  sides  are  unusually  steep  in  cer- 
tain localities  where  hard  rock  ledges  project  into  the  valley.  (U.  S.  G.  S.) 

cutting-off  shortens  the  river,  often  by  many  miles,  and  increases 
its  velocity  along  the  new  course.  The  manner  of  their  forma- 
tion has  been  discussed  in  a  previous  chapter  (p.  417). 


THE  EROSION-CYCLE  ON  A  SIMPLE  COASTAL  PLAIN 

Characters  of  the  Coastal  Plain.  —  A  coastal  plain  such  as  that 
which  faces  the  Atlantic  from  New  Jersey  to  the  Gulf,  with  a 
width  varying  from  a  few  miles  to  over  a  hundred  miles,  represents 
an  emerged  strip 'of  former  sea-bottom.  The  sediments  of  which 
it  is  composed  represent  the  deposit  formed  in  the  littoral  district 
of  the  sea  when  it  covered  this  region,  and  the  shore-line  of  which 


710        The  Sculpturing  of  the  Earth's  Surface 


FIG.  605.  —  The  meanders  and  oxbows  of  the  Mississippi.  (Miss.  River 
Comm.,  1881-82  and  1894 ;  from  Ratzel.)  Lakes  Chicot  and  Lee  represent 
cut-off  portions  of  the  river,  now  forming  oxbow-lakes.  The  portion  shaded 
by  waterlining  represents  the  river  bed  according  to  the  surveys  of  1881-82, 
the  sandbanks  of  that  period  being  dotted.  The  black  line  represents  the  bed 
of  the  river  according  to  the  survey  of  1894,  and  the  sandbanks  of  that  period 
are  outlined  by  dotted  lines.  The  new  landing-places  erected  since  1882  are 
underlined. 


The  Erosion-Cycle  on  a  Coastal  Plain          711 

lay  approximately  along  the  inner  margin  of  the  present  coastal 
plain.  Not  all  coastal  plains  are  composed  of  simple  marine 
deposits;  indeed,  the  example  cited  contains  delta  and  alluvial 
deposits  which  were  formed  above  sea-level,  these  being  especially 
characteristic  of  the  lower  part  of  the  series.  Nor  is  the  marine 
series  always  continuous,  but,  as  in  the  example  cited,  it  is  more 
often  the  product  of  successive  encroachments  and  withdrawals 
of  the  sea.  Nevertheless,  this  series  may  be  treated  as  a  unit,  and 
in  our  discussion  of  erosion  we  may  assume  simplicity  of  structure 
as  typical  of  such  a  coastal  plain. 

Although  appearing  horizontal,  the  strata  of  the  coastal  plain 
are  in  reality  gently  inclined  seaward,  finally  passing  beneath  the 


FIG.  606. — A.  Diagrammatic  section  of  coastal  plain  strata  in  a  trans- 
gressive  or  overlapping  series.  B.  A  similar  series  formed  in  a  retreating  sea 
(off -lapping  series),  a  and  c  are  soft  strata,  b  and  d  are  hard  strata.  The  strata 
are  represented  as  of  the  same  character  throughout,  though  in  reality  they 
change  in  lithic  character  toward  the  shore. 

level  of  the  sea.  This  may  be  in  conformity  with  the  slope  of  the 
surface,  or  it  may  be  somewhat  in  excess  of  that,  depending  upon 
whether  we  are  dealing  with  a  transgressive  series  of  strata  sud- 
denly emerged,  or  with  a  retreatal  one,  the  product  of  slow  emer- 
gence. The  two  types  are  shown  here  in  section  (Fig.  606,  A,  B). 

For  the.  sake  of  simplicity  we  shall  consider  the  transgressive 
series  only,  and  we  shall  further  assume  that  this  series  consists  of 
alternating  deposits  of  hard  and  soft  strata.  The  former  may  con- 
sist of  sands  bound  together  by  calcareous  material  (b,d\  the  latter 
of  muds,  etc.  (a,  c)  (Fig.  606,  A). 

Drainage  Systems  of  the  Simple  Coastal  Plain.  —  Upon  a  sim- 
ple coastal  plain,  where  the  surface  is  comparatively  uniform  and 
regular,  sloping  gently  to  the  sea,  a  definite  type  of  drainage  will 
develop.  The  rain-water  run-off  (see  p.  411)  will  flow  down  the 
slope  to  the  sea,  soon  forming  for  itself  definite  lines  of  drainage, 
which,  following  the  shortest  course,  will  in  general  be  at  right 
angles  to  the  coast.  Such  streams,  the  first  to  be  formed,  are 
called  consequent  streams.  Streams  which  were  in  existence  on 


yi2        The  Sculpturing  of  the  Earth's  Surface 

the  oldland  when  the  coastal  plain  had  not  yet  emerged  will,  if 
persisting,  become  extended  across  the  coastal  plain  to  the  sea. 
These  are  the  extended  consequent  streams,  and  since  they  origi- 
nate in  the  mountains  or  higher  oldland,  they  will  generally 
have  more  water  than  the  newly  formed  consequents  upon  the 
coastal  plain,  and  so  be  able  to  cut  deep  and  wide  channels  more 
quickly  than  these.  Such  larger  streams  are  spoken  of  as  the 
master  consequents. 

Along  the  margin  of  the  consequent  streams  others  will  come 
into  existence  from  the  near-by  run-off,  which  finds  its  shortest 
course  into  the  consequent.  These  streams  are  not  consequent 
on  either  the  initial  slope  of  the  surface,  or  difference  of  rock 
resistance  sufficiently  pronounced  to  be  recognizable.  Hence 
they  are  spoken  of  as  inconsequent,  or,  more  briefly,  insequent 
streams.  They  cut  back  their  channels  from  the  edge  of  the 
consequent  valley  and  prolong  their  own  valleys  by  headward 
cutting. 

If  the  coastal  plain  structure  is  of  the  simple  type  above  out- 
lined, with  its  strata  progressively  overlapping  upon  the  oldland, 
and  if  the  capping  stratum  is  of  uniform  character  throughout, 
the  drainage  pattern  developed  will  consist  of  the  two  types  of 
streams  so  far  described,  namely,  the  consequents  and  the  inse- 
quents.  The  former  cut  downwards,  the  latter  headwards,  in- 
creasing in  length  and  complexity  until  a  complicated  drainage 
system  of  the  multiple-branching  type  is  produced. 

It  is,  however,  conceivable  that  near  the  inner  end  of  the  coastal 
plain  the  conditions  may  be  such  that  the  insequent  streams 
developed  there  may  have  an  advantage  over  those  formed  farther 
down  on  the  coastal  plain.  If  the  oldland  is  abrupt  and  high, 
the  moisture-bearing  winds,  on  striking  them,  may  be  forced  to 
rise  and  part  with  some  of  their  burden  of  water,  and  many  of  the 
mountain  streams  thus  formed  may  find  their  way  into,  or  be 
gathered  up  by,  the  growing  insequents,  as  these  prolong  their 
valleys  by  headward  cutting.  In  this  way  a  portion  of  the  inner 
edge  of  the  coastal  plain  may  be  removed,  the  oldland  being 
stripped  of  its  cover  of  coastal  plain  strata. 

Coastal  plains  are,  however,  seldom,  if  ever,  as  simple  as  here 
outlined.  As  we  have  seen,  they  are  often  composed  of  strata 
due  to  repeated  advances  and  retreats  of  the  sea,  with  the  result 
that  the  inner  edge  of  the  coastal  plain  differs  considerably  from 


The  Erosion-Cycle  on  a  Coastal  Plain         713 

that  of  the  simple  type  first  described.  Moreover,  the  strata  of 
a  coastal  plain  are  never  uniform  throughout,  but,  as  has  been 
shown  in  a  previous  chapter,  their  lithic  character  changes  as  we 
approach  the  shore  of  the  sea  in  which  they  were  formed.  All  of 
these  characters  combined  may  render  the  inner  edge  of  the  coastal 
plain  less  resistant  to  erosion,  and  as  a  result,  a  new  type  of  stream, 
the  subsequent,  will  develop,  which  differs  from  the  insequent  in 
that  it  develops  by  headward  erosion  along  this  belt  of  weaker 
rocks.  If  the  coastal  plain  consists  primarily  of  a  series  of  strata 
deposited  in  a  retreating  sea,  the  ends  of  the  weaker  strata  will 
be  exposed,  as  shown  in  diagram  B,  Fig.  606,  and  along  the  weaker 
strata  subsequent  streams  will  develop.  Again,  if  weaker  strata 
are  exposed  because  of  the  peneplanation  of  a  coastal  plain  series, 
subsequent  streams  will  develop  along  these  exposures;  but  these 
belong  to  a  later  cycle  of  erosion. 

By  the  erosion  of  the  inner  edge  of  the  coastal  plain,  the  valley 
of  the  subsequent  stream  is  produced,  and  this,  in  a  general  way, 
will  be  at  right  angles  to  the  valley  of  the  consequent.  Erosion 
in  this  region  progresses  with  relative  rapidity  because  of  the 
weaker  character  of  the  strata  upon  which  it  takes  place,  and 
thus,  partly  by  the  wearing  activities  of  the  stream,  and  partly 
by  that  of  rain 
water  and  the  rapid 
weathering  of  the 
softer  rock,  the  val- 
ley of  the  subse- 
quent will  be  broad- 
ened until  it  be-  FIG.  607  a.  —  Diagram  illustrating  the  coastal  plain 
of  overlapping  strata,  with  consequent  and  insequent 
comes  wide  enough  drainage. 

to  be  termed  a  low- 
land. As  this  lowland  lies  next  to  the  oldland,  or  on  the  inside 
of  the  coastal  plain,  it  is  called  an  inner  lowland.  It  consists 
in  part  of  a  stripped  belt,  where  the  coastal  plain  strata  have 
been  removed  down  to  the  rock  of  the  oldland,  and  in  part  it  is 
cut  from  the  coastal  plain  strata.  (Compare  Figs.  607  a  and  b.} 

The  inner  lowland  will  be  bordered  on  one  side  by  the  slop- 
ing surface  of  the  oldland,  and  on  the  other,  the  seaward  side, 
by  the  eroded  edge  of  the  coastal  plain.  If  the  top  stratum 
of  this  coastal  plain  is  of  resistant  rock,  a  cliff  will  crown  this 
cut  edge,  especially  if  the  cutting  has  been  deep  enough  to  expose 


714        The  Sculpturing  of  the  Earth's  Surface 

the  softer  bed  just  beneath.  In  this  manner  the  coastal  plain  will 
end  landwards  in  a  steeper  slope  or  in/ace,  and  between  this  in- 
face  and  the  oldland  lies  the  inner  lowland  (see  Fig.  607  b). 
The  remnant  of  the  coastal  plain  thus  separated  from  the  oldland  is 

called  a  cuesta  (pro- 
nounced kwest.a), 
and  this  topographic 
feature  is  character- 
istic of  the  mature 
stage  of  dissection 

FIG.  607  b.  -  The  same  region  shown  in  Fig.  607  a,  °f  the  C°astal  Plam' 

after  the  development  of  subsequent  streams  and  Two      types     of 

the  formation  of  an  inner  lowland.     The  remnant  streams     thus      are 
of    the   coastal  plain  now  forms  a  cuesta  ending 
landward  in  an  inface,  and  transected  by  the  conse-  isary 

quent  stream  valley.  duce  the  cuesta :  the 

consequent  which 

flows  down  the  dip  of  the  strata,  and  the  subsequent  which  flows 
along  the  strike  of  the  strata. 

The  inface  of  the  cuesta  may  also  become  the  site  of  develop- 
ment of  a  fourth  type  of  stream  which  flows  down  it  and  into  the 
subsequent  stream  which  occupies  the  inner  lowland.  These 
streams  likewise  increase  in  length  by  head  ward  erosion,  and  as 
they  flow  in  the  opposite  direction  from  that  of  the  original  con- 
sequent, they  have  been  called  ob sequent  streams.  They  form  the 
third  side  of  a  rectangular  drainage  pattern  thus  established.  By 
their  activities,  portions  of  the  front  of  the  cuesta  may  become 
separated  from  the  main  part  and  constitute  plateau-  or  butte-like 
outliers  —  or  table  mountains,  if  high.  These  are  characteristic 
features  of  many  cuesta  fronts.  (See  Fig.  633,  p.  740.) 

It  must  always  be  borne  in  mind  that  the  depth  of  the  inner 
lowland,  which  is  occupied  by  the  subsequent  streams,  cannot  be 
greater  than  the  depth  of  the  consequent  stream  valley  to  which 
these  subsequents  are  tributary,  and  that  it  is  the  master  stream 
of  the  region  which  determines  the  maximum  depth  to  which  ero- 
sion of  all  the  valleys  may  proceed.  The  present  Atlantic  coastal 
plain  of  eastern  North  America  appears  to  be  in  essentially  this 
state  of  dissection,  although  parts  of  it  have  suffered  great  modifi- 
cations. In  the  southern  part  of  the  plain,  the  normal  cuesta  to- 
pography is  easily  recognizable.  Thus  in  Alabama  we  find  next  to 
the  oldland  which  is  formed  by  the  subdued  mountains  of  the  Appa- 


The  Erosion-Cycle  on  a  Coastal  Plain          715 

lachians,  the  Black  Prairie  Valley,  which  forms  the  inner  lowland, 
and  the  name  of  which  is  derived  from  the  dark  color  of  the  rich 
soil,  weathered  from  the  underlying  weak  limestone.  This  flat 
inner  lowland  includes  the  best  cotton  district  of  the  state,  and  in 
it  lie  the  cities  of  Montgomery  and  Selma.  Proceeding  seaward 
we  note  the  rising  ground  of  the  inface  of  the  cuesta,  locally 
called  the  Chunnemugga  Ridge,  which  is  rather  abrupt,  though  not 
characterized  by  steep  cliffs,  and  upheld  by  a  more  resistant  lime- 
stone stratum.  Numerous  small  obsequent  streams  run  down  this 
inface  of  the  cuesta  and  dissect  it.  At  the  top  of  this  inface,  which 
rises  200  feet  above  the  inner  lowland,  the  Hill  Prairies  and  Pinelands 


Copyright  by  Detroit  Furnishing  Co. 

FIG.    608.  —  The  "Pine-Barrens"  —  on  the  southern  part   of   the   Atlantic 
Coastal  Plain  of  the  United  States.    . 

begin,  and  the  cuesta  surface  falls  with  an  imperceptible  slope  to 
the  sea  (Fig.  608).  The  Alabama,  Tombigbee  and  Chattahoochee 
rivers  are  the  chief  consequent  streams  rising  in  the  oldland  and 
traversing  the  cuesta  to  the  sea. 

Farther  north,  in  Maryland,  Delaware,  and  New  Jersey,  the 
coastal  plain  has  suffered  depression  after  the  erosion  topography 
was  produced.  As  a  result  the  sea  has  entered  the  river  valleys 
and  produced  shallow  inlets  or  bays,  of  which  Chesapeake  and 
Delaware  bays  are  conspicuous  examples,  though  marine  erosion 
has  greatly  modified  and  widened  these  original  river  valleys 
(Fig.  629,  p.  738).  Still  farther  north,  the  drowning  of  the 
coastal  plain  has  been  so  complete  that  it  allowed  the  sea  to 
enter  the  inner  lowland.  In  this  manner  Long  Island  Sound  was 


716        The  Sculpturing  of  the  Earth's  Surface 

produced  between  the  inface  of  the  cuesta  which  forms  the  north- 
ern shore  of  Long  Island  (here  modified  by  the  superposition  upon 
it  of  the  terminal  moraine  of  the  ice  age)  and  the  Connecticut 
shore,  which  is  a  part  of  the  oldland.  Much  of  the  seaward  side 
of  the  old  coastal  plain  has  also  been  submerged,  this  accounting 
in  part  for  the  narrowness  of  the  remaining  portion  which  forms 
Long  Island. 

Completion  of  Cycle  of  Erosion ;  Formation  of  the  Peneplane.  — 
As  erosion  continues,  the  inface  of  the  cuesta  is  pushed  farther  sea- 
ward and  the  inner  lowland  is  widened.  If  a  second  hard  layer  is 
discovered  during  the  progress  of  erosion,  the  inface  will  assume  a 
terraced  condition,  each  hard  layer  forming  a  cliff,  and  the  softer 


FIG.  609  a.  —  A  coastal  plain  in  the  first  cycle  of  erosion.  The  presence  of 
two  hard  layers  produces  a  terraced  cuesta  front.  In  exceptional  cases  the 
upper  terrace  may  weather  back  with  sufficient  rapidity  to  constitute  a  second 
cuesta  with  a  lowland  of  some  width  between  it  and  the  lower  cuesta,  if  the 
weak  stratum  between  the  harder  is  of  sufficient  thickness.  (Drawn  by  Mary 
Welleck.) 

one  between,  a  slope.  Continued  erosion  will  eventually  result  in 
the  complete  destruction  of  the  cuesta  and  the  formation  of  a  pene- 
plane.  As  the  surface  of  this  peneplane  will  have  a  gentler  slope 
than  that  of  the  strata  composing  it,  it  is  evident  that  this  surface 
and  the  strata  will  intersect  at  a  moderate  angle,  with  the  result 
that  the  outcropping  ends  of  the  strata  are  beveled  across  by  the 
erosion  plane.  This  is  shown  in  the  following  diagrams  (Figs.  609 
a,  b),  the  first  of  which  (a)  shows  the  terraced  cuesta  of  two  hard 
layers,  while  the  second  (b)  shows  the  completion  of  the  peneplane 
and  the  beveling  across  of  the  strata.  If  the  coastal  plain  con- 
sists of  a  retreatal  series  of  strata  (Fig.  606  B)  each  weak  stratum 
will  form  a  lowland  and  each  hard  one  a  cuesta. 

It  will  be  observed  that,  as  the  result  of  this  beveling,  the  out- 
crops of  the  strata  are  in  the  form  of  broad  belts,  and  that,  in  con- 


The  Erosion-Cycle  on  a  Coastal  Plain         717 

sequence,  the  outcrops  of  the  two  hard  layers  b  and  d  are  separated 
by  a  broad  belt  of  soft  strata. 

Across  such  a  peneplane  the  original  consequent  streams  will 
continue  to  flow,  but  because  of  the  disappearance  of  the  original 
bounding  walls  of  these  streams,  they  are  no  longer  confined  to  a 
straight  course,  but  may  begin  to  wind  or  meander,  partly  because 


FIG.  609  b.  —  The  same  region  shown  in  Fig.  609  a,  after  peneplanation. 
The  strata  are  now  beveled  across  by  the  peneplain,  and  the  rivers  are  no  longer 
controlled  by  the  topography.  (Drawn  by  Mary  Welleck.) 

of  obstructions  left  by  the  diminished  current  in  dropping  its  load 
of  sediment,  and  partly  for  other  reasons ;  and  as  the  outcrops  of 
the  harder  beds  are  more  and  more  reduced  to  the  general  level 
of  the  peneplane,  they  will  come  to  have  less  and  less  influence 
upon  the  course  of  the  streams. 

Beginning  of  the  Second  Cycle  of  Erosion.  —  If,  after  the  region 
has  been  reduced  to  the  condition  of  a  peneplane,  it  suffers  a  re- 


FIG.  609  c.  —  The  same  district  shown  in  the  two  preceding  diagrams,  after 
elevation  and  renewed  dissection.  Two  cuestas  and  two  lowlands  are  now 
produced  from  the  hard  and  weak  strata,  respectively.  (Drawn  by  Mary 
Welleck.) 

newed  bodily  elevation,  the  streams  will  be  rejuvenated  because 
their  slope  is  increased,  and  they  will  once  more  begin  to  cut  down- 
wards. Here  again  the  consequent  streams,  especially  those  which 
flow  from  the  oldland  to  the  sea,  will  determine  the  rate  and  extent 
of  downward  cutting,  for  all  other  streams  are  tributary  to  these 
consequents,  and  cannot  cut  below  the  level  attained  by  them. 


7iS        The  Sculpturing  of  the  Earth's  Surface 

If  the  consequent  streams  have  begun  to  meander  upon  the  sur- 
face of  the  peneplane,  they  are  apt  to  retain  that  meandering  course, 
and  the  new  channel  or  gorge  which  they  cut  during  the  second 
cycle  will  be  characterized  by  incised  or  intrenched  meanders,  the 
existence  of  which  is  generally  suggestive  of  a  second  or  later  cycle 
of  erosion  (Fig.  610).  (See  also  Figs.  602,  p.  706,  and  603,  p.  708.) 


FIG.  6 10.  —  Intrenched  meanders  of  the  Seine  in  the  old  peneplane  cut  upon 
the  strata  of  the  Paris  Basin ;  near  Rouen.     (After  de  Martonne.) 

As  the  consequent  streams  cut  their  transverse  channels  across 
the  outcropping  belts  of  the  soft  and  the  hard  layers  alike,  the  lat- 
eral streams  will  also  begin  cutting,  but  they  will  be  largely  con- 
fined to  the  softer  belts  which  are  more  easily  eroded.  As  a  result, 
subsequent  valleys  will  be  formed  upon  these  softer  belts,  and  the 
harder  ones  will  gradually  be  modeled  in  relief  as  new  ridges,  and 
eventually  each  will  assume  the  appearance  of  a  cuesta  inface, 
fronting  a  lowland  cut  upon  the  soft  belt.  But  there  will  now  be 
two  such  cuesta  infaces  and  two  broad  lowland  valleys,  one  the 
inner  lowland  between  the  oldland  and  the  inface  formed  by  the 


The  Erosion-Cycle  on  a  Coastal  Plain         719 

lower  hard  stratum,  the  other  between  this  latter  and  the  inface 
formed  by  the  upper  hard  stratum.  The  diagram  (Fig.  609  c) 
illustrates  the  result  thus  produced. 

These  two  cuesta  ridges  may  now  be  many  miles  apart,  but  their 
summit  elevation  will  be  in  the  same  plane,  as  they  formed  part 
of  the  original  peneplane  surface.  They  may,  indeed,  retain  a 
part  of  the  beveling  which  was  produced  during  the  peneplanation, 
as  is  indicated  in  the  diagram. 

Central  England  as  an  Illustration.  (Fig.  611.)  — What  is  gen- 
erally regarded  as  a  good  illustration  of  such  a  belted  structure 
with  two  cuestas  and  lowlands,  is  seen  in  the  topography  of  central 
England.  If  we  consider  the  rugged  hills  and  mountains  of  Wales 


FIG.  611.  —  Block  diagram  of  the  cuestas  and  lowlands  of  Central  England. 
A,  Oldland  of  Wales;  B,  Inner  lowland  (Triassic) ;  C,  Oolite  cuesta,  Cots  wold 
Hills ;  D,  Outer  lowland  (Green-sands  and  Gault) ;  E,  Chalk  cuesta,  Chiltern 
Hills;  F,  London  coastal  lowland.  (After  Davis.) 

as  a  part  of  the  oldland  (^4)  we  find  to  the  east  and  southeast  of 
this  a  broad  inner  lowland  cut  upon  .soft  red  sandstones  and 
marls  (Triassic).  In  this  lowland  (B)  are  situated  the  cities  of 
Bristol,  Nottingham,  and  York.  Rising  from  this  lowland  belt 
upon  the  east  is  the  first  ridge  or  cuesta  (C) ,  which  extends  in  a 
curving  line  across  central  England  from  southwest  to  northeast, 
and  is  formed  by  the  edges  of  the  lower  hard  limestone  strata 
(Oolites)  which  overlie  the  red  sandstones.  Near  Bristol  this  up- 
land belt  forms  the  Cotswold  Hills,  and  it  is  characterized  by  a 
relatively  steep  westward  and  very  gentle  eastward  slope.  Next 
east  of  this  lies  the  second  lowland  (D),  cut  upon  the  second  belt 
of  soft  strata  (Gault  and  Green-sands)  and  in  this  are  situated  the 
cities  of  Cambridge  and  Oxford.  A  second  upland  or  cuesta  (E), 
bounds  the  eastern  and  northeastern  border  of  this  lowland,  this 
being  formed  by  the  resistant  chalk.  It  constitutes  the  Chiltern 


720        The  Sculpturing  of  the  Earth's  Surface 


Hills  on  the  south,  and  from  the  summit  of  the  cuesta  there  is  a 
gentle  slope  toward  the  London  coastal  lowland  (F). 

While  this  illustrates  in  a  general  way  the  double  cuesta  type  of 
topography  produced  in  a  second  erosion  cycle,  there  are  many 
local  modifications,  owing  to  the  disturbances  of  the  strata  by  folds 
and  faults,  and  the  history  of  the  region  is  on  the  whole  more  com- 
plex than  here  outlined. 

River  Capture  in  Coastal  Plain  Dissection.  —  It  has  been  shown 
that  in  every  coastal  plain  undergoing  dissection  there  is  apt  to 
be  a  master  consequent  stream  which,  because  of  its  greater  water 
supply,  cuts  deeper  and  more  rapidly  than  the  other  consequent 
streams.  The  tributaries  to  such  a  stream  have  the  advantage 
over  the  tributaries  to  another  consequent,  in  that  they  are  enabled 


j   i 


FIG.  612.  —  Diagrams  showing  the  progress  of  river  capture  and  beheading. 
At  first  the  divide  is  halfway  between  the  two  consequent  streams  at  A  ;  but 
the  subsequent  tributary  of  the  stronger  consequent  encroaches  upon  the 
territory  of  the  weaker  one,  pushing  the  divide  to  B ;  continuing,  it  beheads  the 
weaker  stream,  capturing  and  diverting  its  headwaters  (C).  The  beheaded 
stream  now  follows  a  wriggling  course  in  its  old  valley,  which  becomes  much 
obstructed  by  the  debris  from  the  sides  which  this  stream  is  no  longer  able  to 
carry  away. 

to  cut  deeper  and  tend  more  rapidly  to  extend  their  territory  by 
headward  cutting.  Of  two  subsequent  streams  flowing  in  opposite 
directions  into  separate  consequents,  the  one  tributary  to  the 
master  will  have  the  steeper  slope,  and  so  be  enabled  to  extend  its 
drainage  territory  at  the  expense  of  the  other  subsequent,  which  is 
progressively  robbed  by  the  more  powerful  stream.  Thus  while 
the  divide  between  the  two  tributaries  (subsequents)  may  at  first 
have  been  at  A  (Fig.  612),  it  has  gradually  been  pushed  to  B,  and 
finally,  at  C,  tapped  the  headwater  portion  of  the  smaller  con- 
sequent, making  it  a  captured  tributary  to  the  master  stream. 
The  beheaded  portion  of  the  smaller  consequent  will  remain  as  a 
much  diminished  stream  in  its  original  valley.  Since  the  lateral 
insequent  streams  continue  to  bring  in  at  least  the  same  amount 
of  sediment  as  before,  this  must  now  be  partly  dropped  upon 


The  Erosion-Cycle  on  a  Coastal  Plain         721 

the  floor  of  the  valley  occupied  by  the  beheaded  consequent,  be- 
cause this  river  is  no  longer  able,  on  account  of  its  diminished 
water  supply,  to  carry  this  material  away.  Because  of  the  ob- 
structions thus  produced,  the  diminished  stream  of  the  valley  must 
frequently  change  its  course.  To  the  extreme  irregular  windings 
thus  assumed,  the  name  "  staggering  course  "  is  applied,  and  such 
a  course  may  in  general  be  said  to  be  characteristic  of  recently 
beheaded  streams. 

Meanwhile  the  area  appropriated  by  the  master  stream  will  be 
deepened  in  conformity  with  the  depth  of  that  stream,  and  so  the 
inner  lowland  floor  may  be  cut  far  below  the  level  at  which  the 
beheaded  consequent  continues  to  flow,  the  valley  of  this  stream 
ending  abruptly  against  the  much  deeper  subsequent  valley.  This 
is  illustrated  in  the  following  diagram  (Fig.  613),  and  may  be  re- 


FIG.  613.  —  Overdeepened  subsequent.     (Drawn  by  Mary  Welleck.) 

garded  as  a  characteristic  feature  of  the  valleys  of  beheaded  streams 
on  dissected  coastal  plains.  Such  conditions  may  develop  during 
the  first,  and  during  any  later  cycle  of  erosion,  and  as  a  result  the 
form  of  the  drainage  system  of  the  master  stream,  after  it  has 
captured  a  number  of  streams,  will  resemble  that  of  a  well-trained 
grape  vine. 

Waterfalls  in  the  Coastal  Plain  Drainage  System.  —  Large 
waterfalls  are  not  a  characteristic  feature  of  the  coastal  plain 
proper,  though  small  ones  are  formed  in  all  the  streams  which 
flow  over  hard  strata  of  the  coastal  plain  deposits.  Along  the 
head  of  the  coastal  plain,  where  the  extended  consequents  and 
other  streams  pass  from  the  oldland  of  hard  rocks  to  the  low- 
land cut  on  the  soft  belt  of  the  coastal  plain,  a  series  of  waterfalls 
is  likely  to  come  into  existence,  for  the  hard  rocks  of  the  oldland 
permit  less  rapid  erosion  than  do  the  softer  rocks  of  the  valley 
adjoining.  The  inner  border  of  the  coastal  plain  thus  becomes  a 


722        The  Sculpturing  of  the  Earth's  Surface 

"  fall  line,"  and  it  is  here  that  water  power  is  developed,  which 
favors  the  location  of  large  cities  and  towns,  Trenton,  Phila- 
delphia, Washington,  Richmond,  Raleigh,  Camden,  Augusta,  and 
Columbus  are  all  located  upon  the  fall  line  of  the  Atlantic  coastal 
plain  where  it  is  crossed  by  large  rivers. 


THE  EROSION  CYCLE  ON  DOMES  AND  BASINS 
The  Black  Hills  Type 

The  Black  Hills  of  South  Dakota  represent  an  uplift  of  the 
strata  in  the  form  of  a  very  regular  dome  of  oval  outline  100  miles 
long  north  and  south  and  50  miles  broad  east  and  west.  The 
strata  on  the  flanks  are  raised  to  angles  of  45  degrees  or  more,  and 
the  original  height  of  the  dome  was  about  5000  feet  above  the 
surrounding  plain.  The  normal  erosion  cycle  upon  such  a  dome 
proceeds  somewhat  as  follows. 

Radial  Consequents.  —  The  first  type  of  stream  to  come  into 
existence  on  a  newly  raised  dome  of  this  kind  is  a  series  of  radial 
consequent  streams  which  flow  down  the  sides  of  the  dome  in  all 
directions.  At  the  base  of  the  dome  they  become  tributary  to 
one  or  more  streams  which  carry  away  the  drainage,  but  these 
have  no  especial  significance  in  the  development  of  the  drainage 
topography  of  the  dome.  As  the  strata  are  removed  from  the 
summit  of  the  dome  by  erosion  they  gradually  begin  to  form  a 
series  of  rimming  outcrop  belts  around  the  center,  the  highest  or 
youngest  stratum  forming  the  outermost,  and  the  oldest  the  inner- 
most belt.  Wherever  a  hard  bed  of  sandstone  or  limestone  alter- 
nates with  softer  shaley  or  marly  beds,  a  ridge  will  be  formed  from 
the  hard  bed,  which,  because  of  the  steep  inclination  of  the  strata, 
will  have  a  triangular  section,  the  outer  slope  being  formed  by  the 
surface  of  the  hard  stratum  and  the  inner  by  the  cut  edges  of  the 
series.  Such  a  uniclinal  ridge  is  called  a  " hog-back"  (Fig.  631, 
p.  739),  and  the  Black  Hills  are  encircled  by  a  number  of  such 
hog-backs  concentrically  placed  around  the  central  region.  Be- 
tween the  hog-backs  are  circling  valleys  cut  upon  the  softer 
strata,  and  one  of  these,  cut  on  red  shales  and  soft  sandstones,  has 
such  a  regular  form  that  the  Indians  called  it  the  "  race  course." 
Because  of  the  red  color  of  the  rock  it  is  also  known  as  the  Red 
Valley.  The  drainage  of  these  valleys  is  carried  through  a  series 


The  Erosion  Cycle  on  Domes  and  Basins      723 

of  water  gaps  in  the  hog-backs  or  uniclines,  and  these  gaps  form 
the  avenues  of  approach  to  the  central  mountainous  area. 

This  central  area  is  formed  by  the  old  crystalline  rocks,  here 
raised  high  above  the  surrounding  country  and  largely  stripped  of 
their  former  covering  of  sediments.  They  have  been  dissected 
into  a  series  of  peaks  and  ridges,  which  to-day  rise  to  2000  or  3000 
feet  above  the  plain  and  which  by  forcing  the  winds  to  rise  com- 
pel precipitation.  They  are  therefore  clothed  with  dark  forests 
(hence  the  name  Black  Hills),  and  they  form  a  marked  contrast  to 
the  more  arid  rolling  treeless  plains  which  surround  the  hills. 
The  presence  of  precious  metals  in  these  ancient  rocks  has  made 
this  a  center  of  active  mining  operations.  The  following  cross- 
section  shows  the  position  of  the  hog-back-forming  strata  and 
their  former  extent  across  the  dome  (Fig.  614). 


FIG.  614.  —  Cross-section  of  the  Black  Hills  dome,  showing  the  present 
structure  and  topography  and  the  former  continuation  of  the  beds,  i,  Archaean 
slates  and  schists;  2,  granite;  3,  basal  Cambrian  sandstone;  4,  limestone 
(Ordovician  at  base,  Carbonic  for  the  most  part) ;  5,  Red  beds  shales  and 
sandstones  (Triassic)  forming  the  Red  Valley ;  6,  Jurassic  shales,  etc. ;  7,  Dakota 
sandstones;  8,  Cretaceous  shales;  9,  Cretaceous  limestone;  10,  Tertiary 
strata ;  DH,  Dakota  hog-back. 

Flat  Domes 

Ozark  Plateau.  —  The  Ozark  Plateau  of  Missouri  represents  a 
low,  flat,  but  somewhat  asymmetrical  dome  of  great  extent, 
steeper  in  the  south,  and  which  has  been  dissected  so  that  the 
strata  which  once  covered  it  are  now  found  chiefly  as  rimming 
belts  around  the  flanks.  Only  a  small  area  of  the  underlying 
crystalline  rocks  is  exposed  in  the  St.  Francis  Mountains  in  the 
northeastern  part  of  the  dome.  Over  the  greater  part  of  the 
Missouri  area,  the  Cambro-Ordovician  strata,  which  immediately 
overlie  the  crystallines,  form  the  surface  rock  of  the  so-called  Salem 
upland.  These  are  dolomitic  limestones  and  sandstones,  dipping 
at  such  low  angles  that  they  appear  horizontal.  They  are  dis- 


724        The  Sculpturing  of  the  Earth's  Surface 


sected  by  streams  which  in  general  have  a  radial  arrangement, 
partly  with  reference  to  the  St.  Francis  Mountain  area,  but  chiefly 
with  reference  to  the  geographic  center  of  the  dome.  Some  of  the 
valleys  cut  by  these  streams  are  250  feet  or  more  in  depth  (Fig.  615). 
The  northern  slope  of  the  dome  is  characterized  by  a  succes- 
sion of  broad,  flat  plateaus,  separated  by  escarpments  of  irregular 

outline  formed  by 
the  .harder  strata, 
which  here  dip 
northward  at  a  very 
low  angle,  so  that 
the  escarpments 
have  the  character- 
istic appearance  of 
cuesta  fronts.  They 
are  partly  formed  by 
Silurian  and  partly 
by  Mississippian 
limestones.  Many 
outliers  are  found  in 
front  of  the  escarp- 
ments and  mark  the 
progress  of  differ- 
ential retreat.  The 
western  and  south- 


Carbonic       Mississippi&n 


Pre-  Cambrian 
Crystallines 


Silur.&nd 
Upper  Ord 


Tertiary 
Cover 


Carnbrp- 


FIG.  615.  —  Geological  map  of  the  Ozark  dome, 
showing  the  concentric  arrangement  of  the  out- 
cropping formations  and  the  radial  drainage. 


western  margins  of 
the  dome  are  formed 
by  the  Springfield 

Plain,  which  faces  the  center  of  the  dome  in  the  Burlington  es- 
carpment and  is  another  cuesta-like  structural  feature.  It  is  pro- 
duced by  the  harder  Mississippian  limestones  which  dip  gently 
to  the  west  in  Missouri  and  to  the  southwest  in  Arkansas,  the 
slope  of  the  surface  of  the  plain  corresponding  to  the  dip  of 
the  strata.  Outliers  also  characterize  the  front  of  this  escarpment. 
On  the  south,  the  margin  of  the  dome  is  formed  by  the  continua- 
tion of  the  Burlington  escarpment,  and  farther  away  by  the  Boston 
Mountains,  the  rocks  of  which  dip  southward  at  a  somewhat 
greater  angle  than  do  those  of  other  portions  of  the  dome.  The 
front  of  this  ridge  is  a  bold  escarpment  capped  by  a  resistant  layer 
of  sandstone,  and  from  it  many  finger-like  prolongations  extend 


The  Erosion  Cycle  on  Domes  and  Basins      725 

northward,  while  dissociated  remnants  form  outliers  to  the  north 
of  the  main  escarpment.  The  topography  becomes  rougher  toward 
the  east,  where  the  plateau  and  escarpment  characters  are  ob- 
scured. On  the  southeast  the  dome  passes  beneath  the  alluvial 
lands  of  the  Mississippi  River. 

This  dome  differs  from  the  Black  Hills  Dome  in  that  its  rimming 
elements  have  a  cuesta-like  character,  owing  to  the  nearly  hori- 
zontal attitude  of  the  strata,  whereas  the  steeply  inclined  strata 
of  the  Black  Hills  Dome  form  rimming  hog-backs.  Like  the 
Black  Hills  case,  however,  the  strata  which  now  end  in  escarp- 
ments on  the  north,  west,  and  south,  formerly  extended  across  the 
Ozark  dome,  and  have  since  been  removed  from  the  central  part 
by  erosion. 

Ontario  Dome.  —  A  less  perfectly  preserved  low  dome  of  the 
Ozark  type  is  seen  in  the  Ontario  region  north  and  east  of  the 
Great  Lakes.  Over  the  greater  part  of  this  dome  only  the  old 
crystalline  rocks  are  now  exposed,  but  around  its  eastern,  south- 
ern, and  southwestern  border,  and  in  part  on  its  northwestern 
as  well,  the  sedimentary  strata  which  once  covered  it  to  a  large 
extent,  if  not  completely,  are  still  visible,  forming  a  series  of  more 
or  less  rimming  escarpments  which  face  the  center  of  this  dome. 
As  is  the  case  with  all  the  domes  of  eastern  North  America,  this  one 
is  not  in  its  first  cycle  of  dissection,  but  has  been  at  least  once 
reduced  to  the  condition  of  a  peneplane  since  the  end  of  the 
Palaeozoic,  when  its  main  elevation  took  place.  As  a  result,  the 
strata  which  dip  from  5  to  15  feet  per  mile  are  beveled  across 
at  a  low  angle  and  their  outcrops  form  broad  belts  upon  the 
surface.  After  the  peneplanation  a  renewed  upward  movement 
of  the  dome  took  place  in  early  Tertiary  times,  and  this  revived 
the  radial  consequent  drainage.  Many  of  the  valleys  of  these 
revived  radial  consequents  are  still  recognizable,  though  some 
have  been  deepened  by  glacial  erosion  and  transformed  into  lakes, 
while  others  are  filled  by  drift,  or  are  partly  occupied  by  streams 
which  flow  in  the  opposite  direction.  Of  the  over-deepened 
valleys,  those  occupied  by  the  "  Finger  Lakes  "  of  New  York  are 
typical  examples.  Their  striking  radial  arrangement  is  the  direct 
result  of  their  origin  as  radial  consequents,  and  a  series  of  lines 
passed  through  them  and  marking  the  former  courses  of  the  rivers 
would- converge  somewhere  near  the  center  of  the  dome  north  of 
Lake  Ontario  (Fig.  616).  The  waters  of  these  rivers,  then  flowing 


726        The  Sculpturing  of  the  Earth's  Surface 


southward,  were  gathered  by  the  various  branches  of  the  Susque- 
hanna  and  carried  to  the  Atlantic.  The  continuity  of  these  old 
valleys  to  their  junction  with  the  old  valley  of  the  Susquehanna 
can  still  be  traced,  and  so  can  others,  like  that  of  the  Genesee,  now 
occupied  by  northward  flowing  rivers.  Where  not  modified  by 
ice  erosion,  these  valleys  are  often  from  one  to  several  miles  in 

width,  with  gently  slop- 
ing sides  and  flat  bot- 
toms, mostly  covered 
to  some  depth,  by  sands 
and  clays  of  later  origin. 
The  master  stream 
valley  of  the  region  is 
now  entirely  filled  in 
by  glacial  drift,  but  has 
been  traced  by  borings. 
It  passed  through  the 
western  end  of  Lake 
Ontario,  where,  near  the 
city  of  Hamilton,  it  was 
about  three  miles  wide 
and  of  great  depth. 
Its  general  direction  of  flow  was  to  the  southwest  (Figs.  617,  6 1 8). 
Another  of  the  radial  consequent  valleys  can  be  partly  traced 
through  the  gap  in  the  ridge,  which  separates  Georgian  Bay  from 
Lake  Huron,  and  its  southwestern  continuation  was  apparently 
in  the  submerged  valley  which  now  forms  Saginaw  Bay.  The 
waters  of  these  streams  were  gathered  and  carried  to  the  Mississippi 
by  paths  as  yet  only  partially  known. 

In  the  later  cycle  of  erosion  the  outcrops  of  the  softer  beds  of 
this  peneplaned  dome  were  eroded  into  lowlands,  and  the  harder 
beds  formed  new  rimming  cuestas.  One  of  these  lowlands  is 
occupied  by  Lake  Ontario  on  the  south  and  by  Georgian  Bay 
on  the  west  of  the  dome,  the  connecting  portion  being  filled  by 
glacial  drift.  Along  the  southern  border  of  Lake  Ontario  ex- 
tends the  Niagara  escarpment,  which  is  the  inface  of  the  revived 
cuesta.  The  upper  part  of  this  cuesta  rises  as  a  ridge  200 
feet  high  above  the  surface  of  the  lake,  but  the  greater  part  is 
submerged.  The  escarpment  can  be  traced  to  and  around  the 
western  end  of  Lake  Ontario  (where  it  is  breached  by  the  valley 


FIG.  616.  —  Map  of  a  portion  of  central  New 
York,  showing  the  radial  arrangement  of  the 
Finger  Lakes. 


FIG.  617.  —  Map  of  the  ancient  drainage  system  of  the  southern  and  western 
part  of  the  Ontario  dome ;  early  stage.  The  present  hydrography  is  dotted ; 
the  principal  cuesta-fronts  are  indicated. 


FIG.  6 1 8.  —  Map  of  the  ancient  drainage  system  of  the  southern  and  western 
part  of  the  Ontario  dome,  showing  a  later  stage  when  by  capture  the  head- 
waters of  the  other  consequents  have  become  tributary  to  the  Dundas  River. 

727 


728        The  Sculpturing  of  the  Earth's  Surface 

of  the  drift-buried  consequent  above  mentioned  —  the  Dundas 
River)  and  throughout  the  peninsula  which  divides  Georgian  Bay 
from  Lake  Huron,  continuing  beyond  the.  gap,  in  Manitoulin 
Island.  A  second  escarpment,  formed  by  a  higher  limestone  bed 
(the  Onondaga),  can  be  traced  east  and  west  through  the  city  of 
Buffalo,  across  western  Ontario  in  a  northwest  direction,  where 
it  is  chiefly  buried  by  glacial  drift,  and  diagonally  across  Lake 
Huron  from  near  Goderich,  Ontario,  to  Mackinac  Island.  Even 
where  submerged,  it  is  clearly  shown  by  soundings  in  Lake  Huron 
to  have  a.  height  of  about  400  feet  (Fig.  619).  A  third  escarp- 
ment forms  the  front  of  the  Alleghany  Plateau  in  southwestern 
New  York,  and  extends  along  the  western  margin  of  Lake 
Huron.  The  lowland  in  front  of  it  is  partly  occupied  by  the  east- 


FIG.  619.  —  Cross-section  of  Lake  Huron  from  Point  au  Sable,  (a)  Michigan, 
across  nine  fathom  ledge^i)  to  Cape  Hurd,  (c)  Canada,  showing  the  submerged 
Onondaga  cuesta  and  the  lowland  in  front  of  it. 

ern  end  of  Lake  Erie.  The  general  relationship  of  the  radial 
streams  and  revived  cuestas  is  shown  in  the  diagram  on  page  760 
(Fig.  653). 

These  three  escarpments  can  be  traced  eastward  through  New 
York  state,  gradually  approaching  one  another  as  the  Mohawk 
River  is  reached,  because  the  softer  beds,  which  form  the  lowlands 
on  the  west,  die  out  eastward.  They  finally  unite  to  form  the 
ridge  known  as  the  Helderberg  Mountains  in  eastern  New  York, 
culminating  in  the  Catskill  Mountains,  which  project  as  a  group 
of  monadnocks  above  the  surface  of  the  peneplane. 

These  various  cuestas  and  lowlands  have  the  relation  to  the 
center  of  the  Ontario  Dome  that  normal  cuestas  and  lowlands  have 
to  their  oldland,  and  they  are  sometimes  referred  to  as  normal 
cuestas  in  the  second  cycle  of  erosion.  They  are,  however,  the 
remnants  of  the  rimming  strata  of  a  peneplaned  dome,  which,  be- 
cause of  further  elevation  of  that  dome,  have  undergone  renewed 
dissection  with  the  revival  of  an  incomplete  rimming  cuesta 
topography. 

Nashville  Dome.  —  As  a  final  example  of  a  dissected  dome  of 
gently  dipping  strata,  the  Nashville  Dome  of  central  Tennessee 


The  Erosion  Cycle  on  Domes  and  Basins       729 

may  be  noted.  Although  a  structural  dome,  it  now  forms  a 
topographic  basin,  the  Central  Basin  of  Tennessee.  This  is  due 
to  the  fact  that  after  the  peneplanation  of  this  dome,  the  central 
area  exposed  only  soft  strata,  and  these  were  largely  cut  away 
down  to  a  lower  series  of  hard  strata  by  the  tributaries  of  the 
radial  streams  which  dissected  the  dome  on  renewal  of  uplift.  The 
chief  of  these  streams  is  the  Tennessee  River,  which  has  cut  a 
narrow,  gorge-like  valley  in  the  surrounding  rim.  The  rim  is  con- 
tinuous except  for  the  breaches  by  the  radial  streams,  which  lie 
chiefly  upon  the  western  side.  It  is  formed  by  the  edges  of  the 
harder  strata  which  overlie  the  softer  ones  not  far  from  their 
position  of  outcrop  after  the  peneplanation.  The  central  de- 
pression (the  topographic  basin)  is  about  70  miles  across,  and 


FIG.  620.  —  Section  of  the  Nashville  dome,  showing  the  center  eroded  into  a 
topographic  basin  or  encircled  lowland,  the  so-called  Central  Basin  of  Tennessee. 
C,  Cambrian ;  Ot,  Ordovician  (Trenton)  limestone ;  On,  Ordovician  (Nashville) 
shale;  Sn,  Silurian  (Niagaran)  limestone;  Db,  Devono-Mississippian  shale; 
Ms,  Mississippian  sandstone;  Mlm,  Mississippian  (Mountain)  limestone; 
C,  coal  measures;  Length  of  section  about  120  miles.  (After  Safford.) 

its  bottom,  which  is  about  600  feet  above  sea-level,  is  formed  by 
the  lower  series  of  hard  rocks  and  has  a  gently  undulating  surface 
(Fig.  620).  Outliers  from  the  surrounding  plateau  characterize  it 
in  many  places  as  we  approach  the  rimming  margin.  The  Cin- 
cinnati Dome  is  a  larger  but  less  perfect  example  of  similar 
character. 

Shallow  Basins 

Shallow  structural  basins  are  the  complements  of  the  flat 
domes  with  which  they  are  commonly  associated.  In  America 
the  best  example  is  found  in  the  lower  peninsula  of  Michigan 
(Michigan  Basin),  while  in  Europe  the  Paris  Basin  is  the  best 
known  example.  Both  of  these  represent  basins  not  in  the  first 
cycle  of  erosion,  for  both  have  suffered  peneplanation  and  the 
present  topography  is  a  revival  in  the  second  or  later  cycle.  As 
a  result  of  this,  the  strata  now  form  a  succession  resembling  a 
nest  of  plates,  the  largest  at  the  bottom  and  the  smallest  at  the 
top,  the  successive  edges  of  the  plates  forming  the  outer  rims  of 
the  several  formations. 


730        The  Sculpturing  of  the  Earth's  Surface 

By  the  excavation,  during  the  second  cycle,  of  subsequent  valleys 
upon  the  outcrops  of  the  softer  beds  there  are  produced  con- 


centric annular  lowlands.  The  outcropping  edges  of  the  hard 
strata  form  cuesta  ridges,  the  escarpments  of  which  face  out- 
ward in  all  directions.  This  is  especially  well  seen  in  the  eastern 


The  Erosion  Cycle  on  Domes  and  Basins      731 

part  of  the  Paris  Basin,  where  a  succession  of  such  outward- 
facing  escarpments,  separated  by  broad  flat  valleys,  marks  the 
approach  from  Germany  towards  Paris,  which  lies  in  the  center 
of  the  basin.  These  escarpments  form,  indeed,  a  series  of  huge 
steps  up  which,  or  through  the  gaps  in  which,  invading  armies 
from  the  east  must  fight  their  way;  while  the  gentle  slope  from  the 
top  of  each  step  toward  the  foot  of  the  next  inner  one,  and  finally 
toward  Paris,  makes  the  approach  from  the  west  an  easy  one 
(Fig.  621).  The  heights  of  Nancy  and  Metz  form  the  eastern- 
most of  these  escarpments  (Middle  Jurassic).  Beyond  them, 
nearer  Paris,  lies  the  Woevre  lowland,  west  of  which  rises  the  second 
escarpment  (Upper  Jurassic),  which  has  to  be  ascended  to  reach 
Verdun.  Still  farther  west  lies  the  Wet  Champagne  lowland  sepa- 
rated by  the  chalk  escarpment  from  the  Dry  Champagne  lowland, 


FIG.  622.  —  Cross-section  of  the  Michigan  Basin  showing  the  rimming 
cuestas  which  are  largely  submerged.  Or,  Ordovician;  N,  Niagaran;  5  & 
M,  Salman  and  Monroan;  On,  Onondaga;  Tr,  Traverse;  A,  Antrim  black 
shale ;  M ,  Mississippian ;  C,  Coal  Measures. 

which  in  turn  is  bounded  on  the  west  by  the  Rheims-Epernay- 
Sezanne  escarpment  (Tertiary),  the  last  to  be  ascended  before  reach- 
ing the  center  of  the  basin  and  Paris.  These  escarpments  are  also, 
though  less  perfectly,  developed  on  the  south  and  west,  but  on  the 
northwest  they  are  obliterated  by  the  English  Channel.  Between 
the  Ardennes  Mountains  and  Calais  on  the  coast  they  are  not 
developed  because,  owing  to  a  tilt  in  the  basin,  erosion  has  not 
gone  deep  enough.  Hence  the  approach  to  Paris  through  Belgium 
is  the  only  one  which  avoids  these  great  step-like  natural 
defenses. 

The  center  of  the  Michigan  Basin,  in  which  Alma  lies,  is  sur- 
rounded by  a  similar  series  of  outward-facing  escarpments,  but 
owing  to  the  heavy  glaciation  of  the  region  the  lowland  valleys 
are  generally  filled  with  drift  and  obliterated,  while  others  are 
occupied  by  the  waters  of  Lakes  Huron  and  Michigan.  If  these 
lakes  were  drained,  central  Michigan  would  be  well  defended  by 
cliffs  from  Canadian  and  Wisconsin  approaches,  though  ,  easily 
accessible  from  the  south  (Fig.  622).  (See  also  Fig.  516,  p.  600.) 


732        The  Sculpturing  of  the  Earth's  Surface 


THE  EROSION  CYCLE  ON  ANTICLINES  AND  SYNCLINES 

The  First  Cycle.  —  The  erosion  of  anticlines  proceeds  much 
after  the  manner  of  that  on  steep  domes,  except  that  the  initial 
consequent  drainage  is  not  radial,  but  consists  of  a  series  of  parallel 
streams  on  either  side  of  the  anticline  and  flowing  away  from 
its  axis.  These  lateral  consequents  carry  the  drainage  into  the 
synclines,  where  the  initial  longitudinal  consequent  streams,  i.e., 
those  flowing  along  the  axis  of  the  fold,  are  situated  (Fig.  623). 
Only  at  the  ends  of  the  anticlines,  where  these  flatten  out  or  pitch, 
is  the  radial  form  of  initial  drainage  partly  realized. 


FIG.  623.  —  A  longitudinal  consequent  valley  in  a  syncline,  with  hog-backs  or 
uniclines  facing  outward  on  both  sides. 

As  the  consequent  streams  cut  into  the  flanks  of  the  anticlines, 
they  will  develop  gorges,  along  the  sides  of  which  insequent  streams 
arise,  especially  where  the  strata  become  flat  on  the  axis.  In  the 
course  of  time  there  will  be  opened  a  longitudinal  breach  in  the 
axial  portion  of  the  anticline  (Fig.  624,  I).  If  a  hard  stratum 
forms  the  capping  rock,  with  a  softer  one  beneath  it,  this  breach 
will  be  enclosed  by  cliffs  broken  only  by  the  narrow  gaps  through 
which  the  consequents  flow.  Continued  erosion  will  not  only 
lengthen  this  breach  on  the  axis  (Fig.  624,  II),  but  will  push  the 
sides  farther  and  farther  apart,  that  is,  down  the  flanks  of  the 
anticline.  If  a  second  hard  stratum  is  discovered  on  the  axis  of 
the  anticline,  this  will  form  a  central  ridge,  flanked  on  either  side 
by  longitudinal  subsequent  valleys. 


The  Erosion  Cycle  on  Anticlines  and  Synclines     733 

Further  erosion  may  open  up  a  new  longitudinal  valley  in  the 
top  of  this  axis,  at  the  same  time  pushing  the  sides  of  the  older 


FIG.  624.  —  Diagrams  showing  the  development  of  river  systems  in  anticlinal 
folds  with  anticlinal  valleys  as  the  end  product  (IV).  As  a  result  of  this  erosion 
the  streams  flowing  in  the  synclinal  valleys  are  diverted  by  capture  and  the 
main  streams  become  anticlinal.  (After  de  Martonne.) 

valleys  farther  down  the  flanks.  Thus  a  series  of  uniclinal  ridges 
is  produced,  facing  the  center  of  the  original  anticline.  The 
drainage  system  produced  will  at  first  comprise  longitudinal  subse- 


734        The  Sculpturing  of  the  Earth's  Surface 

quent  streams  which  occupy  the  valleys  on  the  softer  strata  parallel 
to  the  ridges,  and  transverse  consequents  passing  through  gaps 
in  the  ridges  to  the  longitudinal  consequent  streams  which  flow 
down  the  axis  of  the  syncline.  Some  of  the  larger  transverse 
consequent  streams  may  cut  so  deep  that  their  tributary  sub- 
sequents  will  encroach  upon  the  territory  of  adjoining  weaker 
consequents,  which  they  may  finally  behead,  carrying  the  captured 
drainage  to  the  master  stream.  If  this  happens,  the  original 
water-gap  through  the  ridge  formed  by  the  weaker  consequents, 
will  be  abandoned  below  the  elbow  of  capture  and  become  a  dry 
notch  in  the  ridge.  It  is  then  known  as  a  wind-gap. 

In  the  following  diagram  (Fig.  625)  a  stream  (Q)  flowing  towards 


FIG.    625.  —  Diagram  illustrating  the  imminence  of  river  capture.      (After 

Davis.) 

the  anticlinal  valley  on  the  right  across  two  hard  strata  G,  J,  is 
encroached  upon  by  another  flowing  upon  soft  strata  H,  the  valley 
of  which,  as  shown  by  the  figures,  is  much  lower.  In  the  next 
stage,  shown  in  diagram  Fig.  626,  the  capture  of  the  upper  part 
of  the  stream  (Q)  has  been  completed. 

When  the  breaching  of  the  anticline  has  proceeded  as  far  as 
the  altitude  of  the  land  with  reference  to  the  base-level  of  erosion 
permits,  a  series  of  parallel  uniclines  or  hog-back  ridges  and 
longitudinal  strike- valleys,  with  generally  a  central  axial  or 
anticlinal  valley,  has  been  produced,  the  cut  edges  of  the  ridges 
successively  facing  the  center  of  the  anticline.  Further  erosion 
will  result  in  the  lowering  of  these  ridges,  and  when  they  have 


The  Erosion  Cycle  on  Anticlines  and  Synclines     735 

nearly  disappeared,  a  peneplane  will  be  formed,  the  surface  of 
which  will  be  marked  by  a  series  of  parallel  belts  of  alternating 
hard  and  soft  strata ;  but  none  of  these  produce  marked  relief 
features.  Where  the  ends  of  the  anticline  pitch  beneath  the  erosion 
surface,  the  corresponding  strata  will  curve  around  from  opposite 
sides,  joining  along  the  axial  line,  or  better,  a  continuous  curve 
will  unite  the  corresponding  strata  on  opposite  sides,  forming  canoe- 
shaped  valleys  (see  Figs.  508,  p.  595,  and  514,  p.  598).  As  the  hard 
strata  which  confined  the  longitudinal  subsequents  to  their  re- 
spective valleys  are  reduced  to  the  level  of  the  peneplane,  these 
streams  may  shift  their  location,  meandering  aimlessly  across  hard 


FIG.  626.  —  Diagram  showing  the  same  region  with  the  capture  completed. 

(After  Davis.) 

and  soft  strata  alike.    Thus  the  initial  character  of  the  drainage 
is  destroyed. 

The  Second  Cycle.  —  If  a  peneplaned  region  of  anticlinal  and 
synclinal  structure  is  elevated,  rejuvenation  of  the  streams  will 
take  place,  and  these  will  begin  to  cut  downwards  where  they  were 
located  at  the  time  of  the  uplift  (Fig.  627).  If,  as  generally 
happens,  the  surface  of  the  peneplane  becomes  warped  or  slightly 
tilted  on  uplift,  the  direction  of  the  new  drainage  will  be  deter- 
mined by  the  slope  thus  produced,  and  the  streams  may  begin  to 
cut  downward  across  hard  and  soft  strata  alike.  So  soon  as  a 
master  stream  has  begun  to  incise  its  bed  beneath  the  surface  of 
the  peneplane,  lateral  streams  will  begin  to  open  out  valleys 
along  the  softer  strata,  leaving  the  harder  in  relief,  and  thus  the 


736        The  Sculpturing  of  the  Earth's  Surface 

original  topography  of  longitudinal  valleys  and  parallel  uniclinal 
ridges  will  be  revived  (Fig.  628).  By  capture  of  the  weaker 
streams  which  flow  across  the  strata,  wind  gaps  are  produced, 
and  the  drainage  all  becomes  tributary  to  the  master  stream  or 
streams  of  the  region. 

Recognition  of  the  Second  Cycle.  —  How  can  we  recognize 
that  a  dissected  anticlinal  region  is  in  the  second  cycle,  when  the 
ridges  all  stand  out  in  bold  relief  and  the  valleys  are  occupied  by 
longitudinal  streams,  tributary  to  a  transverse  consequent  which 


FIG.  627. — A  stream  incised  upon  the  axis  of  a  broad  anticline,  where  it 
happened  to  be  located  at  the  beginning  of  a  new  cycle  of  erosion. 

has  cut  gaps  for  its  passage  across  the  successive  ridges?  If  we 
were  dealing  with  a  single  anticline,  it  would  probably  be  difficult 
to  determine  the  cycle,  though  here,  too,  certain  guiding  principles 
might  help  to  solve  the  problem.  In  the  first  cycle,  the  drainage 
from  the  corresponding  longitudinal  valleys  on  opposite  sides  of 
the  axis  is  carried  outward  in  opposite  directions.  It  is,  however, 
conceivable  that  by  capture  the  entire  drainage  system  of  one  side 
might  be  reversed  and  carried  out  in  the  opposite  direction.  While 
this  might  happen  in  the  case  of  one  anticline,  it  is  extremely 
doubtful  that  by  capture  the  entire  drainage  can  become  tributary 
to  a  stream  which,  as  the  result  of  such  capture,  crosses  two  anti- 
clines and  all  the  uniclinal  ridges  derived  from  them.  Still  more 


The  Erosion  Cycle  on  Anticlines  and  Synclines     737 

doubtful  does  it  become  when  the  number  of  parallel  anticlines 
is  more  than  two.  If  a  master  stream  has  cut  gap  after  gap  across 
all  the  uniclinal  ridges  of  several  parallel  anticlines,  carrying  the 
drainage  of  all  the  longitudinal  valleys  and  of  the  original  syncline 
as  well,  it  is  practically  certain  that  this  stream  was  not  built  up 
from  progressively  captured  portions  of  other  streams,  but  that 
it  represents  a  new  stream  formed  upon  the  surface  of  a  sloping 
peneplane.  This  stream  cuts  its  channel  regardless  of  hard  and 
soft  strata;  for  there  is  no  chance  for  readjustment  during  the 
cutting  of  the  channel,  because  such  a  stream  controls  the  drain- 


FIG.  628.  —  Diagram  showing  the  development  of  a  peneplane  from  anti- 
clinal ridges  and  the  reappearance  of  the  uniclinal  topography  after  elevation 
and  renewed  erosion.  This  is  the  type  of  structure  found  in  the  Appalachians. 
(After  Davis.) 

age  of  the  region,  and  no  valley  can  be  cut  by  any  of  its  tributaries 
which  is  lower  than  the  floor  of  the  valley  of  the  master  stream. 

It  should,  however,  be  noted  that  many  if  not  most  of  the 
phenomena  here  cited  may  be' explained  by  the  hypothesis  of  an 
antecedent  stream  in  the  first  cycle;  but  there  are  other  criteria 
which  will  serve  to  determine  the  applicability  of  that  hypothesis. 
Antecedent  streams  are  discussed  in  a  later  section. 

It  is  because  of  the  existence  of  a  number  of  such  transverse 
streams,  which  cut  across  all  the  ridges  of  the  region,  and  more- 
over, have  a  meandering  course,  that  the  Appalachian  Mountains 
are  believed  to  be  in  at  least  a  second  cycle  of  erosion  (Fig.  629). 
The  Susquehanna  is  the  most  prominent  of  these  transverse  streams, 
and  its  tributaries  have  opened  up  again  a  large  part  of  the  valley 
system  of  the  present  Appalachians  of  Pennsylvania,  etc.  This 
opening  of  the  Appalachian  valleys  was  essentially  coincident  with 
the  opening  of  the  cuesta-like  topography  of  the  Ontario  and  other 


738        The  Sculpturing  of  the  Earth's  Surface 

domes  to  the  north,  and  the  drainage  accomplishing  the  latter 
was  in  part  carried  out  by  the  Susquehanna. 

As  a  result  of  this  erosion  in  the  second  cycle,  the  axes  of  the 
original  synclines,  now  high  above  the  new  base-level  of  erosion, 
have  often  been  left  in  relief  by  the  cutting  of  the  valleys  on  either 
side  to  a  greater  depth.  Thus  synclinal  mountains  are  actually 
produced,  these  being  a  characteristic  feature  of  some  parts  of  the 
Appalachian  system. 

When  the  center  of  the  anticline  consists  of  hard  rocks,  erosion 
progressing  in  the  manner  outlined  will  leave  a  central  ridge  in- 


FIG.  629.  —  Map  of  Northern  Appalachians.     (U.  S.  G.  S.) 

stead  of  a  valley.  This  central  ridge  seen  in  parts  of  the  Appala- 
chians will  be  bordered  on  either  side  by  uniclinal  ridges  formed 
by  the  hard  strata  of  the  sedimentary  series  (Fig.  628).  Such 
uniclinal  ridges  or  hog-backs,  as  they  are  called  (Fig.  630,  I,  II), 
which  have  been  developed  in  the  first  cycle  of  erosion,  may  be- 
come obliterated  again  when  the  region  approaches  a  condition 
of  peneplanation.  Renewed  elevation  will  steepen  the  inclination 
of  the  layers  (Fig.  630,  III),  after  which  erosion  will  once  more 
model  out  the  hog-back,  or  uniclinal  topography  (Fig.  630,  IV). 

This  has  been  essentially  the  later  history  of  the  Rocky  Moun- 
tain Front  Range,  though  by  the  further  development  of  thrust 
faults  this  history  has  been  rendered  more  complex.  The  central 


The  Erosion  Cycle  on  Anticlines  and  Synclines     739 


axis  of  the  range  consists  of  crystalline  rocks  (granites,  etc.),  and 
is  margined  on  either  side  by  pronounced  hog-backs,  formed  by 
the  harder  strata  of  the 
Mesozoic  series  which 
once  extended  across 
the  crystallines.  Typical 
examples  of  such  hog- 
backs are  shown  in  Figs. 
63 1 ,  63  2 .  The  upturned 
strata  along  the  moun- 
tain front,  will,  if  they 
are  sandstones  or  other 
porous  rocks,  take  in 
surface  waters  and  so 
form  a  head  for  artesian 
conditions  farther  out 
in  the  plains,  as  shown 
in  the  section  of  the 
Dakota  artesian  system 


FIG.  630.  —  Diagrammatic  sections  represent- 
ing the  development  of  hog-backs.  I,  a  simple 
anticlinal  ridge  consisting  of  hard  and  soft  .sedi- 
ments overlying  crystalline  rocks ;  II,  the  same 
after  erosion  and  development  of  hog-backs 
(uniclines)  on  opposite  sides;  III,  the  same 
region  after  peneplanation  and  further  arching ; 
IV,  Development  of  the  hog-backs  in  the  second 
cycle  of  erosion. 


on  p.  424  (Fig.  353). 

As    the    strata    are 
flexed  only  near  the  mountains,  and  change  toward  horizontality 
away  from  these,  it  is  evident  that  as  the  hog-back  is  pushed 


FIG.  631. — Hog-back  of  Dakota  sandstone,  near  Canyon  City,  Colorado. 
The  ridge-making  hard  stratum  dips  steeply  to  the  right.  A  second,  minor 
ridge  is  formed  by  another  hard  sandstone  on  the  right.  Valley  of  the 
Graneros  River.  (C.  D.  Walcott,  photo,  U.  S.  G.  S.) 


740        The  Sculpturing  of  the  Earth's  Surface 

farther  away  from  the  mountain  by  erosion,  the  steepness  of  the 
dip  of  its  strata  will  decrease,  until  it  becomes  so  low  that  the 


FIG.  632.  —  Hog-back  near  Colorado  City,  Colorado.     (I.  C.  Russell,  photo.) 

beds  have  the  appearance  of  gently  dipping  coastal  plain  strata, 
and  the  hog-back  changes  to  a  cuesta.  Still  farther  away,  where 
the  strata  are  absolutely  horizontal,  a  step  topography  is  produced 


FIG.  633.  —  Hog-backs  RR,  changing  into  cuestas  (CC)  and  these  into 
steps  (HH)  by  progressive  flattening  of  strata ;  UU,  oldland ;  7T,  inner  low- 
land; SS,  second  lowland;  P,  plateau  between  steps;  V,  nose;  Z,  outlier. 
(After  Davis.) 

by  the  edges  of  the  hard  formations.  In  some  cases  all  of  these 
stages  may  be  observed  along  a  line  of  outcrop,  as  shown  in  the 
preceding  diagram  (Fig.  633)  where  the  hog-backs  (RR)  pass  into 


The  Erosion  Cycle  on  Anticlines  and  Synclines     741 

cuestas  (CC)  and  these  into  steps  (HH).  Such  a  change  is  also 
seen  along  the  present  eastern  front  of  the  Appalachians,  where 
the  Helderberg  front,  of  the  cuesta  type,  passes  southward  into 
the  uniclinal  ridge  through  which  the  Delaware  River  has  cut  its 
water-gap  (Fig.  634). 

Complexly  Folded  Strata.  —  When  erosion  attacks  complexly 
folded  strata,  the  process,  though  following  the  general  laws  ob- 
served in  the  case  of  simple  folds,  is  correspondingly  more  com- 
plicated, and  the  erosion  forms  produced  are  more  diverse.  In- 


FIG.  634.  —  View  of  Delaware  Water-Gap  from  the  Great  Valley,  showing 
the  even  sky  line  of  the  mountains  and  the  abrupt  cut  of  the  water  gap.  (U.  S. 
G.S.) 

stead  of  long,  regular  ridges,  such  as  those  of  the  Appalachians, 
individual  peaks  and  irregular  ridges,  such  as  those  of  the  White 
Mountains,  the  Alps,  the  Caucasus,  and  others,  result.  In  these, 
moreover,  glacial  erosion  has  aided  in  the  sculpturing  process,  as 
will  be  set  forth  later.  Peneplanation  of  such  a  region  is  apt  to 
leave  peaks  of  harder  rock  standing  above  the  peneplane  level  as 
monadnocks.  Such  is  Mt.  Monadnock  in  southern  New  Hamp- 
shire, the  type  of  residual  erosion  peaks,  which  from  some  points 
of  view  has  almost  the  regularity  of  outline  of  a  volcanic  cone 
(Fig.  635). 


742        The  Sculpturing  of  the  Earth's  Surface 


FIG.  635.  —  The  New  England  peneplane  with  Mt.  Monadnock  (New 
Hampshire)  rising  above  it.  From  Be^ch  Hill,  Keene,  N.  H.  Tertiary  valleys 
in  the  foreground.  (Gardner  Collection  of  Photographs,  2634.  Courtesy 
Geological  Department,  Harvard  University.) 


FIG.  636.  —  Glen  Coe,  a  glaciated  stream  valley  in  the  Scottish  upland. 
The  glens  of  Scotland  are  splendid  glacial  troughs,  of  which  Glen  Coe  is  a  typical 
example.  (Courtesy  of  D.  W.  Johnson.) 


The  Erosion  Cycle  on  Anticlines  and  Synclines     743 


The  New  England  peneplane  was  uplifted  with  a  tilt,  so 
that  there  is  a  rise  of  the  surface  inland.  Large  portions  of 
the  peneplane  are  still  intact,  forming  broad  uplands  beneath 
which  the  streams  have  intrenched  their  courses.  The  Scottish 
Highlands  region,  on  the  other  hand,  represents  a  much  more 
thoroughly  dissected  peneplane,  so  that  the  upland  has  been 
reduced  to  narrow  ridges  between  the  glens,  which  are  the 
product  of  a  later  cycle  of  erosion  by  streams  and  glaciers 
(Fig.  636). 

Antecedent  Streams.  —  Streams  cutting  across  anticlinal  or 
complex  mountain  systems  are  not  necessarily  inherited  from  a 
former  cycle  or  devel- 
oped upon  a  peneplane 
surface.  They  may 
represent  streams 
across  the  path  of 
which  a  series  of  folds 
or  fault  blocks  have 
arisen  at  such  a  slow 
rate  that  the  down- 
ward cutting  of  the 
stream  kept  pace  with 
the  rising  of  the  folds 
or  other  structures 
which  would  otherwise 
have  diverted  the 
stream.  Such  streams 
are  called  antecedent, 
because  they  existed 
in  the  region  before  the  structure  in  question  made  its  appearance. 
The  Columbia  River  is  believed  to  have  such  an  antecedent  re- 
lation to  the  Cascade  Mountains  which  rise  across  its  course  and 
are  cut  by  it.  The  Meuse  River  of  France  is  also  believed  to  be 
antecedent  with  reference  to  the  Ardennes  highland,  through  which 
it  cuts  a  deep,  narrow  gorge  or  canon.  Although  the  uprising 
of  this  highland  of  ancient  strata  across  the  river  path  was  so 
slow  that  the  Meuse  was  able  to  keep  pace  with  it  by  cutting 
downward,  and  so  maintain  its  course,  it  lost  its  tributaries  which 
were  diverted  by  capture  to  other  streams  not  so  handicapped 
(Figs.  637,  638). 


FIG.  637.  —  Map  of  the  ancient  Meuse  when 
the  Toul,  now  diverted,  formed  a  part  of  it.  The 
Moselle  of  that  time  included  the  Meurthe  (a 
tributary  to  the  modern  Moselle)  and  the  Pom- 
pey,  a  tributary  of  that  time,  but  now  with  the 
Toul  forming  a  part  of  the  modern  Moselle  (see 
Fig.  638).  (After  Davis.) 


744        The  Sculpturing  of  the  Earth's  Surface 


Superimposed  Streams.  —  There  is  still  another  way  in  which 
streams  can  be  forced  to  cut  across  rock  structures,  hard  and  weak 
alike,  without  being  able  to  adjust  themselves,  by  selective  erosion, 
to  the  weaker  structures.  This  is  effected  where  a  country  of 
diverse  structure  is  covered  by  a  deposit  of  coastal  plain  strata 
which  present  a  new  surface  upon  which  rivers  may  develop  irre- 
spective of  the  character  of  the  underlying  rock.  Having  ac- 
quired a  certain  course,  determined  by  the  slope  of  the  coastal 

plain  strata,  the  river 
will  be  forced  to  con- 
tinue downward  cut- 
ting into  the  underly- 
ing rocks,  no  matter 
how  diverse  their  struc- 
ture. Eventually  the 
coastal  plain  strata 
may  be  largely  or 
wholly  removed  by 
eiosion,  revealing  the 
underlying  rock  and  the 
topographic  features 
which  marked  its  sur- 
face before  the  coastal 
plain  strata  were  de- 
posited upon  it,  and  these  may  be  entirely  out  of  harmony  with 
the  course  of  the  river  superimposed  upon  them.  The  lower  course 
of  the  Connecticut  through  the  crystalline  rocks  of  the  New 
England  upland  is  believed  to  be  due  to  superimposition  of  the 
course  acquired  upon  a  former  covering  of  coastal  plain  strata, 
now  wholly  removed  from  that  region  by  erosion. 

Special  cases  of  superimposed  streams  are  occasionally  met  with. 
One  of  the  most  interesting  is  that  of  the  famous  Lake  District  of 
Northwestern  England.  This  is  a  roughly  oval  area,  formed  of 
complexly  folded  older  Palaeozoic  shales,  sandstones,  and  igneous 
rocks,  with  many  high  mountain  peaks  formed  by  resistant  strata, 
between  which  the  river  valleys  expand  in  a  series  of  beautiful 
long  and  narrow  lakes  which  have  made  the  region  famous.  The 
area  is  almost  completely  surrounded  by  belts  of  late  Palaeozoic 
and  younger  strata,  including  the  Carboniferous  limestone  of  the 
English  geologists  (Mississippian  age),  the  Coal  Measures  which 


/W  E,  U 


FIG.  638.  —  Map  of  the  Meuse  and  Moselle, 
showing  the  formation  of  the  modern  Moselle 
by  capture  of  former  tributaries  to  the  Meuse. 
(After  Davis.) 


The  Erosion  Cycle  on  Anticlines  and  Synclines     745 

overlie  these,  the  next  higher  Permian  sandstones,  and  finally, 
on  the  west,  the  New  Red  sandstone  beds  (Triassic).     Wherever 


FIG.  639.  —  Map  of  the  Lake  District,  showing  radial  drainage.  The  dotted 
area  represents  Carboniferous  and  younger  rocks,  gently  dipping  away  from 
the  center  of  the  dome.  The  plain  area  is  formed  by  Silurian  and  Ordovician 
strata  much  deformed  and  with  a  general  N.  E.  and  S.  W.  strike.  (From  Lake 
and  Rastall.  Textbook  of  Geology.) 

these  rocks  rest  upon  the  older  Palaeozoics,  they  are  found  to  have 
an  unconformable  relationship,  these  younger  rocks  being  but 
slightly  disturbed,  while  the  older  ones  are  strongly  so.  More- 
over, with  negligible  exceptions,  these  younger  rocks  dip  away  in 


746        The  Sculpturing  of  the  Earth's  Surface 

all  directions  from  the  center  of  the  area  which  they  surround,  and 
this  indicates  that  the  region  was  a  dome  once  covered  by  these 
younger  strata  which  have  since  been  eroded  from  the  central 
area.  The  radial  drainage  developed  upon  this  dome  has  become 
superimposed  upon  the  underlying  rocks  of  complex  structure  as 
they  were  uncovered,  and  hence,  despite  this  complex  structure 
and  the  general  northeast  and  southwest  strike  of  the  folds  of 
the  older  strata,  the  drainage  is  a  radial  one,  wholly  out  of  har- 
mony with  the  rock  structure  of  the  region,  except  locally  where 
adjustment  has  taken  place.  There  can  be  no  doubt  that  the 
general  direction  of  these  streams  is  inherited  from  their  courses  on 
the  domed  strata  which  formerly  covered  this  region,  and  along  the 
margins  of  the  area,  where  they  still  lie  within  these  younger 
strata,  they  have  the  normal  arrangement  of  radial  consequents 
(Marr).  The  lakes  in  these  river  valleys  owe  their  origin  in  part  to 
glacial  erosion  and  obstruction  by  drift. 

The  map  on  the  preceding  page  (Fig.  639),  copied  from  Lake 
and  Rastall,  shows  the  radial  arrangement  of  the  lakes  of  this 
district  and  the  streams  which  determined  their  position. 


CHAPTER  XXIII 

THE    SCULPTURING     OF    THE    EARTH'S     SURFACE 

(Continued) 

THE  EROSION  CYCLE  IN  A  FAULTED  REGION 

A  REGION  that  has  -been  subjected  to  extensive  faulting,  presents 
some  interesting  conditions  which  exercise  a  controlling  influence 
upon  the  topography  produced  by  erosion.  We  can  consider  only 
the  simpler  types  of  features  produced  by  faulting,  but  they  will 
serve  to  point  the  general  principles  of  which  cognizance  must 
be  taken  in  the  interpretation  of  the  land  forms  produced  in  a 
faulted  region  by  the  agencies  of  erosion. 

The  Fault  Block  or  Block  Mountain 

Characteristics  of  Block  Mountains.  —  In  certain  regions  the 
crust  of  the  earth  appears  to  have  been  broken*  into  a  series  of 
parallel  blocks,  and  by  what  appears  to  be  a  tilting  movement  of 
these  blocks  one  side  has  been  raised  and  the  other  depressed, 
resulting  in  the  formation  of  parallel  ridges  'and  valleys  of  tri- 
angular cross-section,  each  characterized  by  a  long,  gentle  slope 
on  one  side  and  a  short  abrupt  slope  on  the  other.  This  type  of 
block  faulting,  as  it  is  called;  may  be  illustrated  by  placing  upon 
the  table  a  row  of  books  or  blocks  of  the  same  size  and  thickness, 
and  then  tilting  the  whole  series  in  one  direction,  when  the  upper 
edges  of  the  books  or  blocks  will  produce  a  series  of  ridges  and  val- 
leys of  the  type  described  (Fig.  640  a,  b),  although  it  must  be  clearly 
borne  in  mind  that  such  regularity  does  not  exist  in  nature.  When 
these  tilted  fault  blocks  are  large  enough,  they  constitute  block 
mountains.  In  a  region  of  horizontal  strata  such  block  faulting 
will  produce  a  succession  of  hog-back-like  ridges  (Fig.  640  c  d.) 
but  these  differ  from  the  hog-backs  of  erosion,  the  true  uniclines 
produced  upon  the  side  of  an  anticline,  in  the  fact  that  each  of  the 
tilted  fault  blocks  shows  the  same  succession  of  strata,  whereas 

747 


748        The  Sculpturing  of  the  Earth's  Surface 

in  a  series  of  hog-backs  of  erosion,  the  stratum  capping  each  suc- 
cessive ridge  will  be  a  different  one,  either  higher  or  lower,  accord- 
ing to  the  order  in  which  the  successive  hog-backs  are  examined. 
The  capping  bed  of  the  inner  hog-back  (i.e.,  that  nearest  the  center 


FIG.  640.  —  Diagrams  illustrating  block  faulting,  a,  b,  ridges  produced  by 
tilting  of  blocks ;  c,  region  of  horizontal  strata  intersected  by  vertical  fissures ; 
d,  the  same  after  tilting,  —  note  the  repetition  of  the  strata  in  the  successive 
blocks ;  e,  the  same  series  with  the  fault  faces  of  the  blocks  eroded ;  /,  a  series 
of  normal  uniclines  showing  a  similar  appearance,  but  each  ridge  is  capped  by 
a  different  bed. 


of  the  original  anticline)  will  pass  under  that  capping  the  next 
outer  one  and  the  capping  bed  of  this  one  again  under  that  of  the 
one  next  outward  from  it.  This  is  shown  in  diagram  /  in  the 
illustration  (Fig.  640)  which  should  be  compared  with  the  diagram 
e  of  the  same  figure.  A  more  complex  and  irregular  type  of  block 
faulting  has  already  been  illustrated  in  Fig.  557  a,  p.  633.  An  actual 


The  Erosion  Cycle  in  a  Faulted  Region        749 


example  of  complex  block  faulting  on  a  small  scale,  from  Nevada, 
is  illustrated  in  the  following  map  and  section  (Figs.  641,  642). 

Typical  Examples.  —  Typical  examples  of  mountains  formed 
by  the  tilting  of  large  blocks  which  themselves  are  broken  into 
minor  blocks,  are  found  in  the  Sandia  and  Magdalena  mountains 


FIG.  641.  —  Plan  of  the  principal  faults  in  the  Bullfrog  district,  Nevada. 
(Emmons,  U.  S.  G.  S.) 

of  New  Mexico.  The  displacement  occurs  in  originally  hori- 
zontar strata  underlain  by  crystalline  rocks  (Fig.  643,  Johnson). 
Block  faulting  in  regions  of  more  complex  structure  is  seen  in  the 
Basin  Ranges  of  Utah  and  Nevada.  On  the  eastern  edge  of  the 
Great  Basin  stands  the  block  which  forms  the  Wasatch  Mountains. 


Horizontal  Scale 


FIG.  642.  —  Diagrammatic  section  illustrating  fault  block  displacements  in 
the  Bullfrog  district,  Nevada.     (Emmons,  U.  S.  G.  S.) 

Its  surface  is  tilted  so  as  to  slope  eastward  and  its  fault  surface 
faces  westward.  On  the  opposite  side  of  the  Great  Basin  stands 
the  block  which  forms  the  Sierra  Nevada  Range.  Its  fault  face 
is  on  the  east,  and  its  long  back  slope  drops  off  gently  to  the  Valley 
of  California  on  the  west.  Between  the  two  are  many  narrow  ridges 
from  i  o  to  50  miles  in  length  which  represent  modified  fault  blocks 
of  this  type.  The  fault  faces  of  some  are  turned  east,  of  others  west. 


750        The  Sculpturing  of  the  Earth's  Surface 

Complex  block  faulting  has  also  occurred  in  a  number  of  cases. 
The  general  relationship  is  shown  in  the  lower  diagram  on  this 
page,  after  Le  Conte  (Fig.  644),  the  faulting  being  interpreted  as 
the  breaking  down  of  a  great  arch. 


FIG.  643.  —  Block  faulting,  Sandia  Mountains.  The  mass  consists  of  crys- 
talline rocks,  shown  by  dots,  covered  by  limestones  and  other  sediments, 
which  originally  were  horizontal.  A  single  great  fault  forms  the  left-hand  face 
of  the  mountain,  which  consists  of  a  tilted  block,  broken  into  minor  blocks 
by  lesser  faults.  (After  D.  W.  Johnson.) 

The  block  mountains  of  the  Great  Basin  ranges  are  not  generally 
formed  from  horizontal  strata,  but  are  complex.  The  Wasatch 
block  contains  many  crystalline  and  metamorphic  rocks;  the 
Sierra  Nevada  block  is  in  large  part  igneous.  In  the  ranges  within 
the  Basin,  the  strata  are  part  of  an  old  folded  mountain  system 
similar  to  that  of  the  Appalachians  and,  like  that,  peneplaned. 
Instead  of  simple  elevation  at  the  beginning  of  the  second  cycle, 
the  Great  Basin  region  then  suffered  block  faulting.  The  direction 

of  the  faults  does  not  conform 
to  that  of  the  old  folds,  i.e.,  to 
the  strike  of  the  strata,  but  is 
often  obliquely  across  them. 
This  relationship  is  shown  in 
the  following  diagram  (Fig. 
645).  In  some  of  the  Basin 


FIG.  644.  —  Generalized  cross-section 
of  the  Great  Basin  from  the  Sierra 
Nevada  to  the  Wasatch,  showing  the 
origin  of  the  block  mountains  of  the 
Basin  Ranges  by  collapse  of  a  former 
arched  surface.  (After  Le  Conte.) 


Ranges  slight  faulting  or  up- 
lift of  the  fault  block,  has 
occurred  in  comparatively 

recent    times,  and    some   are    probably    still    undergoing   move- 
ment (Fig.  646). 

Erosion -of  Block  Mountains.  —  The  streams  which  come  into 
existence  upon  a  faulted  block  mountain  are  of  two  types,  one 
flowing  down  the  back  slope  of  the  tilted  block,  that  is,  the  original 


The  Erosion  Cycle  in  a  Faulted  Region        751 


FIG.  645.  —  Diagram  illustrating  the  development  of  the  block  mountain 
topography  of  the  Basin  Ranges.  The  region  is  one  of  folded  Palaeozoic  strata, 
their  folds  forming  the  ancient  Palaeocordilleran  mountains.  Peneplanation 
ensued,  followed  by  block-faulting,  after  which  erosion  carved  the  present 
mountain  topography,  while  the  depressions  between  the  blocks  were  partly 
filled  by.  the  waste  from  the  mountains.  (After  Davis.) 


FIG.    646.  —  Recent    fault    crossing   moraine.      Wasatch    Mountains,   Utah. 
(F.  J.  Pack,  photo.) 


752        The  Sculpturing  of  the  Earth's  Surface 
\ 

surface  of  that  part  of  the  land,  whether  it  was  of  horizontal  strata 
or  a  peneplaned  surface ;  the  other  flowing  down  the  much  steeper 
fault  scarp.  The  first  type  may  be  compared  to  the  consequent 
streams  of  an  anticline,  or  if  the  tilting  is  slight,  to  the  normal 
consequent  of  the  coastal  plain.  The  second  type,  that  flowing 
down  the  fault  scarp,  may  be  compared  to  the  obsequent  stream 
of  the  cuesta,  or  the  similar  obsequent  stream  on  the  inner  side 
of  the  hog-back.  In-  a  measure,  of  course,  both  stream  types  de- 
veloped on  the  fault  block  are  consequent,  one  being  the  dip-slope 
consequent,  the  other  the  fault-scarp  consequent,  and  they  are 
so  classed  by  many  physiographers. 

Because  of  the  greater  steepness  of  the  fault  scarp  of  the  tilted 
block,  the  streams  upon  it  will  have  greater  erosive  power  than 


FIG.  647.  —  Diagram  showing  mountains  and  valleys  due  to  block  faulting 
in  the  background,  and  the  dissection  of  the  blocks  and  filling  of  the  valleys, 
in  the  foreground.  A  stage  of  maturity  has  been  reached  in  the  development 
of  these  mountains.  (After  Davis.  Erklarende  Beschreibung  der  Landformen.) 

those  on  the  back  slope.  In  consequence,  the  divide  between 
the  two  types  of  streams  flowing  in  opposite  directions,  which  in 
the  beginning  was  at  the  crest  of  the  tilted  block,  is  pushed  away 
from  the  edge  of  the  fault  scarp,  and  the  drainage  basin  of  the 
fault-scarp  stream  will  encroach  more  and  more  upon  that  of  the 
outer-slope  stream.  This  is  true  of  course,  primarily,  where  the 
two  stream  systems  receive  an  approximately  equal  supply  of 
water.  Equilibrium  will  be  established  when  the  gradients  of  the 
two  streams  are  essentially  alike,  though  of  course  differences  in  rock 
character  and  structure  on  opposite  sides  of  the  block,  as  well  as 
differences  in  the  amount  of  water  supplied  and  other  factors,  enter 
to  complicate  the  process.  The  material  worn  from  the  blocks 
may  be  deposited  in  the  valleys,  which  thus  take  on  a  level  surface 
(Fig.  647). 


The  Erosion  Cycle  in  a  Faulted  Region        753 

The  Wasatch  Mountains  as  an  Example.  —  The  Wasatch  Moun- 
tains, which,  as  already  stated,  form  the  eastern  fault  block  that 
bounds  the  Great  Basin,  have  a  gentle  slope  toward  the  east  for  a 
distance  of  15  to  20  miles.  The  western  slope,  the  fault  scarp, 
is  an  extremely  abrupt  one,  elevations  of  10,000  feet  being  attained 
within  one  or  two  miles  of  the  western  base,  where  the  mountain 
rises  abruptly  from  the  broad  flat  plains  of  the  Utah  Basin.  These 
plains  are  formed  by  an  unknown  thickness  of  alluvial  deposits 
which  cover  the  valley  floor  (Fig.  648).  In  spite  of  this  difference 
of  slope,  the  main  crest  of  the  mountains  produced  by  erosion  lies 


FIG.  648.  —  Diagram  illustrating  several  stages  in  the  development  of  the 
Wasatch  Mountains  from  the  original  fault  block  (A)  through  rugged  mountain 
topography  (B) ;  to  subdued  mature  topography  (C) ;  and  final  obliteration 
(in  the  future)  and  formation  of  a  peneplane  (Z>).  (After  Davis.  Erklarende 
Beschreibimg  der  Landformen). 

near  the  eastern  border,  and  the  westward  flowing  streams  are 
generally  from  two  to  three  times  the  length  of  the  eastward  flow- 
ing ones.  Thus  the  divide  has  been  pushed  by  erosion  two  thirds 
or  more  of  the  way  across  the  original  block.  The  various  stages 
in  dissection  of  the  fault  block  are  illustrated  in  the  diagram  (Fig. 
648). 

On  the  crystalline  and  metamorphic  rocks  of  this  block,  peaks  from  11,000 
to  12,000  feet  in  height  have  been  developed,  and  they  have  rugged  pinnacle- 
like  forms.  On  the  slightly  tilted  sedimentary  rocks,  peaks  of  pyramidal  out- 
line with  cliffs  and  slopes  are  developed.  These  erosion  forms  are  not  wholly 
due  to  river  wear  and  weathering,  for  ice  erosion  has  also  played  an  important 
part  in  the  higher  regions.  Below  the  level  of  glaciation,  however,  that  is,  in 
summits  less  than  9000  feet  in  height,  the  outlines  are  rounded  and  softened  by 
heavy  slopes  of  land  waste. 


754        The  Sculpturing  of  the  Earth's  Surface 

The  Sinai  Type  of  Fault-Bounded  Cuesta  Block 

A  striking  example  of  a  block  isolated  by  graben-faulting,  but 
having  otherwise  the  character  of  a  normal  cuesta,  is  seen  in  the 
Sinai  Peninsula.  This  is  of  triangular  outline,  the  apex  being 
on  the  south,  where  two  rift  valleys  converge,  that  of  the  Gulf  of 
Suez  on  the  west  and  that  of  the  Gulf  of  Akaba,  continued  in  the 
Vale  of  Araba  and  the  Dead  Sea,  on  the  east.  The  apex  of  the 
peninsula  is  formed  of  a  series  of  much  dissected  peaks  of  crystalline 
rocks  (granite,  porphyry,  diorite,  gneiss,  etc.).  Some  of  the  peaks 
rise  to  considerable  heights,  notably  the  Jebel  Musa  group,  which 
culminates  in  Mt.  Catherine  (8540  ft.).  This,  or  the  more  iso- 
lated Mt.  Serbal  (6750  ft.),  is  to  be  identified  as  Mt.  Sinai  or  the 
"  Mountain  of  the  Law."  North  of  this  oldland  mountain  group 
lies  a  series  of  great  valleys  which  represent  the  inner  lowland, 
and  from  which  arises  on  the  north  the  great  cuesta  front  of  Jebel 
El-Tih,  which  attains  a  height  of  3000  ft.,  and  is  much  dissected 
into  fantastic  forms.  (See  map,  Fig.  649.)  This  great  cliff  is  formed 
of  the  cut  edges  of  nearly  horizontal  strata,  comprising  chiefly 
the  Nubian  Sandstone,  Cretaceous  limestones,  and,  farther  north, 
Tertiary  Nummulitic  limestones,  the  strata  dipping  at  a  gentle 
angle  northward  to  the  Mediterranean.  From  the  summit  of 
this  cuesta-inface  the  surface  descends  northward  in  conformity 
with  the  dip  of  the  strata  for  250  kilometers  (about  160  miles) 
passing  beneath  the  level  of  the  Mediterranean  like  a  normal  coastal 
plain.  The  surface  thus  has  an  average  slope  of  less  than  20  ft. 
per  mile,  and  though  under  the  prevailing  climatic  conditions  it  is 
partly  a  desert  region  (the  desert  El-Tih),  it  is  dissected  by  wadis 
which  form  a  consequent  drainage  system  descending  from  near 
the  south  edge  of  the  cuesta  to  the  sea.  On  the  east  the  edge  of 
the  cliff  is  dissected  by  numerous  insequent  valleys  tributary  to 
the  Araba  Graben.  (See  map,  Fig.  649.) 

The  Rift  Valley  or  Graben 

The  name  Graben  (ditch)  has  been  given  in  Germany  to  valleys 
formed  by  the  down-faulting  of  a  long,  narrow  block  of  the  earth's 
crust,  and  the  typical  example  of  such  a  Graben  is  the  Rhine 
trough  north  of  Mayence  (Mainz)  already  described.  The  region 
was  formerly  a  peneplane  eroded  on  crystalline  rocks,  and  upon 
this  were  deposited  red  sandstones  and  shales  (Triassic  with  some 


The  Erosion  Cycle  in  a  Faulted  Region        755 


Permian),  followed  by  shales  (Lias)  and  limestone  (Jurassic), 
which  extended  uninterruptedly  across  this  region,  uniting  the 
remnants  of  these  beds  now  seen  on  the  one  side  in  the  cuestas  of 
eastern  France  and  on  the  other  in  the  Swabian  Alp,  etc.,  of  Ger- 
many. Later  this  region  arose  as  an  anticline  or  arch,  the  south- 
ward continuation  of  which  was  formed  by  a  group  of  anticlines 
and  synclines  which  constitute  the  Jura  Mountains.  (See  Fig. 


FIG.  649.  —  Topographic  map  of  the  Sinai  Peninsula  and  the  Nile  region. 

(From  Ratzel.) 

552,  p.  631.)  At  that  time,  or  somewhat  later,  the  center  of  the 
arch  on  the  north  collapsed,  sinking  down  as  a  long,  narrow  block, 
bounded  by  a  series  of  parallel  fault  planes  on  both  sides  (Fig. 
650  A).  Peneplanation  of  the  region  followed,  and  in  this  pro- 
cess the  crystalline  rock  was  uncovered  in  several  places  on  each 
side  of  the  fault  block.  This  peneplanation  appears  to  have 
extended  widely  over  this  entire  region,  being  the  extension  of  the 
peneplane  which  truncated  the  strata  of  the  Paris  Basin.  We 
know  that  the  faulting  took  place  before  the  peneplanation,  be- 


756        The  Sculpturing  of  the  Earth's  Surface 

cause  upon  the  floor  of  the  Rhine  Graben  are  found  the  strata 
(Triassic  and  Jurassic)  which  this  peneplanation  removed  on  both 
sides,  cutting,  as  we  have  seen,  in  places,  even  to  the  crystallines. 

Conditions  at  the  time  of  the  completion  of  the  peneplane  were 
somewhat  like  those  shown  in  the  diagram  Fig.  650  B.     After  the 


FIG.  650.  —  Diagrammatic  sections  to  represent  the  development  of  the 
Rhine  Graben  and  its  present  structural  and  topographic  features.  A  (upper), 
conditions  after  the  first  faulting,  which  formed  the  original  Graben  which  is 
occupied  by  the  sea  in  which  the  Tertiary  strata  (7)  were  deposited ;  B  (middle), 
the  same  region  after  peneplanation;  C  (lower),  the  same  region  after  renewed 
faulting  and  dissection;  V,  Vosges  Mountains;  R.G.,  Rhine  Graben;  B.F., 
Black  Forest  (Schwarzwald) ;  N,  Neckar  valley ;  o,  ancient  crystallines ;  c, 
folded  Carbonic  beds;  i.  Buntsandstein  (Triassic,  including  some  Permian); 
2.  Muschelkalk;  3.  Keuper;  4.  Lias;  5.  Dogger  shales  and  sandstones; 
6.  Malm,  forming  the  Swabian  Alp ;  7.  Tertiary  (chiefly  Oligocene) ;  8.  Quater- 
nary and  modern  alluvium. 

peneplanation,  renewed  faulting  set  in,  the  movements  continuing 
in  some  places  even  to  the  present  time.  The  northward  continu- 
ation of  the  Graben  was  blocked  by  the  outbreak  of  a  series  of  vol- 
canoes, especially  between  Darmstadt  and  Kassel,  and  in  conse- 
quence the  Rhine,  which  had  begun  to  occupy  this  trough,  had 
to  cut  its  path  across  the  slate  mountains  and  so  form  its  famed 
gorge  (Fig.  601,  p.  703).  In  the  south,  the  uplifted  sides  of  the 


The  Erosion  Cycle  in  a  Faulted  Region        757 

Graben  carried  the  crystallines  to  a  high  altitude,  and  these  consti- 
tute to-day  the  Vosges  Mountains  on  the  west  and  the  Black  Forest 
on  the  east.  Because  of  the  hard  character  of  these  rocks,  they 
have  resisted  erosion  and  now  constitute  prominent  barriers  on 
both  sides  of  the  Rhine  Graben.  A  steep  fault  scarp,  somewhat 
modified  by  later  erosion,  faces  the  Rhine  Valley  on  each  side, 
while  from  the  top  of  each  range  a  gentle  slope  extends  backward 
to  the  foot  of  the  first  cuesta  formed  by  the  erosion,  in  the  second 
cycle,  of  the  softer  beds  beneath  the  harder  cliff-making  limestones 
(Fig.  650  C). 

Recent  rift  valleys  of  this  type  exist,  as  we  have  seen  (Fig. 
554,  p.  632),  in  east  Africa,  where  volcanic  eruptions  likewise 
have  blocked  the  continuity  of  the  valley  in  many  places,  with  the 
formation  of  lakes,  of  which  Tanganyika  is  one  (Fig.  555,  p.  633). 
The  Palestine  rift  valley,  occupied  partly  by  the  Dead  Sea,  the 
River  Jordan,  and  the  Sea  of  Galilee,  and  continued  southward 
in  the  Gulf  of  Akabah,  is  also  a  very  recent  one.  In  this,  erosion 
is  apparently  still  in  the  first  cycle,  having  modified  only  the  sides 
of  the  trough  to  a  certain  extent  (Fig.  649,  p.  755).  Among  the 
minor  erosion  phenomena  along  the  Dead  Sea  are  the  salt  pillars 
carved  from  ancient  salt  beds  exposed  by  the  faulting,  and  one  or 
another  of  which  has  been  identified  since  time  immemorial  as  the 
statue  of  Lot's  wife. 


Checker-board  Fault  Structure 

Where  a  country  is  traversed  by  many  parallel  master  joints 
arranged  in  two  intersecting  series,  dislocation  of  the  blocks  thus 
produced,  results  in  the  elevation  of  some  and  the  depression  of 
others,  the  amounts  being  variable  in  the  different  blocks.  Such 
a  checker-board  dislocation  of  a  country  produces  a  series  of  more 
or  less  rectangular  elevated  and  depressed  fields,  and  on  these 
erosion  will  produce  a  most  complex  series  of  river  systems.  If 
such  a  region  is  finally  peneplaned,  a  very  complicated  rock  surface 
will  result,  some  squares  consisting  of  one  kind,  others  of  different 
material,  and  all  abruptly  bounded  by  fault  lines.  In  the  second 
cycle  of  erosion,  the  softer  beds  will  be  attacked  and  a  complicated 
topography  is  produced. 

Southern  Sweden  is  one  of  the  best  known  examples  of  this 
structure.  Here  one  passes  abruptly  from  rounded  hills  of  crystal- 


The  Sculpturing  of  the  Earth's  Surface 


line  rock  to  buttes  or  table  mountains  cut  on  horizontal  strata, 
and  from  these  to  regions  covered  with  more  or  less  inclined  strata, 
and  on  again  to  hills  of  crystalline  rock,  there  being  absolutely  no 
regularity  of  structure.  The  geologist  working  in  such  a  field 
passes  abruptly  from  rocks  of  Archaean  age  to  those  of  Mesozoic. 
These  may  in  the  next  block  be  replaced  by  Cambrian  and  Ordovi- 
cian  strata,  beyond  which  a  block  exposes  Tertiary  beds  at  the 


FIG.  651.  —  Diagram  showing  block  faulting  of  the  checkerboard  type ;  pene- 
planation  of  the  region,  and  the  development  of  a  complicated  topography  by 
erosion  in  the  second  cycle.  Illustration  of  the  geology  of  central  Sweden. 
(After  Davis.) 

surface,  and  then  follows  perhaps  one  with  Silurian  strata.  Along 
the  fault  lines,  valleys  are  often  carved,  and  some  of  these  may  be 
occupied  by  lakes.  Constant  diversity  appears  to  be  the  keynote, 
not  only  of  the  rocks  and  rock  structures,  but  of  the  topography  as 
well  (Fig.  651). 

Fault-Line  Valleys 

When  a  country  traversed  by  faults  of  some  extent  has  become 
peneplaned  during  the  first  cycle  of  erosion,  a  valley  may  be  cut 
by  streams  along  the  fault  line  during  the  succeeding  cycle.  For 
not  only  is  the  fault  line  one  of  weakness  in  the  earth's  crust,  but 
along  it  are  apt  to  be  found  many  remnants  of  weaker  strata 
which,  being  faulted  below  the  level  of  peneplanation,  escaped 
removal  during  the  first  cycle  but  are  readily  attacked  in  the 
second  cycle.  As  a  result,  valleys  will  be  formed  which  have  no 
marked  adjustment  to  the  kinds  of  rocks,  and  which,  moreover, 
may  extend  in  a  continuous  manner  for  great  distances.  Such 


The  Erosion  Cycle  in  a  Faulted  Region        759 

a  valley  is  the  Great  Glen  which  traverses  the  Scottish  Highland 
region  from  Loch  Linnhe  on  the  southwest  to  the  Moray  Firth  on 
the  northeast,  and  which,  on  account  of  the  succession  of  beautiful 
lochs  situated  along  it, 
makes  this  not  only  the 
most  picturesque  but 
also  the  easiest  line  of 
travel  across  Scotland 
from  Oban  to  Inverness. 
Such  fault-line  "valleys 

must    be    distinguished 

FIG.  652.  —  A  fault-line  valley.    (From  Pnn- 
from    fault    valleys    or  dpks  of  Stratigraphy.) 

troughs,  the  latter  rep- 
resenting purely  a  structural   feature,   the  valley  always  being 
in  the  first  cycle.     The  fault-line  valley,  on  the  other  hand,  is  an 
erosion  feature,  often  in  a  later  cycle  after  peneplanation  has  de- 
stroyed the  original  fault  topography  (Fig.  652). 

Renewal  of  Fault-Scarp  Topography 

If  in  a  faulted  region  the  original  fault  topography  has  been 
destroyed  by  peneplanation,  erosion  in  the  second  cycle  may  renew 
the  fault-scarp  topography  by  removing  the  softer  rock  on  one  side 
of  the  fault  and  leaving  the  harder  rock  on  the  other  side  in  relief. 
The  scarp  or  cliff  thus  produced  along  the  fault-line  is  called  a 
fault-line  scarp,  in  distinction  from  the  fault  scarp  which  is  the  orig- 
inal cliff  due  to  dislocation.  According  to  the  nature  of  the  strata 
uncovered  on  either  side  of  the  fault-line  by  peneplanation,  the 
fault-line  scarp  may  face  in  the  same  direction  as  the  original  fault 
scarp,  that  is,  face  the  down- throw  block,  in  which  case  it  is  called 
resequent,  since  it  retakes  a  consequent  position  [re (con) sequent]. 
On  the  other  hand,  it  may  face  in  the  opposite  direction,  i.e.,  face 
the  original  upthrow  block,  if  the  strata  exposed  by  peneplanation 
upon  the  surface  of  this  block,  prove  to  be  the  softer.  In  such  a 
case,  the  fault-line  scarp  is  said  to  be  obsequent.  Neither  fault 
scarp  nor  fault-line  scarp  will  long  maintain  its  position  along 
the  fault-line,  but  erosion  will  progressively  push  them  back  until 
the  cliff  may  be  many  miles  from  the  fault-line,  though  essentially 
parallel  to  it.  Such  cliffs  are  characteristic  topographical  features 
of  the  plateau  country  west  of  the  Rocky  Mountains,  though  in  most 
cases  they  have  not  weathered  back  very  far  from  the  fault  plane. 


760        The  Sculpturing  of  the  Earth's  Surface 

SOME  ILLUSTRATIONS  OF  COMPLICATED  RIVER  EROSION 

We  may  here  introduce  a  few  typical  examples  of  complexly 
adjusted  rivers  and  the  topographical  features  which  accompany 
them,  in  order  that  the  student  may  appreciate  the  interrelations 
of  the  various  forces  operative  in  producing  a  complex  series  of 
land  forms.  We  will  select  the  Niagara,  the  Genesee,  and  the 
Colorado  rivers. 

Niagara  River  and  Falls 

The  Niagara  River  differs  from  the  normal  form  of  river,  which 
we  have  been  considering,  in  so  many  respects  that  it  may  be  taken 
as  the  type  of  a  special  class,  that  of  the  spillway  of  one  water 
body  into  another. 


FIG.  653.  —  Block  diagram  illustrating  the  formation  of  three  cuestas  and 
lowlands  by  normal  drainage  on  the  peneplaned  strata  which  surround  the 
old  Ontario  dome  on  the  south  and  west.  Three  principal  streams  are  indi- 
cated, of  which  the  middle  one  is  the  master  stream  (Dundas  river).  (Drawn 
by  Mary  Welleck.)  See  also  maps,  Figs.  617,  618,  p.  727. 

Pre-Niagara  Topography.  —  Before  Niagara  River  came  into 
existence,  the  region  was  the  site  of  normal  dissection  of  part  of 
a  low  dome  that  had  been  peneplaned,  as  has  previously  been  out- 
lined (p.  725).  As  a  result,  three  lowlands  and  cuestas  had  come 
into  existence,  of  which  the  northernmost  was  the  most  profound 
and  the  middle  one  the  shallowest  (Fig.  653).  These  three  cuestas 
are  formed  by  the  three  resistant  formations  of  the  region,  and  are, 
respectively,  the  Niagara  cuesta  (Fig.  654,  a),  the  Onondaga  cuesta 
(b),  and  the  Portage-Chemung  cuesta  or  front  of  the  Allegheny 


Illustrations  of  Complicated  River  Erosion      761 

plateau  (c),  while  the  lowlands  in  front  of  these  are  the  Ontario 
lowland  (A),  the  Salina  lowland  (5),  and  the  Erie  lowland  (C). 
The  peneplane  is  indicated  by  the  beveled  surfaces  of  the  Niagara 
and  Onondaga  limestones  at  the  edges  of  the  respective  cuestas,  and 
the  summit  elevation  of  the  Alleghany  front  south  of  Buffalo. 
When .  these  points  are  connected  by  a  line  which  restores  the 
peneplane  surface,  it  is  seen  that  this  old  surface  now  rises  to  the 
south,  and  this  indicates  that  the  peneplane  has  been  tilted  to 
the  north  with  possibly  some  warping  which  carried  the  southern 
region  to  greater  altitudes.  Such  tilting  occurred,  however,  only 
after  the  cuesta  and  lowland  topography  had  been  etched  out  of 
the  hard  and  soft  strata,  respectively,-  by  a  revival  of  the  conse- 
quent and  subsequent  drainage  from  the  old  Ontario  dome  toward 
the  southwest  (p.  726).  Indeed,  this  tilting  appears  to  have  been 
due  to  the  depression  of  the  land  on  the  north  during,  and  prob- 


FIG.  654.  —  North-south  section  across  western  New  York,  showing  the 
three  cuestas  and  lowlands  and  the  tilting  of  the  peneplane.  a,  Niagara 
cuesta;  b,  Onondaga  cuesta;  c,  Portage-Chemung  cuesta;  A,  Ontario  low- 
land; B,  Salina  lowland;  C,  Erie  lowland;  LO,  Lake  Ontario;  NF,  Niagara 
Falls;  LE,  Lake  Erie. 

ably  because  of,  the  accumulation  of  the  continental  ice  sheet  of 
Pleistocene  time.  As  a  further  result  of  this  northward  depression, 
the  floor  on  the  Ontario  lowland  was  carried  several  hundred  feet 
below  the  present  sea-level,  from  its  position  some  distance  above 
that  at  the  time  of  its  formation  by  river  erosion  (Fig.  654). 

As  these  various  lowlands  extend  in  a  direction  at  right  angles 
to  that  of  the  movement  of  the  great  ice  sheet  in  Pleistocene  time, 
they  suffered  no  appreciable  deepening,  though  the  tops  of  the 
cuestas  may,  in  some  cases,  have  been  planed  down  to  a  slight 
extent. 

In  addition  to  the  depression  of  the  country  on  the  north,  the 
valley  of  the  master  consequent  stream  had  been  rilled  by  glacial 
drift,  and  the  same  thing  happened  to  the  shallow  Salina  lowland. 
Thus,  with  the  outlet  of  the  inner  lowland  choked  by  drift,  and 
the  actual  slope  of  the  country  reversed  by  depression,  the  Ontario 
lowland  became  a  circumscribed  basin  and  was  filled  with  water 


762        The  Sculpturing  of  the  Earth's  Surface 

up  to  the  level  of  the  lowest  point  in  the  rim,  which  is  at  present 
at  the  Thousand  Islands,  a  region  that  was,  in  the  days  of  former 
great  elevation,  the  head  of  the  St.  Lawrence  River  and  the  divide 
between  it  and  a  westward  flowing  stream  which  joined  the  subse- 


FIG.  655.  — Map  of  Lake  Iroquois,  with  the  modern  outlines  of  Lake  Ontario, 
etc.,  shown  in  dotted  lines.     (After  Gilbert.) 

quent  by  which  the  Ontario  lowland  was  carved  (see  maps,  Figs. 
617  and  618,  p.  727).  Before  the  present  St.  Lawrence  outlet  came 
into  action,  however,  the  waters  of  the  lake  in  the  Ontario  lowland 
overflowed  along  the  line  of  the  old  Ontario  River  and  across  the 
divide  at  Little  Falls  into  the  Mohawk  and  Hudson.  This  was 


L   CM, 


FIG.  656.  —  The  edge  of  the  Niagara  escarpment.  NS,  showing  the  posi- 
tion of  the  ancient  Iroquois  Beach  (IB)  and  the  relation  of  the  Iroquois  level 
to  that  of  Lake  Ontario. 

necessitated  by  the  fact  that  a  part  of  the  great  ice  sheet  still 
lingered  in  the  region  of  the  Thousand  Islands  and  so  blocked 
that  outlet  (Fig.  655).  As  a  result,  the  level  of  the  lake,  called 
Lake  Iroquois,  stood  higher  than  the  present  Ontario  level  and 
extended  farther  over  the  flat  terrace  which  lies  between  the  Niag- 
ara escarpment  and  the  submerged  part  of  the  cuesta.  The 


Illustrations  of  Complicated  River  Erosion      763 


ancient  shore-line  is  distinctly  marked  by  one  or  more  abandoned 
beaches,  which  can  be  traced  along  the  southern  border  of  Lake 
Ontario  but  distant  from  it  some  miles  (Fig.  656).  The  principal 
one  is  followed  by  the  so- 
called  Ridge  Road. 

Because  of  the  northward 
tilting  of  the  land  and  the 
choking  of  the  old  outlets, 
the  Erie  lowland  also  was 
transformed  into  a  lake 
basin,  its  level  being  likewise 
determined  by  the  lowest 
place  of  outlet  in  its  rim, 
which  happened  to  be  across 
the  Onondaga  cuesta  where 
Buffalo  now  stands.  Spilling 
over  at  this  point,  the  waters 
took  their  course  across  the 
country  in  a  northward  di- 
rection, this  being  the  only 
avenue  of  escape  open  to 
them,  until  they  reached  the 
edge  of  the  Niagara  escarp- 
ment where  Lewiston  stands 
to-day,  and  there  fell  over  it 
as  the  newborn  Niagara  Falls. 
That  no  fall  or  only  a  slight 
one  came  into  existence  over 
the  Onondaga  cuesta  was  due 
to  the  filling  of  the  lowland 
in  front  of  it  by  glacial  drift, 
and  also  because  at  the  point 
of  overflow  there  appears  to 
have  been  an  old  notch  cut  in 
the  escarpment  by  some  ob- 
sequent  stream  during  the 
period  of  normal  valley 
erosion. 

Peculiar  Course  of  Niagara  River.  —  If  we  examine  the  course 
of  Niagara  River,  we  find  certain  peculiarities,  especially  repeated 


FIG.  657.  — Map  of  the  Niagara  Gorge, 
showing  variable  course  and  physical 
features.  (After  Gilbert.)  Old  river 
banks  are  shown  by  dotted  lines;  shell 
localities,  by  crosses. 


764        The  Sculpturing  of  the  Earth's  Surface 

changes  in  direction,  unlike  what  we  might  expect  in  a  normal 
spill-over  from  one  water  body  to  another  across  a  comparatively 
level  plain  (Fig.  657).  From  its  head  at  Buffalo,  northward,  the 
river  is  a  broad  placid  stream  dividing  into  two  arms  which  reunite 
and  enclose  a  large  flat  island  called  Grand  Island.  Beyond  this 
it  turns  almost  due  westward  and  soon  becomes  a  turbulent,  though 
still  very  broad,  stream  in  which  no  boat  can  make  headway. 
Then  it  plunges  over  a  succession  of  low  limestone  ledges  as  a 
magnificent  series  of  rapids  until  it  has  descended  about  50  feet 
vertically,  when  it  reaches  the  present  falls,  of  which  there  are  two, 
the  American,  parallel  to  the  line  of  rapids,  and  the  Horseshoe 


FIG.  658.  —  Niagara  Falls. 

Falls,  opening  in  a  curve  almost  due  north.  Between  the  two  lies 
Goat  Island,  a  drift-covered  rock  mass  which  rises  nearly  to  the 
level  of  the  river  above  the  upper  rapids  (Fig.  658). 

From  the  foot  of  the  Horseshoe  Falls  the  gorge  extends  in  a 
direction  somewhat  east  of  north  for  about  three  miles,  and  it  is 
virtually  over  the  side  of  this  gorge  that  the  American  cataract 
falls.  The  width  of  this  part  of  the  chasm  varies  from  1250  to 
1700  feet  at  the  top,  its  banks  being  for  the  most  part  nearly  verti- 
cal. The  depth  to  water-level  is  nearly  200  feet,  and  the  water 
itself  is  from  150  to  200  feet  deep. 

Just  before  it  passes  under  the  railroad  bridge  at  Clifton,  the 
gorge  suddenly  turns  to  the  northwest,  making  almost  a  right 
angle.  It  also  contracts  to  a  width  of  about  700  or  750  feet  at  the 
top  and  550  feet  at  the  water-line,  and  the  water  becomes  only 


Illustrations  of  Complicated  River  Erosion      765 


about  35  feet  deep.  This  is  the  beginning  of  the  Whirlpool  Rapids, 
the  waters  here  tumbling  over  each  other  with  indescribable  fury 
and  descending  50  feet  in  the  space  of  less  than  a  mile  (Fig.  659). 
This  portion  of  the  gorge  ends  at  the  Whirlpool,  a  deep  circular 
basin  a  thousand  feet  or  more  in  diameter  which  has  a  depth  of  water 
ranging  from  150  to  200 
feet  (Fig.  660).  At  the 
great  swollen  elbow  of 
the  Whirlpool,  the  gorge 
makes  another  right- 
angled  turn,  extending 
northeast  for  nearly  two 
miles.  This  part  is  again 
a  wide  gorge,  the  width  at 
the  top  varying  from  1300 
to  1700  feet,  though  it  is 
less  at  the  water-line. 
The  depth  of  water  is, 
however,  not  great,  being 
at  the  Whirlpool  outlet 
50  feet  and  ranging  from 
35  to  70  feet  in  other 
parts.  In  this  portion  of  FIG.  659.  —  The  Whirlpool  Rapids  gorge 

the  gorge  on  the  Canadian     *   Niasara-     (^m  Grabau,   Geology  of 

Niagara    Falls,   N.    Y.    State    Educational 
side  is  an  interesting  table     Department.) 

rock,  known    as   Foster's 

Flats,  to  which  reference  will  be  made  again.  At  the  end  of  this 
part  of  its  course,  the  river  turns  due  north,  retaining  about  the 
same  width  to  the  edge  of  the  escarpment  at  Lewiston,  and  thence 
continuing  in  the  same  direction  as  a  broad  navigable  stream  with 
relatively  low  banks  for  seven  miles  more  to  Lake  Ontario. 

The  three  bends  in  the  gorge,  together  with  the  changing  width 
and  depth  (Fig.  66i)>  are  its  most  striking  feature,  while  the  sudden 
bend  in  the  river  at  the  Horseshoe  Falls  is  another  feature  of  sig- 
nificance. Tracing  the  lower  part  of  the  gorge,  which  extends  due 
north,  southward  to  the  bend,  we  find  it  aligned  with  a  shallow 
trench  in  the  upland  extending  for  a  mile  or  more  due  southward 
and  occupied  by  a  small  stream,  the  Bloody  Run,  famous  as  the 
scene  of  an  Indian  massacre  during  the  wars  of  1763,  and  which 
forms  the  chasm  of  the  Devil's  Hole  where  it  enters  the  present 


766        The  Sculpturing  of  the  Earth's  Surface 

gorge.  The  valley  of  this  stream  is  readily  recognized  as  a  pre- 
glacial  one,  and  it  appears  that  it  is  the  upper  part  of  the  valley 
of  an  obsequent  stream  which  flowed  over  the  edge  of  the  escarp- 


FIG.   660.  —  The  Whirlpool  in  Niagara  gorge.      (From  Grabau,  Geology  of 
Niagara  Falls.     Courtesy  N.  Y.  State  Educational  Department.) 

ment  during  the  earlier  period  of  normal  erosion  and  long  before 
Niagara  came  into  existence  (Fig.  662).  Thus  the  course  of  the 
lower  gorge  and  that  of  the  river  beyond  to  Lake  Ontario  appears 
to  have  been  predetermined  by  the  cutting  of  a  shallow  obsequent 


FIG.  66 1.  —  Longitudinal  section  of  Niagara  gorge  from  the  Falls  (F)  to 
Queenston  Heights  (£)  showing  the  west  bank  and  the  depth  of  water  (in 
black)  at  the  several  points,  W,  Whirlpool ;  R,  railroad  bridges.  (After  Gilbert.) 

valley  in  the  surface  of  the  cuesta,  though  it  probably  was  not  cut 
back  very  far  from  the  edge  of  the  escarpment  at  Lewiston. 

The  gorge  of  the  Whirlpool  Rapids  south  of  the  Whirlpool 
presents  another  such  case.  As  noted,  this  extends  almost  due 


Illustrations  of  Complicated  River  Erosion      767 

northwest  to  the  Whirlpool.  An  examination  of  the  northwestern 
border  of  the  Whirlpool  discloses  the  fact  that  it  is  formed  by  glacial 
drift  instead  of  rock,  and  a  series  of  borings  has  shown  that  an  old 
drift-filled,  gorge  extends  from  the  Whirlpool  northwestward  to 
the  edge  of  the  Niagara  escarpment  at  St.  Davids,  gradually  widen- 
ing in  this  direction  (Fig.  662).  Beyond  St.  Davids  it  has  not 
been  traced,  but  without  doubt  it  extends  to  Lake  Ontario.  This 
ancient  St.  Davids  Gorge  appears  to  represent  another  obsequent 
stream  which  had  cut  back  from  the  edge  of  the  cuesta,  probably, 
to  the  head  of  the  Whirlpool  Rapids  gorge  at  the  Clifton  railroad 


FIG.  662.  —  Bird's-eye  view  of  Niagara  gorge,  showing  the  course  of  the  river, 
the  falls,  the  railroad  bridges,  Whirlpool,  location  of  Foster's  Flats,  escarp- 
ments at  Queenston,  pre-glacial  valley  of  Bloody  Run,  and  flaring  mouth  of 
pre-glacial  St.  Davids  Gorge.  (Modified  after  Gilbert.) 

bridge.  At  the  inlet  of  the  Whirlpool,  where  the  water  is  less  than 
50  feet  deep,  the  gorge  is  crossed  by  a  heavy  bed  of  white  sandstone 
called  the  Whirlpool  sandstone,  and  just  beyond  this  the  depth 
of  the  water  becomes  150  to  200  feet.  Evidently  there  was  here  a 
waterfall  of  that  height  in  the  ancient  obsequent  river  determined 
by  the  presence  of  this  bed  of  sandstone. 

These  facts  adequately  account  for  the  direction  of  the  gorge 
of  the  Whirlpool  Rapids  and  for  its  narrowness  and  shallowness 
as  compared  with  other  parts.  Evidently  it  was  largely  drift- 
filled  to  its  head  when  the  overflow  from  Lake  Erie  was  directed 
into  it,  and  followed  it  for  some  distance  before  the  drift-filling 
became  so  high  that  the  waters  were  forced  to  spill  over  the  side  of 


768        The  Sculpturing  of  the  Earth's  Surface 

the  shallow  channel  and  find  their  way  across  the  country  to  another 
ancient  and  partly  drift-filled  obsequent  valley.  The  following 
section  (AB,  Fig.  663)  gives  the  approximate  profile  of  this  old 
St.  Davids  valley.  When  the  falls  had  cut  back  nearly  to  the  site 
of  the  Whirlpool  (section  CD,  Fig.  663)  only  a  narrow  rock  wall 
upheld  the  drift-filling  of  the  Whirlpool  and  Whirlpool  Rapids 
Gorge,  and  when  this  barrier  broke  the  drift  was  quickly  cleared 
out  and  the  falls  became  suddenly  transferred  to  near  the  head  of 
the  Whirlpool  Rapids  Gorge. 


FIG.  663.  —  Sections  of  the  Whirlpool  at  Niagara  before  the  former  drift- 
filling  was  cleared  out  by  the  river,  i,  Queenston  shales;  2,  Whirlpool  sand- 
stone; 3,  Medina  shales  and  sandstones;  4,  Thorold  quartzite;  5,  Clinton 
shale  and  limestone ;  6,  Rochester  shale ;  7,  Lockport  dolomite ;  x,  the  narrow 
rock  barrier  northeast  of  the  St.  Davids  Gorge,  the  destruction  of  which  caused 
the  sudden  clearing  of  drift  from  the  Whirlpool  and  the  transference  of  the  falls 
for  some  distance  upstream. 

The  upper  portion  of  the  gorge,  for  three  miles  down  stream 
from  the  falls,  was  also  directed  by  a  shallow  valley  in  the  upland 
across  which  the  Erie  waters  spilled.  This  appears  to  have  been 
the  valley  of  an  old  consequent  stream  originally  flowing  in  a  south- 
westerly direction,  but  with  the  present  slope  reversed  owing  to  tilt- 
ing and  to  partial  drift-filling.  It  was  cut  into  the  upland  to  a 
depth  not  much  over  50  feet,  but  this  was  sufficient  to  guide  the 
waters  as  they  spilled  into  it.  The  western  side  of  this  valley  is 
seen  in  the  cliff  opposite  the  Horseshoe  Falls  on  the  Canadian  side, 


Illustrations  of  Complicated  River  Erosion      769 

and  the  eastern  in  the  rapids  above  the  falls,  which  here  extend 
continuously  across  the  river. 

We  thus  see  that  the  course  of  the  Niagara  River  is  due  to  the 
spilling  over  of  the  waters  from  one  ancient  channel  to  another, 
and  that  the  falls  cut  back  along  these  channels,  sometimes  as  a 
single  fall,  at  others  probably  as  a  succession  of  falls,  with  a  sudden 
transference  upstream  for  about  a  mile  when  the  barrier  which 
held  in  the  drift  filling  of  the  upper  end  of  the  St.  Davids  Gorge 
(Whirlpool  Rapids  Gorge)  was  broken  and  the  drift  cleared  from 
this  part  of  the  old  channel. 


FIG.  664.  —  Map  of  glacial  Lake  Algonquin  and  the  discharge  by  the  Trent 
river.     (After  Taylor.) 

1  There  are  some  other  larger  geographic  factors  which  must 
be  taken  into  account  in  the  history  of  Niagara.  Thus  it  has 
been  clearly  shown  that,  for  a  time  at  least,  the  waters  of  the 
upper  Great  Lakes  discharged,  not  as  they  do  now  into  Lake  Erie 
by  way  of  the  Detroit  River  and  thence  over  Niagara  Falls,  but  by 
a  more  direct  route  across  Ontario  along  a  valley  now 'occupied  by 
the  Trent  River  (Fig.  664).  This  meant,  of  course,  that  the  water 
flowing  over  Niagara  was  only  the  spill-over  from  the  much  smaller 
Lake  Erie  of  that  time,  and  therefore  the  Falls  were  not  able  to 
cut  so  deep  and  broad  a  gorge  as  they  do  to-day,  nor  retreat  at  so 
rapid  a  rate.  It  has  been  held  that  during  the  existence  of  these 


770        The  Sculpturing  of  the  Earth's  Surface 


conditions  the  shallower  part  of  the  gorge  north  of  the  Whirlpool 
was  cut.  Still  later,  when  the  ice  had  melted  away  from  the  St. 
Lawrence  outlet,  which  was  then  even  lower  than  at  present,  owing 
to  the  greater  depression  of  the  land,  and  was,  moreover,  because 
of  this  depression,  probably  flooded  by  water  entering  from  the 
sea  (Fig.  665),  the  upper  Great  Lakes  discharged  directly  to  the 
St.  Lawrence  or  the  sea  which  occupied  its  valley,  along  a  course 
now  partly  occupied  by  the  Ottawa  River.  This  still  left  the 
Niagara  only  the  drainage  from  Lake  Erie,  and  it  is  held  by  some 
that  during  this  period  the  Whirlpool  Rapids  Gorge  was  cut,  or 


FIG.  665.  —  Map  of  the  Nipissing  Great  Lakes,  and  the  marine  submer- 
gence of  the  Champlain  and  Ontario  valleys.  The  outlet  of  the  Great  Lakes 
is  by  the  Ottawa  River.  (After  Taylor.) 

at  least  deepened,  and  that  because  of  the  small  quantity  of  water 
carried  over  the  falls,  this  deepening  and  widening  was  much  less 
than  it  would  have  been  with  the  present  volume  of  water. 

After  this  period,  the  land  in  the  north  began  to  recover  from  its 
depression,  and  as  we  have  seen  (p.  695)  may  still  be  slowly  rising, 
as  indicated  by  the  abandoned  beaches  in  the  north  and  the  en- 
croachment of  the  waters  of  the  lakes  on  the  south  shores.  When 
the  tilting  back  toward  the  original  position  had  progressed  so  far 
that  the  upper  Great  Lakes  began  to  spill  over  by  way  of  the 
Detroit  River  into  Lake  Erie,  Niagara  was  forced  to  carry  out  the 
combined  drainage  of  these  lakes  and  assumed  its  present  volume 


Illustrations  of  Complicated  River  Erosion      771 


of  22,440,000  cubic  feet  or  more  than  7,000,000  tons  per  minute. 
Since  this  time  it  has  cut  its  broad  deep  gorge  from  the  railroad 
bridge  to  the  present  Falls.  This  cutting,  judging  by  the  measured 
rate  of  retreat  during  the  last  three  quarters  of  a  century,  must 
have  taken  about  3000  years.  Thus  the  beginning  of  the  great 
cataract  about  three  miles  north  of  its  present  site,  dates  back  to 
about  the  eleventh  century  B.C.,  or  to  about  "  300  years  before  the 
time  of  Romulus,  or  to  the  reign  of  King  David  at  Jerusalem  " 
(Hitchcock).  How  long  it 
took  to  cut  the  other  parts 
of  the  gorge  is  more  difficult 
to  determine,  because  of  the 
variable  factors  above  re- 
ferred to.  It  is  probably  safe 
to  say  that  it  was  at  least 
50,000  years  ago  when  the 
falls  began  near  Lewiston, 
and  it  may  have  been  several 
hundred  thousand  years  ago. 
Mode  of  Cutting  of  Ni- 
agara Falls.  —  Niagara  River 
carries  no  sediment.  Where 
it  leaves  Lake  Erie  the  water  FIG.  666.  —  Section  of  the  Horseshoe 
is  so  pure  that  the  intake  Falls,  Niagara,  to  show  the  depth  of  water 

of  the  water  supply  of  the  ^elowf  *e  faU!  ^d,.the  arra*gement  °f 

f  the  strata.     7  b,  Medina  sandstones  and 

City  01  Buffalo  is  located  at  shales  with  Whirlpool  sandstone  at  base 

that  point.     What  sediment  (resting  on  Queenston  shales)  and  Thorold 

is     brought     in    by    lateral    ?uartfite  ab°8v";  *a>  C^ton  stoile  and 

limestones;    8b,   Rochester    shale;     &c, 

streams  or  picked  off  the  bank    Lockport  dolomite.    (After  Gilbert.) 
is   dropped   in    the    quieter 

water  long  before  the  falls  are  readied,  over  which  the  water 
passes  as  a  pure  stream.  The  erosion  accomplished  by  the 
cataract  is  entirely  due  to  undermining  by  spray  and  to  the  force 
of  the  falling  water.  The  greatest  amount  of  water  passes  over 
the  Horseshoe  Falls,  and  the  section  here  is  shown  in  the  above 
diagram  (Fig.  666).  It  will  be  observed  that  a  heavy  bed  of  lime- 
stone (about  80  feet  thick)  (8c)  caps  the  cliff  and  is  underlain  by 
softer  shales  with  some  limestone  and  sandstone  layers.  It  is  these 
softer  beds  which  are  slowly  removed  by  the  water  and  spray 
driven  against  their  edges  and  by  the  quarrying  activity  of  the 


772        The  Sculpturing  of  the  Earth's  Surface 

frost  in  winter.  Thus  the  limestone  ledge  comes  to  overhang  more 
and  more,  and  eventually  it  breaks  down  from  its  own  weight  and 
that  of  the  water  above  it.  Thus  the  gorge  is  lengthened  by 
periodic  rock  falls.  The  fallen  blocks  are  broken  and  ground  up 
by  the  enormous  force  of  the  descending  water,  and  in  this  process 
they  dig  into  the  soft  red  shales  and  sandstones  of  the  river  bot- 
tom (yft),  thus  producing  a  depth  of  water  approaching  200  feet. 
In  other  words,  the  force  of  the  plunging  water  is  so  great  that  the 
gorge-cutting  goes  on  for  a  depth  of  200  feet  below  the  level  of 
the  river  in  front  of  the  falls,  producing  a  "  plunge  basin  "  of  un- 
usual magnitude  and  depth. 


FIG.  667.  —  Section  of  the  American  Falls  at  Niagara ;  the  rapids  above  the 
Falls  are  formed  by  the  thin-bedded  Guelph  dolomites  (7) ;  the  edge  of  the 
falls  is  formed  by  the  Lockport  dolomite  (6) ;  the  Cave  of  the  Winds  is  cut  by 
the  spray  from  the  Rochester  shale  (5) ;  and  its  floor  is  formed  by  the  Clinton 
limestone  (4) ;  beneath  this  lie  the  Clinton  shale  (3),  the  Thorold  quartzite  (2), 
and  the  Medina  sandstones  and  shales  (i).  In  front  of  the  falls  lie  the  larger 
blocks  of  Lockport  dolomite,  broken  from  the  edge  of  the  falls. 

The  American  Falls,  on  the  other  hand,  having  so  much  less 
water,  are  not  able  to  cut  such  a  plunge  basin.  Indeed,  the  water 
is  unable  to  move  or  destroy  the  larger  fallen  blocks  of  limestone, 
and  so  the  foot  of  these  falls,  as  seen  from  the  Canadian  side,  is 
lined  by  rows  of  such  large  blocks  (Fig.  349,  p.  418).  The  section 
of  these  falls  is  shown  in  the  preceding  diagram  (Fig.  667).  Here 
too,  the  soft  shale  (Rochester)  immediately  below  the  capping  lime- 
stone is  removed  by  spray  and  frost,  so  that  the  limestone  overhangs. 
Another  limestone  series  (Clinton),  about  40  feet  thick,  lies  beneath 
this  shale,  and  this  is  not  destroyed  by  the  spray.  Thus  the 
"  Cave  of  the  Winds  "  is  produced,  the  floor  of  which  is  formed 
by  the  lower  (Clinton)  limestone  and  the  roof  by  the  upper  (Lock- 


Illustrations  of  Complicated  River  Erosion      773 


port)  limestones,  the  cave  itself  being  due  to  the  retreating  face  of 
the  shale  (Rochester),  which  is  about  80  feet  thick. 

Unequal  Retreat  of  the  Falls.  —  It  is  readily  seen  that  because  of 
the  greater  amount  of  water  which  flows  over  the  Horseshoe  Falls, 
these  retreat  more  rapidly  than  do  the  American  Falls.  Between 
the  years  1842  and  1890,  the  measured  retreat,  according  to  sur- 


FIG.  668.  —  Successive  crest  lines  of  the  Horseshoe  Falls,  Niagara,  from  1842 
to  1890.  i,  Terrapin  rocks;  2,  Former  Table  Rock.  (After  Grabau,  Geology 
of  Niagara  Falls.) 

veys  (Fig.  668),  when  distributed  over  the  entire  falls,  gave  for  the 
American  Falls  a  mean  total  recession  of  30.75  feet,  or  a  mean  annual 
recession  of  0.64  feet,  and  for  the  Horseshoe.  Falls  a  mean  total 
recession  of  104.51  feet,  or  a  mean  annual  recession  of  2.18  feet. 
Figured  in  area,  this  gives  a  removal  of  rock  surface  from  the 
edge  of  the  falls  between  these  years  of  0.755  acres  for  the 
American  and  6.32  acres  for  the  Horseshoe  Falls.  The  greater 
volume  of  water  is  due,  of  course,  to  the  fact  that  the  Horseshoe 


774        The  Sculpturing  of  the  Earth's  Surface 

Falls  lie  in  the  path  of  the  main  current  of  the  river  which,  strik- 
ing the  left  bank  above  the  falls,  is  deflected  so  as  to  carry  its  full 
measure  over  the  Horseshoe  Falls.  The  outline  of  the  crest  of  the 
American  Falls,  as  it  appeared  at  the  beginning  of  this  century,  is 
shown  in  the  photograph  (Fig.  669)  from  Goat  Island. 


FIG.  669.  —  Crest  of  American  Falls  at  Niagara,  as  seen  from  Goat  Island 
in  1911,  showing  irregular  recession  by  the  fall  of  large  blocks  which  have  be- 
come undermined.  (Photo  by  author.)  (See  also  Ing.  349,  p.  418.) 

It  is  easy  to  see  that  when  the  Horseshoe  Falls  have  retreated  to 
beyond  the  head  of  Goat  Island, -the  American  Falls  will  become 
entirely  dry.  Such  an  event  has  happened  once  before  in  that 
part  of  the  gorge  which  lies  below  the  Whirlpool.  Only  here, 
because  the  bend  at  the  Whirlpool  was  a  sharper  one,  the  main 
current  was  thrown  over  to  the  right  side,  and  this  side  was  there- 
fore deepened.  An  abandoned  platform,  once  the  site  of  a  fall 
similar  to  the  American  Falls,  is  now  seen  on  the  Canadian  side, 
forming  Foster's  Flats  (Ongiara  Park)  (Fig.  670).  At  the  foot  of 


Illustrations  of  Complicated  River  Erosion      775 

the  cliff  over  which  the  cataract  once  fell  are  found  huge  blocks  of 
limestone  similar  to  those  seen  at  the  foot  of  the  present  American 
Falls,  and  in  some  of  these  blocks  the  falling  waters  have  bored 


FIG.  670.  —  View  of  Niagara  Glen  or  Foster's  Flats  looking  south.     Forests 
omitted.     (After  Gilbert.)     This  is  the  site  of  .a  former  fall. 

large  pot-holes.  This  platform  and  its  cliffs  present  precisely 
what  will  be  seen  in  the  American  Falls  when  its  waters  are  drawn 
off  by  the  retreating  Horseshoe  several  thousand  years  from  now, 
or  by  man  in  the  less  distant  future. 


The  Genesee  River 

As  a  second  example  of  a  complicated  river  history  we  will  select 
the  Genesee  River,  which  traverses  New  York  state  from  south 
to  north,  rising  in  the  uplands  of  the  Alleghany  Plateau  in  Pennsyl- 
vania and  ending  in  Lake  Ontario  north  of  Rochester,  New  York. 

Preglacial  Character.  —  The  Genesee  River  lies  in  a  region  char- 
acterized by  ancient  valleys  formed  during  the  period  of  normal 
river  erosion  on  the  peneplaned  strata  which  flank  the  Ontario  Dome 
on  the  south.  The  erosion,  which  occurred  in  Tertiary  time,  resulted 
in  the  formation  of  numerous  consequent  valleys  with  subsequent 
branches.  Many  of  the  consequents  were  beheaded  in  the  deep- 
ening of  the  Ontario  lowland  by  the  subsequent  tributary  to  the 
master  stream  of  the  region  (see  map,  Fig.  618,  p.  727).  The 


776        The  Sculpturing  of  the  Earth's  Surface 


map  (Fig.  671)  shows  the  location  of  several  of  these  ancient 
valleys  which  are  concerned  in  the  history  of  this  river.  The  prin- 
cipal ones  are  the  Dansville  Valley  on  the  east  with  several  tribu- 
taries, the  upper  Genesee  Valley,  and  the  Oatka  Valley  with  the  Dale 
Valley  as  a  tributary.  The  largest  of  these  is  the  Dansville  Valley, 
which  near  Dansville  is  ahout  a  thousand  feet  deep  and  from  two  to 


,  .    Rochester 
I  I 
I  I 


Waisa: 


FIG.  671. — Map  of  the  Genesee  River  country;    showing  old  valleys  and 
modern  drainage.     Modern  lakes  in  solid  black. 

three  miles  wide.  Traced  southward,  it  becomes  merged  with  other 
ancient  valleys,  and  it  is  possible  to  trace  the  entire  drainage  sys- 
tem to  the  Susquehanna.  But  the  most  significant  feature  of  the 
system  is  that  the  Dansville  Valley  north  of  that  town  is  much 
deeper  than  the  valley  which  continues  the  drainage  system  south- 
ward. Indeed,  south  of  Dansville,  the  rock  floor  of  the  valley 
rises  very  rapidly  for  several  hundred  feet  until  it  reaches  the  ele- 


Illustrations  of  Complicated  River  Erosion      777 

vation  of  the  rock  floor  of  the  more  southerly  valleys.  This  is 
illustrated  in  the  following  diagram  (Fig.  672),  which  shows  the 
appearance  of  the  valleys.  The  contrast  would  be  greater  if  all 
the  drift  and  other  unconsolidated  deposits  were  removed.  This 
relationship  is  most  readily  explained  by  regarding  the  Dansville 
Valley  as  overdeepened  by  ice-erosion  during  the  period  when  the 
great  Pleistocene  ice  sheet  was  still  in  active  motion  in  this 
region.  Corroboration  of  this  hypothesis  is  furnished  by  the  form 
of  the  Dansville  Valley,  the  sides  of  which  above  Dansville  are 
much  steeper  than  is  characteristic  of  a  mature  valley  due  to  normal 
river  erosion  and  weathering  in  rocks  of  such  soft  nature.  Rocky 


FIG.  672.  —  Diagram  illustrating  the  relation  of  the  Dansville  to  the  Way- 
land  Valleys.  The  floor  of  the  Wayland  Valley  at  Wayland  has  an  elevation 
of  1372  feet.  Its  continuation  on  the  extreme  left  hand  of  the  diagram  is  1300 
feet.  The  elevation  of  the  Dansville  Valley  at  Dansville  is  700  feet.  Both 
valleys  are  partly  filled  by  drift.  The  streams  which  dissect  the  Wayland 
Valley  have  cut  narrow  gorges  in  the  rock  bottom.  (Drawn  by  Mary  Welleck.) 

spurs  on  the  valley  side  have  been  truncated,  a  very  characteristic 
feature  of  glacial  erosion  (see  p.  799),  and  finally,  a  vast  amount 
of  morainal  material  dug  up  from  this  valley  has  been  deposited 
in  the  valley  south  of  Dansville  and  in  the  others  which  continue 
southward  from  this,  such  deposition  being  largely  due  to  the  waters 
from  the  melting  ice.  This  is  shown  by  the  fact  that  the  drift- 
covering  of  the  valley-floor  presents  the  stratification  and  surface 
characters  of  river-laid  deposits. 

The  other  north-south  valleys  have  also  been  deepened  by 
glacial  erosion,  but  not  to  the  same  extent.  This  is  well  illustrated 
by  the  Oatka  Valley,  the  floor  of  which  lies  more  than  100  feet 
below  that  of  the  undeepened  Dale  Valley  which  joins  it  at 
Warsaw.  Formerly  the  floor  of  the  Dale  Valley  joined  the 


7  78        The  Sculpturing  of  the  Earth's  Surface 

Oatka  at  grade,  that  is,  the  two  levels  were  essentially  in  ac- 
cord. To-day  the  point  of  junction  is  marked  by  a  cliff  of  rock 
and  a  descent  to  the  level  of  the  Oatka  Valley,  which  is  here 


FIG.  673.  —  Diagram  showing  the  junction  of  the  Oatka  and  Dale  Valleys 
near  Warsaw.  The  Dale  is  a  hanging  valley  on  the  side  of  the  over-deepened 
Oatka.  (Drawn  by  Mary  Welleck.) 


filled  by  more  than  100  feet  of  alluvial  material  deposited 
upon  it  after  the  overdeepening.  Thus  the  Dale  Valley  is  a  hang- 
ing valley  on  the  side  of  the  Oatka  Valley,  a  type  characteristic 

of  glaciated  regions 
(see  p.  801).  This 
is  illustrated  in  the 
diagram,  Fig.  673. 
The  sides  of  the 
Oatka  Valley  are, 
moreover,  steepened 
and  its  spurs  are 
truncated.  The 
overdeepening  con- 
tinues for  some  dis- 
tance south  of  the 
town  of  Warsaw,  and 
beyond  that  are 
heavy  morainal  de- 
posits composed  in 
part,  at  least,  of  the 
material  scraped  by 

the  ice  from  the  Oatka  Valley.  This  moraine  crosses  the  line  of 
junction  of  the  Oatka  with  the  ancient  Genesee  Valley  and  com- 


^-^ 


FIG.  674.  —  Map  of  the  junction  of  the  ancient 
valleys,  now  partly  drift-filled,  in  the  Portageville 
region,  showing  the  approximate  character  of  the 
valleys  before  they  became  modified  by  glacial 
erosion;  the  course  of  the  modern  Genesee  with 
its  gorges  and  falls  is  represented. 


Illustrations  of  Complicated  River  Erosion      779 


pletely  divides  this  valley  into  two 
parts  by  a  huge  dam  of  drift  at  Port- 
ageville  (Fig.  674). 

Post-Glacial    Development.  —  The 
subsequent  history  of  the  region  was 
somewhat   as  follows.     The  land,  it 
will   be   remembered,  had   been   de- 
pressed on   the   north,   so   that   the 
slopes  of  the  valleys,  which  formerly 
were  southward,  now   descended  to 
the  north.      For  a  time,  the  Ontario 
Valley  was  occupied  by  ice,  blocking 
the  outlets  to  the  east.     This  resulted 
in  the  damming  of  all  the  north-south 
valleys  and  their  conversion  into  lakes 
up  to  the  lowest  point  of  outlet  on  the 
south,  since  drainage  to  the  north  was 
prevented  by  the  ice.     The  outlets 
and  levels  of  practically  all  of  these 
lakes  have  been  traced  with  consid- 
erable detail.     When    the    outlet  of 
the  Ontario  waters  into  the  Mohawk 
(Rome  outlet)  was  uncovered  by  the 
melting  of  the  ice,  the  waters  were 
lowered  to  the  level  of  Lake  Iroquois 
(p.  762),  and,  as  will  be  recalled,  the 
birth  of  Niagara  took  place.     This  re- 
sulted  also   in    the  draining  of   the 
Dansville  and  Oatka  Valleys,  but  the 
upper  Genesee  Valley,  being  dammed 
by  heavy  moraines  at  Portageville,  re- 
mained  a   lake.     Instead   of  south- 
ward drainage,  however,  the  overflow 
of  this  lake  took  place  northward  at 
a  lower  level,  and  thus  was  born  the 
modern  Genesee  (Fig.  675).  The  over- 
flow of  the  waters  of  the  lake  occurred 
on  the  west,  then  they  turned  north 
through  a  depression  in  the  moraine, 
crossed  the  buried  valley  of  the  Oatka 


FIG.  675.  —  Map  of  the 
Genesee  River  from  south  of 
Portageville  to  about  9  miles 
south  of  Mt.  Morris.  The 
1 1  oo  and  1200  foot  contours 
represent  the  rock  topography. 
The  broken  contour  lines  rep- 
resent the  same  elevations  on 
the  drift  filling.  Note  the 
broad  and  regular  character  of 
the  old  valley  north  and  south 
of  St.  Helena.  This  has  been 
glaciated.  The  valley  south 
of  Portageville  shows  rocky 
spurs  and  has  been  less  gla- 
ciated. The  modern  stream 
has  cut  a  narrow  gorge  in  the 
bottom  of  the  St.  Helena  valley 
as  shown  in  the  cross  section 
near  St.  Helena,  given  in  Fig. 
683,  p.  783.  Drifts-fillings  of 
old  valleys  dotted.  A ,  Lower 
Falls,  70  ft. ;  B,  Middle  Falls, 
107  ft. ;  C,  Upper  Falls,  71  ft. 

above  its  junction  with  the 


780        The  Sculpturing  of  the  Earth's  Surface 


old  Genesee  Valley,  and  finally  spilled  into  the  northern  part  of 
the  old  Genesee  Valley  which  Had  been  separated  from  the  southern 

part  by  the  heavy  Port- 
ageville  moraine.  The 
new  river  was  prevented 
from  following  this  val- 
ley to  Lake  Ontario 
(Iroquois)  by  other 
heavy  morainal  ma- 
terial, and  so  the  waters 
spilled  over  the  side 
and  flowed  across  the 
plateau  surface  to  the 
Dansville  Valley,  plung- 
ing into  it  over  the 


FIG.  676.  —  The  lower  falls  of  the  Genesee 
at  Rochester  —  over  the  upper  hard  Medina 
(Thorold)  sandstone.  (Photo  by  author.) 


rocky  side  of  that  valley 
at  the  site  of  the  modern 
town  of  Mt.  Morris,  and 
thence  continuing  for  60 

miles  more  to  the  Lake,  before  reaching  which,  however,  they  had 

to  descend  over  the  exposed  part  of  the  inf  ace  of  the  Niagara  cuesta, 

i.e.,  the  Niagara  escarpment  (Fig.  679,  A). 

Gorge  cutting  by  retreating  waterfalls  began  at  two  points  and 

occurred  simultaneously  with  the  cutting  of  the  Niagara  gorge 

farther  west.    One  gorge  was  cut  by  the  new  Genesee  from  the 

Niagara    escarpment 

southward,    and    the 

other  from  Mt.  Morris 

southwestward.     In  the 

cutting    of    the    gorge 

through     the     Niagara 

escarpment  three  hard 

layers  were  discovered, 

the     lowest     a     white 

quartzite     (Thorold) 

overlying     softer     red 

sandstones      (Medina), 

the  next  a  heavy  lime- 
stone (Clinton)  overlying  shales,  and  the  third  another  limestone 

(Lockport)  overlying  a  still  heavier  shale  bed  (Rochester).    These, 


FIG.  677.  —  The  lower  and  middle  falls  of 
the  Genesee  River  at  Rochester,  the  latter  over 
Clinton  limestones.  (Photo  by  the  author.) 


Illustrations  of  Complicated  River  Erosion      781 


it  will  be  observed,  are  the  same  beds  as  those  the  Niagara  River 
had  to  cut  through,  but  because  that  river  had  so  much  more 
water,  it  has  long  since  accomplished  this  cutting  process  and  is 
now  at  work  upon  the 
single  upper  hard  lime- 
stone (Lockport)  at  the 
present  cataract.  The 
Genesee,  on  the  other 
hand,  is  still  in  an  early 
stage,  because,  although 
it  has  been  at  work  just 
as  long  as  Niagara  has, 
and  on  the  same  rocks, 
the  smaller  amount  of 
water  determined  a 
slower  rate  of  cutting. 
Hence  there  are  still 
three  falls  in  existence 
in  the  Rochester  Gorge,  one  about  100  feet  high,  over  the  Thorold 
quartzite  (Fig.  676),  a  second  40  or  50  feet  high,  over  the  Clinton 
limestone  (Fig.  677),  and  a  third  about  100  feet  high,  over  the 


FIG.  678.  —  The  upper  falls  of  the  Genesee 
River  at  Rochester  —  over  the  lower  beds  of 
the  Lockport  limestone.  (Photo  by  the  author.) 


FIG.  679.  —  A.   Section  of  the  lower  Genesee  River  from  Lake  Ontario  to 
Rochester,  N.  Y.     B.  Section  of  Niagara  river  from  Lake  Ontario  to  the  Falls. 

Lockport  limestone  (Fig.  678).  In  the  Niagara  gorge  the  lower 
two  of  these  falls  have  long  since  disappeared,  because  the  beds 
which  produced  them  have  passed  below  water-level  on  account 


782         The  Sculpturing  of  the  Earth's  Surface 

of  the  southward  dip.     The  relative  position  of  the  waterfalls  of 

the  two  rivers  is  shown  in  the  diagram  (Fig.  679). 
The  waterfall  which  began  at  the  same  time  at  Mt.  Morris, 

cut  more  rapidly  because  it 
had  chiefly  soft  shales  and 
some  sandstones  to  work 
upon.  Hence,  in  the  same 
period  of  time,  it  cut  back 
not  only  across  the  plateau, 
where  it  formed  a  magnificent 
gorge  500  or  more  feet  in 
depth  and  of  a  meandering 
character,  but  all  the  way 
back  to  Portageville,  a  dis- 
tance of  1 8  miles,  completely 
draining  the  Portageville 
lake.  This  cutting  was  not 
uniform,  but  wherever  hard 
layers -existed  a  separate  fall 

' 

FIG.  680.  —  The  Upper 
falls  of  the  Genesee  River  at 
Portage.  (Photo  by  author.) ' 

came  into  being.  To- 
day there  are  three  of 
these,  located  at  A,  B, 
and  C,  respectively  (Fig. 
675),  the  upper  one  at  C 
(Fig.  680)  carrying  the 
water  down  to  the  floor 
of  the  Oatka  Valley,  but 
the  other  two  cutting 
below  that  because  of  the 
greater  depth  of  the  val- 
ley at  Mt.  Morris  (Figs. 
681,  684). 

Contrasting     Features 
of  the  Modern  Stream. 
-It   is    thus   seen    that       FlG    581.— The  middle  falls  of  the  Genesee 
the  Genesee  River  is  not         River  at  Portage.     (Photo  by  the  author.) 


Illustrations  of  Complicated  River  Erosion      783 


a  normal  stream,  but 
one  which  has  come  into 
existence  as  the  result 
of  a  complicated  series 
of  changes  in  the  land 
surface,  by  reversal  of 
the  direction  of  drain- 
age, and  by  the  appro- 
priation of  parts  of  val- 
leys of  several  older 
streams  and  the  cutting 
of  connecting  gorges. 
Where  the  stream  flows 


FIG.  682.  —  The  open  valley  of  the  Genesee 
at  Portageville  above  the  upper  falls.  (Photo 
by  the  author.) 


in  the  ancient  valleys,  the  floors  of  which  are  always  covered  to  a 

considerable  depth  with 
deposits 


FIG.  683.  —  Cross-section  of  the  old  valley  of 
the  Genesee  River,  north  of  Portageville,  N.  Y., 
showing  the  mature  broad  valley,  now  partly 
drift  obstructed,  in  the  center  of  which  the  modern 
stream  (reversed  in  flow)  has  cut  a  narrow  gorge. 

ley  with  sloping  soil-covered  sides  (Fig. 
the  stream  has  cut  a  connect- 
ing gorge,  it  is  confined  in 
a  narrow  chasm  with  steep 
rock  walls.  These  are  ver- 
tical wherever  the  current 
strikes  the  bank  but  form  a 
talus-covered  slope  on  the 
opposite  side.  A  succession 
of  such  cliffs  and  slopes  alter- 
nates on  each  side  of  the 
gorge.  (Seep.  416.) 

Where  the  stream  has  be- 
come incised  below  the  level 
of  the  old  valley-floor,  a  steep- 
sided  narrow  gorge  occupies 
the  center  of  a  broad,  open, 
shallow  valley,  as  shown  in 
the  above  diagram  (Fig.  683). 


made  when 
these  valleys  were  lakes, 
the  stream  itself  has 
the  aspect  of  maturity, 
winding  across  the  flat 
bottom  of  a  broad  val- 
682).  Where,  however, 


>.  684.  — The  lower  falls  of  the 
Genesee  River  at  Portage  as  it  appeared 
in  1892.  (Photo  by  the  author.) 


784        The  Sculpturing  of  the  Earth's  Surface 


FIG.  685.  —  The  lower  falls  of  the  Genesee  at 
Portage  in  1900. 


If  the  old  valley  was  much  encumbered  by  drift,  some  of  this  is 

also  found  in  the  gorge,  whither  it  has  been  carried  by  landslides. 

Finally,  where  the  stream 
is  still  actively  cutting, 
a  series  of  waterfalls 
(A-C)  exist  in  narrow, 
steep-sided  gorges,  each 
waterfall  being  caused  by 
a  hard  stratum  of  rock. 
Of  the  two  portions  flow- 
ing in  mature  valleys, 
the  one  above  Portage- 
ville  lies  at  a  level  which 
is  about  500  feet  above 
that  of  the  mature  por- 
tion below  Mt.  Morris. 
This  difference  of  depth 
is  taken  up  by  the  falls 

and  rapids  in   the  intervening  gorge  portions. 

In  the  lower  of  the  three  falls  in  the  Portage  gorge  (Fig.  684) 

the  phenomenon  of  the  table  rock  or  abandoned  waterfall  seen  at 

Foster's  Flats,  Niagara,  is 

repeated.      Owing   to   a 

bend    in    the    river,    the 

current  was  deflected  to 

the   right    side  and   cut 

there  a  deeper  gorge  (Fig. 

685),  leaving  a  triangular 

table  rock,  the  site  of  the 

former  fall,  upon  the  left 

(Fig.  686).     The  follow- 
ing woodcut  (Fig.  687), 

represents  the  conditions 

as  they  existed  in   1840, 

while  at  present  the  falls 
the    narrower    gorge 


in 


FIG.  686.  —  The  table  rock  and  narrow  lateral 
gorge  at  the  lower  falls  of  the  Genesee  River, 
Portage,  N.  Y.  1892.  (Photo  by  the  author.) 


have  moved  many  hun- 
dred feet  upstream. 

Owing  to  the  jointing  of  this  rock,  the  peculiar  "  Sugar  Loaf  " 
rock,  shown  in  that  illustration  (and  shown  from  below  in  Fig. 


Illustrations  of  Complicated  River  Erosion      785 


FIG.  687.  —  View  of  the  lower  falls  of  the  Genesee  at  Portage  as  it  appeared 
in  1840.     (After  Hall.) 

688),  was  formed.     The  steps  by  which  these  features  were  devel- 
oped are  shown  in  the  following  series  of  diagrams  (Fig.  689). 


FIG.  688.— The  "Sugar 
Loaf"  rock  seen  from  be- 
low (1892). 


FIG.  689.  —  Diagram  showing  the  de- 
velopment of  the  characters  at  the  lower 
falls  of  the  Genesee  River  at  Portage. 
I-III,  hypothetic;  IV,  conditions  in  1840 
(after  Hall) ;  V,  conditions  in  1892  (from 
a  survey  by  the  author). 


786        The  Sculpturing  of  the  Earth's  Surface 

The  Grand  Canon  of  the  Colorado  River 

General  Characters  of  the  Grand  Canon.  —  The  Grand  Canon 
of  the  Colorado  River  is  the  most  stupendous  example  of  a  river 


FIG.  690  a.  —  Sketch  map  of  the  Grand  Canon  district,  showing  the  suc- 
cession of  plateaus.  (See  fig.  6906.)  EC,  Echo  Cliff.  EK,  East  Kaibab 
double  monocline.  WK,  West  Kaibab  fault.  S,  Sevier  fault;  T,  Toroweap 
fault,  and  scarp.  H,  Hurricane  fault  and  ledge.  G  W,  Grand  Wash  Cliffs  and 
fault.  (After  Johnson.) 

gorge  to  be  found  anywhere  on  earth.  It  is  more  than  a  mile  in 
depth  in  its  principal  division,  and  its  average  width  is  eight  miles, 
though  some  portions  are  wider.  "  Its  sides  are  a  succession  of 


Illustrations  of  Complicated  River  Erosion      787 

rocky  slopes  and  sinuous  cliffs,  some  of  which  are  huge  steps  with 
from  300  to  500  feet  sheer  descent"  (Darton).  The  main  section, 
the  Grand  Canon  proper,  has  a  length  of  125  miles,  and  it  is  divided 
into  the  Kaibab  or  eastern  and  the  Kanab  or  western  portions. 
The  Kaibab  is  the  deepest  and  most  complex  portion  of  the  canon, 
and  in  this  the  river  has  cut  an  inner  gorge  into  the  crystalline 


FIG.  690  b.  —  Block  diagram  and  section  of  a  part  of  the  Colorado  Plateau 
(after  Powell).  The  section  is  drawn  north  of  the  Colorado  and  shows  the 
principal  formations.  These  are,  at  the  base,  the  pre-Cambrian  crystallines 
and  ancient  sediments  (in  -white) ;  the  Tonto  group  of  sandstones  and  shales 
(Cambrian)  (in  circles) ;  the  Carboniferous  beds  (limestones,  shales,  sand- 
stones and  limestones)  (blocked) ;  and  the  Permo-Triassic  sandstones,  etc.  (fine 
dots). 

On  the  right  is  Echo  Cliff,  an  erosion  ridge  of  Permo-Triassic  sandstones  in  a 
monocline,  the  flexure  of  which  is  downward  on  the  east.  Along  the  section-line 
this  forms  a  triangular  ridge  with  a  westward  slope  in  conformity  with  the  dip, 
almost  as  steep  as  the  bold  erosion  scarp  on  the  west.  Next,' west  of  this  cliff 
lies  the  Marble  Plateau  formed  "by  the  nearly  horizontal  Carbonic  (Kaibab) 
limestone  and  transected  by  the  Marble  Canon.  This  was  formerly  also 
covered  by  the  Permo-Triassic  sandstone,  a  remnant  of  which  forms  the  high 
Paria  Plateau  which  rises  above  it  in  the  background  denned  by  the  Vermilion 
Cliff.  West  of  the  Marble  Plateau  lies  the  Kaibab  Plateau,  which  rises  2500 
to  4000  feet  higher  and  is  formed  of  the  same  Carboniferous  limestone  (Kaibab). 
This  plateau  is  bounded  on  the  east  by  the  East  Kaibab  monocline,  which  is 
generally  a  double  flexure.  On  the  west  is  the  West  Kaibab  fault,  here  shown 
as  a  flexure.  Southward  this  splits  into  three  secondary  faults  with  associated 
scarps,  and  they  extend  almost  to  the  brink  of  the  canon.  This  fault  lowers 
the  formation  and  the  surface  to  the  Kanab  Plateau  still  of  the  same  limestone 
series.  This  is  transected  by  the  Kanab  Canon.  In  the  distance  is  seen  the 
Vermilion  Cliff  of  Permo-Triassic  sandstones  and  shales.  The  Kanab  is  sepa- 
rated from  the  Uinkaret  Plateau  by  the  Toroweap  fault  with  downthrow  on 
the  west  amounting  to  600  or  700  feet  near  the  Canon.  The  Uinkaret  is 
separated  from  the  Shivwitz  Plateau,  still  farther  west,  by  the  Hurricane  fault 
ledge,  with  a  recent  displacement  up  to  1400  feet  in  addition  to  an  older  displace- 
ment. (See  map  on  opposite  page,  Fig.  690  a.) 

basement-rocks.  In  the  Kanab  portion,  which  is  much  less  diversi- 
fied and  picturesque,  the  inner  gorge  is  cut  into  the  lower  member 
of  the  late  Palaeozoic  series  (the  Red  Wall  limestone),  the  surface 
of  which  forms  a  broad  platform  about  two  miles  wide  on  either 
side  and  a  thousand  feet  below  the  summit  of  the  plateau,  which 
stands  nearly  7000  feet  above  sea-level.  Above  the  Kaibab  portion 
of  the  Grand  Canon  the  river  has  cut  the  Marble  Canon,  which  is 


788        The  Sculpturing  of  the  Earth's  Surface 

65  miles  in  length  and  extends  nearly  north  and  south,  being  joined 
nearer  its  lower  end  by  the  canon  of  the  Little  Colorado.  (See 
map,  Fig.  690  a,  and  the  more  detailed  map,  Fig.  691.)  The 
Marble  Canon  is  cut  through  the  Carbono-Mississippian  lime- 
stones, shales,  and  sandstones  down  to  and  into  the  Cambrian 
(Tonto  beds),  but  to  the  west  of  it  a  broader  open  valley  exposes 
the  older  pre-Cambrian  rocks,  here  raised  to  greater  altitude.  The 
reason  for  these  differences  in  the  rock  sections  will  become  apparent 
when  the  general  structure  of  the  country  is  understood. 


FIG.  691.  —  Map  of  the  Grand  Canon  and  part  of  the  Marble  Canon  on 
the  right.     (U.  S.  G.  S.) 

Rocks  Exposed  in  the  Grand  Canon.  —  In  the  immediate  vicinity  of 
the  canon  the  strata  which  compose  the  plateaus  are  of  Palaeozoic  and  older 
age.  At  the  base  of  the  series  lie  the  granite  and  other  crystalline  base- 
ment rocks,  these  being  exposed  in  the  bottom  of  the  canon  from  about  the 
Grand  View  Point  westward  for  35  miles  or  more  (map,  Fig.  691).  The 
river  has  cut  an  inner  gorge  in  this  resistant  rock,  to  a  depth  of  800  to 
1000  feet,  as  is  shown  in  the  photograph  taken  from  El  Tovar  and  repro- 
duced in  Fig.  693,  and  in  the  cross-section  of  the  canon  from  the  same 
point  (Fig.  694).  These  rocks,  which  form  dark  rugged  ledges  in  the 
inner  gorge,  terminate  upwards  in  a  very  level  surface  of  erosion  (a  pene- 
plane)  and  upon  them,  over  wide  areas,  rest  the  horizontal  Tonto  sand- 
stones, about  150  feet  thick,  followed  by  the  Tonto  shales  of  greenish  color 
and  about  800  feet  in  thickness.  These  formations  belong  to  the  Cambrian 
series  of  rocks.  -The  surface  of  the  Tonto  sandstone  forms  a  plateau  or 
terrace  on  either  side  of  the  inner  gorge,  averaging  a  mile  in  width,  while 
the  shales  which  overlie  it  form  long  slopes  interrupted  by  subordinate 
limestone  and  sandstone  ledges.  The  platform  and  inner  gorge  are  char- 


Illustrations  of  Complicated  River  Erosion      789 

acteristic  features  and  are  well  shown  in  the  topographic  map  of  the  canon 
(Fig.  691)  and  in  the  photograph  (Fig.  693). 

In  the  broad  part  of  the  canon  which  lies  northeast  of  Grand  View 
(west  of  the  Marble  Canon)  and  in  some  other  sections  (Shinumo  basin, 
parts  of  Bright  Angel  canon,  etc.)  another  series  of  old  rocks  appears 
between  the  Tonto  sandstone  and  the  basal  granites  and  other  crystallines. 
This  pre-Cambrian  series  is  known  as  the  Grand  Canon  Series,  and  is  divis- 
ible into  two  parts.  The  lower  division  (Unkar  group}  consists  of  basal 
conglomerates,  limestones  in  thick  dark  beds,  bright  red  shades  and  upper 
heavy  quartzites  and  brown  sandstones.  The  higher  division  (Chuar 
group]  is  seen  only  in  the  region  west  and  northwest  of  the  mouth  of  the 
Little  Colorado,  and  consists  of  sandstones,  shales,  and  limestones.  The 


FIG.  692.  —  General  view  of  the  Grand  Canon. 

beds  of  the  Grand  Canon  series,  which  have  a  total  aggregate  thickness 
of  12,000  feet,  dip  rather  steeply  to  the  north  and  northeast,  and  are  in 
marked  contrast  with  the  nearly  horizontal  Tonto  beds  which  overlie  them 
unconformably.  The  relation  is  shown  in  the  section  (Fig.  694).  The 
surface  of  the  inclined  Grand  Canon  beds  was  beveled  across  by  erosion  and 
worn  into  an  undulating  plain  before  the  Tonto  beds  were  deposited  upon  it. 
Next  above  the  Tonto  shales,  and  like  these  nearly  horizontal,  is  a 
massive  hard  compact  limestone  about  500  feet  thick  and  of  pale-blue 
gray  color  when  freshly  broken.  It  forms  great  cliffs  the  faces  of  which 
are  stained  red  from  the  wash  of  the  red  beds  which  overlie  it,  and  on  this 
account  the  rock  has  been  named  the  Red-wall  limestone.  Its  cliffs  form 
prominent  features  in  the  scenery  of  the  canon,  and  they  are  well  indicated 
by  the  heavy  shading  above  the  Tonto  platform  in  the  map  (Fig.  691) 


790        The  Sculpturing  of  the  Earth's  Surface 

(see  also  the  section  Fig.  694).  In  the  photograph  (Fig.  693)  the  prominent 
dark  cliff  above  the  middle  of  the  view  is  formed  by  this  limestone.  This 
limestone  also  forms  the  caps  of  many  flat-topped  spurs  and  buttresses,  and 
of  outlying  buttes  and  towers  such  as  Cheops  Temple,  Newberry  Butte, 
Sheba  Temple,  Solomon's  Temple,  and  many  others. 

Next  above  the  Red-wall  limestone  lies  a  series  of  red  shales  interbedded 
with  thick  layers  of  red  and  red-brown  sandstone,  the  whole  forming  the 


FIG.  693.  —  Telescopic  view  of  the  Grand  Canon  of  the  Colorado,  from  El 
Tovar  Hotel,  showing  the  crystalline  rocks  (granite  and  gneiss)  at  the  base 
unconformably  overlain  by  the  horizontal  Palaeozoic  strata.  Resting  directly 
upon  the  granite,  the  Tonto  sandstone  forms  a  platform  in  the  foreground ; 
the  first  cliff  beyond  the  gorge  is  formed  by  Unkar  quartzites  covered  by  Tonto 
shale.  The  prominent  cliff  three  fourths  the  way  up  is  the  Red-wall  limestone, 
and  it  is  covered  by  the  Supai  Red  Beds.  The  Red-wall  limestone  forms  the 
prominent  butte,  Cheops  Pyramid,  near  the  center  of  the  picture,  while  the  higher 
butte  is  Buddha  Temple,  a  detached  part  of  the  Kaibab  Plateau  which  forms 
the  background.  A  section  across  this  part  of  the  canon  is  given  in  Fig.  694. 

Supai  formation,  which  has  a  total  thickness  of  noo  feet.  Because  of 
their  hardness,  the  sandstones  cause  a  succession  of  steps  in  the  middle  of 
the  slope  formed  by  the  red  shales.  These  red  beds  constitute  a  large 
part  of  many  fine  promontories  and  ridges  which  project  far  out  into  the 
canon.  Many  of  the  buttes  and  towers  are  also  formed  oi  them.  The 
next  higher  division  is  the  heavy  Coconino  sandstone,  300  feet  thick,  and 


Illustrations  of  Complicated  River  Erosion      791 

this  forms  a  vertical  gray  cliff  in  the  canon  wall,  800  feet  below  the  top. 
It  is  massive  and  strongly  cross-bedded.  On  top  of  it  lies  the  Kaibab  lime- 
stone, a  slabby  light  gray  limestone  nearly  800  feet  thick,  which  everywhere 
causes  the  terminal  cliff  and  the  summit  of  which  forms  great  forest- 
covered  plateaus.  This  is  the  highest  rock  exposed  in  the  canon  walls,  and 
it  and  the  underlying  formations  to  the  top  of  the  Tonto  group  belong 
to  the  "  Carboniferous  "  series  of  rocks.  Between  them  and  the  under- 
lying Tonto  group  is  a  great  break  in  succession,  i.e.,  a  hiatus,  which  leaves 
the  great  middle  portion  of  the  Palaeozoic  series  unrepresented. 

This  rock  series  was  once  covered  by  higher  beds,  but  these  have  all 
been  eroded  from  the  immediate  vicinity  of  the  canon.  They  comprise 
vari-colored  shales  and  red  sandstones  of  Permian  and  Triassic  age,  por- 
tions of  which  are  now  seen  in  the  Vermilion  Cliffs  and  in  Echo  Cliff  (see 
Fig.  690  a  and  the  stereogram,  Fig.  690  b}.  A  still  higher  series  of  white 
sandstones  (Jurassic)  is  seen  in  the  White  Cliffs,  which  form  the  edges  of 
more  elevated  plateaus,  far  to  the  north  of  the  canon. 


FIG.  694.  —  Section  across  the  Grand  Canon  west  of  Bright  Angel  Creek. 
From  El  ToVar  to  Kaibab  Plateau,  showing  the  unconformity  between  the  basal 
crystallines  and  the  Unkar  Group,  and  that  between  the  latter  and  the  Tonto 
formation  of  the  Palaeozoic.  (U.  S.  G.  S.)  A,  Kaibab- limestone;  B,  Coconino 
sandstone;  C,  Supai  (Red  Sandstones  and  shales);  D,  Red-wall  limestone; 
E,  Tonto  shales  and  Sandstones  (at  base) ;  F,  Granite,  etc. 


General  Character  and  Development  of  the  Region. — Taking  a 
broad  view  of  the  country  into  which  the  Grand  Canon  is  cut,  we 
recognize  that  it  consists  of  a  series  of  great  plateaus  composed  of 
nearly  horizontal  strata.  These  plateaus,  known  collectively  as  the 
Colorado  Plateaus,  are  separated  from  one  another  by  monoclinal 
flexures  or  by  cliffs  located  along  lines  of  faulting.  The  general 
trend  of  the  axes  of  the  flexures,  and  that  of  the  fault  lines,  is  north 
and  south,  or  approximately  at  right  angles  to  the  Grand  Canon. 
(See  map,  Fig.  690  a,  and  diagram,  Fig.  690  b.)  Originally  the 
deformation  was  entirely  by  monoclinal  flexures  which  ascended 
westward,  so  that  the  region  was  composed  of  a  series  of  huge 
steps  rising  westward,  each  separated  from  the  others  by  flexures, 
and  elevated  from  one  to  several  thousand  feet  above  the  one  next 
below.  During  or  shortly  after  the  flexing,  the  monoclines  of  the 
western  half  of  the  series  broke  and  the  elevated  blocks  settled 


792        The  Sculpturing  of  the  Earth's  Surface 

down  again,  the  depression  in  general  being  progressively  more 
pronounced  westward,  so  that  a  descending  series  of  steps  was 
produced.  Thus  originated  the  peculiar  type  of  faulting  of  this 
region  previously  referred  to  (p.  600),  in  which  the  up-flexed  portion 
became  the  downthrow  block,  the  strata  of  which  bend  down  near 
the  fault-plane,  while  the  strata  of  the  upthrow  block  bend  up  near 
that  plane,  this  being  the  reverse  of  the  bending  produced  by  drag 
in  ordinary  faulting.  (See  Fig.  518  b,  p.  602.)  In  the  eastern  half 
of  the  region,  the  original  monoclinal  flexures  remained  unbroken 
so  that  the  steps  descended  both  eastward  and  westward  from  the 
high  or  'central  area:  eastward  by  a  succession  of  monoclinal 
flexures ;  westward  by  a  succession  of  faults. 

This  period  of  disturbance  was  followed  by  extensive  erosion, 
during  which  the  country  was  reduced  to  a  peneplane,  both  the 
fault  and  the  flexure-steps  being  obliterated.  It  was  into  this 
peneplaned  country,  the  surface  of  which  was  now  formed  of  dif- 
ferent rocks  in  the  various  blocks,  that  the  westward-flowing 
Colorado  began  to  cut  its  canon.  Over  parts  of  this  peneplaned 
surface  basaltic  lavas  were  poured  out,  covering  adjoining  portions 
of  contiguous  fault-blocks,  in  which  the  surface  was  often  cut  on 
different  formations.  A  second  period  of  faulting,  in  the  same 
direction,  ensued,  but  this  affected  chiefly  the  western  faults  (Grand 
Wash  and  Hurricane).  Where  these  were  crossed  by  basalt  flows, 
the  part  resting  upon  the  western  block  was  lowered,  relatively, 
with  that  block,  during  the  faulting.  In  the  ensuing  second  cycle 
of  erosion,  the  softer  strata  were  stripped  from  off  the  various 
blocks,  except  where  protected  by  the  basalt  layers,  and  thus 
the  step-like  topography  was  reproduced,  but  this  time  the  surfaces 
of  the  several  plateaus  in  the  vicinity  of  the  canon  were  formed 
by  the  Kaibab  limestone,  the  resistant  upper  member  of  the 
Carbonic  series.  Portions  of  the  higher  formations,  however, 
remained  to  form  the  more  elevated  plateaus  north  of  the  canon, 
as  seen  in  the  stereogram  (Fig.  690  b)  and  in  the  map  (Fig.  690  a). 
The  principal  westward-facing  scarps  of  the  region  thus  represent 
resequent  fault-scarps,  though  parts  of  the  Hurricane  and  the  Grand 
Wash  cliffs  are  new  fault-scarps,  produced  by  a  third  period  of 
faulting,  after  the  cutting  of  the  Grand  Canon  had  progressed  to  a 
considerable  extent. 

As  the  Colorado  had  learned  to  flow  westward  upon  the  old  pene- 
planed surface,  formed  during  the  first  cycle  of  erosion,  it  continued 


Land  Forms  Due  to  Glacial  Sculpture         793 


in  this  direction  upon  the  renewed  great  uplift  of  the  country. 
This  latest  period  of  erosion  witnessed  the  cutting  of  the  great 
canon,  which,  though  of  such  magnitude,  is  only  the  latest  erosion 
feature  in  a  region  which  has  under- 
gone   a    succession    of    uplifts   and 
down  cutting  of  vastly  more  stupen- 
dous proportions  and  all  within  the 
space  of  time  which  has  elapsed  since 
the  close  of  the  early  Tertiary  period. 
Because  of  the  differential  elevation 
of  the  several  blocks  into  which  the 
canon  is  cut,  the  bottom  of  the  canon 
in  the  several  sections  is  formed  by 
different  members  of  the  rock  suc- 


FIG.  694  a.  —  Major  John 
Wesley  Powell. 


cession. 

The  characters  of  the  Grand  Canon 
were  practically  unknown  to  the 
world,  until  a  heroic  exploring  party 
traversed  it  from  end  to  end  in  1869, 

under  the  leadership  of  that  intrepid  geologist,  Major  John 
Wesley  Powell  (portrait,  Fig.  694  a),  one  of  the  organizers  and 
early  directors  of  "the  United  States  Geological  Survey.  Since  that 
day  it  has  become  a  favorite  region  for  study  and  inspirational  con- 
templation, having  in  recent  times  been  made  readily  accessible 
to  the  general  tourist.  To  the  student  of  geology  it  serves  as  the 
most  wonderful  object  lesson  in  canon  cutting  which  our  country 
affords. 


LAND  FORMS  DUE  TO  GLACIAL  SCULPTURE 

Glaciers  are  very  important  agents  in  the  sculpturing  of  the 
landscape,  their  mode  of  cutting  being  that  already  set  forth  in 
an  earlier  chapter  (pp.  433-435).  We  shall  here  summarize  only 
the  characteristics  of  the  main  types  of  land-forms  due  to  glacial 
sculpturing. 

The  Cirque,  Arete,  and  Horn.  — One  of  the  most  characteristic 
erosion  features  of  glaciated  mountain  districts  is  the  cirque  or 
glacial  amphitheater  at  the  head  of  the  glacier  (Fig.  303,  p.  366; 
Figs.  695  a,  b).  Where  glaciers  have  entirely  disappeared,  these 
steep-walled  semicircular  indentations  remain  as  eloquent  witnesses 


794        The  Sculpturing  of  the  Earth's  Surface 

of  former  glacial  occupancy,  for  no  agent  but  the  glacier  is  known 
to  be  capable  of  producing  such  a  feature.  Where  several  cirques 
occur  side  by  side,  they  may  be  separated  by  narrow  ridges,  which, 
with  the  growth  of  the  cirque  by  continued  cutting,  will  become 
sharper  and  sharper,  with  knife-edge  crests,  and  this  eventually 
results  in  the  production  of  a  strongly  serrated  ridge  or  arete 
(Fig.  6q6  a). 


FIG.  695  a.  —  South  slope  of  Mt.  Rainier,  with  Paradise  Park  in  the  fore- 
ground. Note  the  glacial  cirques  which  are  being  cut  back  into  this  maturely 
dissected  volcano.  (Photo  by  A.  H.  Barnes,  courtesy  of  D.  W.  Johnson.) 

As  we  have  seen  in  an  earlier  chapter,  the  cirque  is  produced 
by  the  plucking  action  of  the  lower  portion  of  the  glacier  mass  which 
occupies  the  valley  in  the  mountain  side.  The  bergschrund  or 
crevasse  at  the  head  of  the  glacier  formed  by  its  forward  movement 
permits  the  thawing  and  refreezing  of  the  ice  at  the  bottom,  and 
this  brings  about  a  loosening  of  rock  masses  from  the  base  of  the 
cirque.  Its  walls,  which  originally  were  those  of  a  sloping  valley, 
become  more  and  more  steepened.  The  floor  of  the  cirque  also  may 
be  deepened  into  one  or  more  basins  by  the  scouring  action  of  the 
moving  ice,  and  such  basins  may  remain  filled  with  water  after 
the  ice  has  melted  away  entirely,  forming  rock  basin  lakes  or  tarns. 
Lakes  may  also  be  held  in  an  ice-abandoned  cirque  by  morainal 
matter  deposited  across  its  mouth  (Fig.  696  b). 


Land  Forms  Due  to  Glacial  Sculpture          795 

Where  cirques  have  been  cut  into  an  old  table-land  from  various 
directions,  the  region  after  the  disappearance  of  the  ice  presents 
a  remarkably  scalloped  character  which  has  been  aptly  compared 
with  a  heavy  piece  of  'dough  from  which  biscuits  have  been  cut, 
the  hollows  left  by  the  biscuit  cutter  representing  the  cirques,  and 
the  remnant  of  the  dough,  the  irregular  ridges  of  upland  remaining. 


FIG.  695  b.  —  Glacier  at  Cascade  Pass,  —  Glacier  Peak  quadrangle,  Wash- 
ington ;  showing  steep  walls  of  cirques  separated  by  narrow  aretes ;  marginal 
crevasses  on  the  ice  and  abundant  morainal  material  are  shown  in  the  fore- 
ground. (Photo  by  B.  Willis,  for  U.  S.  G.  S.,  courtesy  of  Popular  Science 

Monthly.) 

At  first,  the  cirque  is  scarcely  wider  than  the  valley  below  in  which 
the  glacier  lies,  but  by  continual  sapping  it  becomes  enlarged  in 
all  directions,  so  that  its  form,  with  reference  to  that  of  the  valley 
extending  from  it,  has  been  aptly  compared  to  that  of  the  large 
rounded  head  of  a  nail.  Further  growth  of  the  cirques  may  pro- 
duce compound  or  scalloped  forms  with  subordinate  cirques  around 
the  margin  of  the  principal  one.  In  this  manner  a  complex 


796        The  Sculpturing  of  the  Earth's  Surface 


"  grooved  "  upland  is  produced,  which  later,  by  the  sharpening 
and  partial  destruction  of  the  dividing  ridges,  produces  a  sharply 


FIG.  696  a.  —  Diagram  of  a  mountain  region  from  which  the  former  glaciers 
have  melted  away,  leaving  the  cirques  to  testify  to  their  former  presence.  Some 
of  the  cirques  diverge,  leaving  a  broad  rounded  ridge  between  them,  others  con- 
verge, being  separated  only  by  a  serrated  knife-edge  or  arete  (grat).  (After 
Davis.) 

"  fretted  "  upland  surface,  such  as  is  characteristic  of  the  summit 
of  Mont  Blanc  and  other  regions  of  the  Alps.  Sharp-toothed 
comb-like  ridges,  called  aretes  or  grats,  dividing  the  cirques,  are 


FIG.  696  b.  —  A  glacial  lake  or  tarn  in  an  old  cirque.     Utah.     (Photo  by 

F.  J.  Pack.) 


Land  Forms  Due  to  Glacial  Sculpture         797 


characteristic  of  this 
stage,  and  at  the  junc- 
tion points  of  such  comb- 
ridges  sharp  pyramidal 
peaks  or  "  horns  "  arise 
as  the  erosion  remnants 
of  the  highest  portion  of 
the  original  mountain 
mass.  Of  these,  the 
Matterhorn  (Fig.  697), 
the  Aletschhorn,  and 
many  others  are  typical 
Alpine  examples.  Mount 
Sir  Donald  in  the  Selkirk 
Mountains  of  Canada  is 
a  typical  American  ex- 
ample. 

The  U-shaped  Valley. 
—  The  normal  section  of 
a  young  river  valley  is  more  or  less  V-shaped,  the  greatest  cutting 
taking  place  at  the  bottom  and  downward,  while  weathering  pushes 


FIG.  697.  —  The  Matterhorn,  a  typical  Al- 
pine peak  or  "horn"  in  Switzerland,  as  seen 
from  the  Gornergrat.  (Photo  by  D.  W. 
Johnson.)  „ 


FIG.  698. — The  Yosemite  Valley,  a  typical  U-shaped  young  glaciated  val- 
ley, much  over-deepened  so  that  the  side  streams  now  flow  in  Hanging  Valleys 
and  enter  the  main  valley  by  Waterfalls. 


798        The  Sculpturing  of  the  Earth's  Surface 


FIG.  699.  —  The  Lauterbrunnen  Valley ;  a  young  glacial  trough,  similar  to  the 
Yosemite  Valley.  It  is  especially  noted  for  the  splendid  cataracts  which  enter 
it  from  hanging  valleys.  (Courtesy  D.  W.  Johnson.)  (See  Fig.  310,  p.  372.) 


FIG.  700.  —  Upper  part  of  Kern  Canon,  California.  A  splendid  mature 
glacial  trough  showing  the  typical  catenary  curve.  (Photo  by  Gilbert,  U.  S. 
G.  S.  Courtesy  D.  W.  Johnson.) 


Land  Forms  Due  to  Glacial  Sculpture         799 


back  the  sides  of  the  valley  most  rapidly  in  the  upper  portion. 
With  increasing  age  the  floor  of  the  valley  is  widened  to  a  flat  sur- 
face, but  the  sides  retain  their  slopes.  When  such  valleys  are  oc- 
cupied by  moving  glaciers,  cutting  takes  place  not  only  at  the 
bottom,  but  on  the  sides  as  well,  and  the  section  of  the  valley 
changes  to  a  U-form,  with  precipitous  cliffs  often  of  great  height. 
This  constitutes  the  stage 
of  youthful  deepening  of 
river  valleys  by  ice  and 
presents  the  most  pic- 
turesque aspect  of  such 
erosion.  It  is  well  shown 
in  the  great  Yosemite 
Valley  of  California  (Fig. 
698),  which  has  been 
scoured  out  by  ice  to  a 
depth  of  more  than  two 
thousand  feet,  and  in  the 
beautiful  Lauterbrunnen 
Valley  of  Switzerland 
(Fig.  699),  which  has  a 
very  similar  character. 
(See  also  Fig.  310^.372.) 
Where  a  valley  has  long 
been  occupied  by  a  large 
ice  stream,  the  walls  of 
the  resulting  trough  are 
smooth  and  somewhat 
flaring,  and  the  bottom 
beautifully  rounded  (Figs.  700,  701).  It  no  longer  has  the  parallel 
sides  of  a  U-shaped  valley  and  is  better  called  a  glacial  trough. 
Most  glaciated  valleys  belong  to  this  category. 

If  the  original  river  valley  is  an  irregular  one,  with  rocky  spurs 
projecting  from  alternate  sides,  these  spurs  will  gradually  be  worn 
off  by  the  ice,  and  their  truncated  faces  will  form  a  characteristic 
feature  of  such  a  valley  that  has  been  glaciated.  Hence  the 
steepened  sides  and  the  truncated  spurs  form  characteristic 
topographic  features  by  which  young  glaciated  valleys  may  be 
recognized,  while  the  flaring  sides  and  general  smoothness  of  form 
indicate  a  valley  long  occupied  by  ice  (Fig.  702  a,  6). 


FIG.  701.  —  Catenary  curves  of  glacial 
trough  in  Norway.  View  from  Moldestadt. 
(Courtesy  D.  W.  Johnson.) 


•51^ 


FIG.  702  a.  —  River  valley  with  nu-  FIG.  702  b.  —  The  same  valley  as 

merous  spurs  and  a  V-shaped  form.       that  shown  in  Fig.  702  a  after  glacia- 


(U.  S.  G.  S.) 


tion.  The  spurs  have  been  truncated 
and  the  valley  is  broadly  U-shaped. 
(U.  S.  G.  S.) 


AL 

C      C  Irque 

GT    Glacial  Trou 

HV    Hanging 

MG  Main  Glacier 

MP  Matterhorn  Peak 

R     Rock  Step 

T     Tarn 

TG  Tributary  Gl elder 

us   Unylaeiafect  Summii 

uv   UnglaciateJ  Va/ley 


FIG.  703.  —  Land  forms  due  to  ice  sculpture.  A .  A  mountain  mass  occupied 
by  a  glacier  system.  The  surfaces  of  the  main  and  tributary  valleys  are  in 
accord.  B.  The  same  mountain  mass  after  melting  of  the  glaciers.  The 
floor  of  the  main  valley  lies  far  below  that  of  the  tributary  valleys,  which  now 
form  hanging  valleys  upon  its  side.  (After  Davis.)  C.  Sketch  of  a  glaciated 
valley  showing  the  U-shaped  form.  (Military  Geology.) 

800 


Land  Forms  Due  to  Glacial  Sculpture          801 

Overdeepening ;  Hanging  Valleys.  —  A  normal  river  valley 
will,  in  general,  be  joined  by  its  tributary  valleys  at  grade,  that  is, 
the  floors  of  the  two  valleys  will  be  essentially  in  accordance  at 
the  point  of  junction.  In  glacially  eroded  valleys,  however,  this 


FIG.  704.  —  Diagrams  illustrating  the  transformation  of  a  lateral  stream 
valley  into  a  hanging  valley  by  glacial  erosion  of  the  main  valley:  A,  by 
widening  of  the  main  valley  without  deepening;  and  B,  by  over-deepening  of 
the  same  without  essential  widening. 

is  frequently  not  the  case,  for  the  main  valley  which  is  occupied 
by  the  larger  glacier  may  be  greatly  deepened  by  it,  while  the 
lateral  one,  occupied  by  smaller  glaciers  or  by  none,  may  be  deep- 
ened slightly  or  not  at  all.  Tributary  glaciers  have  their  surfaces 


FIG.  705.  —  The  Norwegian  fjords  are  partially  submerged  glacial  troughs 
into  which  side  streams  fall  in  cataracts  from  hanging  valleys.  The  Naero 
Fjord,  from  Gudvangen.  (Courtesy  D.  W.  Johnson.) 

in  accord  with  the  surface  of  the  main  glacier  at  the  point  of 
junction,  but  their  floors  may  be  at  very  different  levels  (Fig.  703). 
This  might  be  due  to  the  widening  of  the  main  valley  at  the  bot- 
tom (A)  (Fig.  704),  but  is  more  commonly  produced  by  overdeep- 
ening  (B)  or  by  both.  When  the  ice  finally  melts  away  from  the 


802        The  Sculpturing  of  the  Earth's  Surface 


valleys,  the  point  of  junction  of  the  lateral  with  the  main  valley 
is  found  to  be  high  up  on  the  side  of  the  main  valley,  the  lateral 

valley  then  being  spoken  of 
as  a  hanging  valley.  Such 
hanging  valleys  are  very  char- 
acteristic of  glaciated  dis- 
tricts, as  is  so  well  shown  in 
the  falls  of  the  Yosemite  and 
Lauterbrunnen  valleys,  which 
drop  from  hanging  valleys 
high  up  on  the  side  of  the 
main  trough  (Figs.  698,  699), 
and  in  many  of  the  beautiful 
falls  of  the  Norwegian  fjords 
(Fig.  705).  Hanging  valleys 
may  also  be  produced  in 
other  ways.  Thus,  where  a 
waterfall  in  the  main  valley 
cuts  beyond  the  mouth  of  the 
small  tributary  valley  which 
formerly  joined  it  at  grade 
above  the  waterfall,  this  val- 
ley will,  for  a  time  after  the 


FIG.  706.  —  Falls  of  a  hanging  valley 
on  the  side  of  the  Genesee  Gorge  at 
Portage,  N.  Y.  The  difference  in  eleva- 
tion between  the  floors  of  the  two  valleys 
is  about  200  feet,  and  is  due  to  the  re- 
cession of  the  cataracts  of  the  main  val- 


ley.   (Photo  by  author.)  recession  of  such  a  waterfall, 

remain  as  a  hanging  valley  on 

the  sides  of  the  main  gorge.     The  valley  of  the  Bloody  Run  on 
the  side  of  the  Niagara  Gorge  is  such  a  hanging  valley,  and  there 
are  many  examples  of  this  type 
in  the  gorges  of  young  streams, 
as  for  example  in  the  Genesee 
(Fig.    706).     In    broad,    open 
valleys,   however,   such   a   re- 
lationship generally  points  to 
glacial    overdeepening  of    the 
main  valley. 


FIG.  707.  —  Longitudinal  profile  of 
a  river  valley  (a) ;  and  of  the  same 
valley  after  it  has  been  deepened  by  a 
glacier  (6).  Note  that  the  valley  in 
the  second  case  is  deeper  above  the 
mouth,  so  that  it  would  hold  a  lake 
after  the  melting  of  the  glacier. 


Glacial    Lakes,    Tarns,   and 
Fjords.  —  Glaciers   do   not 
deepen  their  channels  in- a  uni- 
form manner  any  more  than  rivers  do.     Near  the  head  of  the 
glacier,  as  well  as  near  its  front,  the  deepening  is,  as  a  rule,  to  a 


Land  Forms  Due  to  Glacial  Sculpture         803 


lesser  degree  than  between  these  two  points.  In  consequence, 
the  profile  of  a  glacial  trough  will  be  more  strongly  concave  than ' 
that  of  a  normal  river  valley  (Fig.  707).  When  such  a  valley 
is  uncovered  by  the  melting  of  the  ice,  a  rock  tarn  or  lake  will 
remain  behind  in  the  deepened  part.  This  lake  may  attain  addi- 
tional depth  by  the  building  < 

of  a  morainic  dam  across  the 
mouth  of  the  valley.  Many 
of  the  beautiful  Scottish  lochs 
are  examples  of  such  lake 
basins  of  glacial  origin,  and 
a  similar  origin  is  a,scribable 
to  the  Finger  Lakes  of  New 
York  state. 

When  the  deepening  by  ice 
has  proceeded  to  a  level  be- 
low that  of  the  sea,  or  if  an 
over-deepened  valley  near  the 
coast  is  carried  down  by  a 
subsidence  of  the  land,  the 
sea  will  occupy  a  part  of 
such  a  valley,  and  a  fjord  is 
produced.  Such  fjords  are 
very  characteristic  of  the  bold 
coast  of  Norway  (Figs.  705, 
708),  and  they  are  found  in  FIG.  708.  —  A  typical  Norwegian  fjord, 
Scotland  and  elsewhere  as  showing  U-shaped  glacial  trough  and  the 
11  rrii  TT  j  T>-  relatively  flat  upland  of  the  Norwegian 

well.      The    Hudson    River    penep]an^    (From  D.  W.  Johnson's  sL* 
(Fig.  709)  has  the  character    Processes,  etc.    John  Wiley  and  Sons.) 
of  a  fjord,  the  depth  being 

over  600  feet  from  water  level  to  rock  bottom  in  the  Highlands, 
whereas  opposite  New  York  City,  it  is  only  somewhat  over  300  feet. 
That  this  is  not  due  to  differential  subsidence  or  downward  bowing 
of  the  land  is  shown  by  the  regular  rise  of  the  crest-line  of  the 
gorge  (the  surface  of  the  old  peneplane),  for  any  unequal  sub- 
sidence affecting  the  river  bottom  would  also  affect  this  crest-line. 
The  relationship  is  shown  in  the  diagram  on  page  804  (Fig.  710). 

There  are  many  drowned  river  valleys  along  the  Maine  coast, 
but  it  is  not  known  whether  these  have  the  characteristics  of  true 
fjords,  that  is,  whether  they  are  deepened  by  glacial  erosion,  less  at 


804        The  Sculpturing  of  the  Earth's  Surface 

the  mouth  than  some  distance  back  of  it  (Fig.  711).     A  map  of  a 
typical  fjorded  coast,  that  of  New  Zealand,  is  shown  in  Fig.  712. 

The  Tinds  and  Nunataks.  —  Around  the  margins  of  ice  caps 
which  cover  elevated  tracts  of  country,  a  special  form  of  erosion 
takes  place.  Where  the  ice  spills  through  notches  in  the  rock 


FIG.  709.  —  The  gorge  of  the  Hudson  in  the  Highlands.     North  of  West  Point. 

margin,  active  erosion  goes  on  at  those  points,  and  the  result  is 
often  the  production  of  a  conical  rock  hill  by  the  reduction  of  a 
mass  of  rock  enclosed  by  the  spill-overs  of  such  an  ice  cap.  So 
long  as  such  a  mass  is  enclosed  by  ice,  above  which  it  projects,  it 
is  called  a  nunafak,  as  are  all  rock-masses  projecting  above  the 


FIG.  710.  —  Profile  of  the  Hudson  River  from  New  York  City  through  the 
Highlands,  showing  the  fjord-like  excavation  above  the  mouth  and  the  gradual 
northward  rise  of  the  peneplane. 

marginal  portion  of  the  ice-field.  When  freed  by  the  melting  of 
the  ice,  such  a  conical  hill  due  to  glacial  erosion  is  called  a  tind. 
Unlike  the  conical  hills  or  horns  produced  at  the  head  of  the 
mountain  glaciers  by  sapping,  the  tind  is  produced  at  the  lower 
end  of  the  glaciers  by  lateral  erosion.  Such  tinds  are  fre- 
quently met  with  in  higher  latitudes  where  ice-caps  are  still  in 
existence. 


The  Sculpturing  of  the  Edge  of  the  Land      805 

Erosive  Work  of  Valley  and  Mountain  Glaciers  Contrasted  with  That 
of  Ice-Caps  and  Continental  Ice-Sheets 

Valley  and  mountain  glaciers  tend  to  accentuate  the  relief  of 
the  country,  especially  in  their  middle  and  upper  courses.  Ice- 
caps and  continental  glaciers,  on  the  other  hand,  tend  to  plane 
down  the  relief  of  the  country  which  they  cover  and  produce  an 
accentuation  of  the  topography  only  at  their  margins. 

% 

THE  SCULPTURING  OF  THE  EDGE  OF  THE  LAND 

The  edge  of  the  land,  that  is,  the  sea-coast  and  the  coasts  of  large 
lakes,  are  subjected  to  a  sculpturing  process  by  the  waves  and 


FIG.  711.  —  Map  of  a  section  of  the  coast  of  Maine,  showing  the  topography 
produced  by  drowning  of  river  valleys. 


806        The  Sculpturing  of  the  Earth's  Surface 

currents,  with  the  production  of  special  erosion  forms,  the  more 
important  of  which  we  may  consider. 

Young  and  Mature  Coasts.  —  A  young  coast-line  is  one  but 
recently  established  by  the  relative  change  in  the  level  of  land 
and  sea.  Such  a  coast-line  is  a  regular  one  if  it  is  formed  upon  a 
young  surface  of  a  recently  emerged  sea  or  lake-bottom,  for  in  such 
a  case  the  coast  marks  merely  the  intersection  of  two  planes,  that 


FIG.  712.  —  Fjord  coast  of  Dusky  Sound,  New  Zealand.  A  young  coast- 
line upon  a  strongly  dissected  land  surface.  (British  Admiralty  Chart,  from 
Ratzel,  Die  Erde.) 

of  the  sea  and  that  of  the  young  coastal  plain.  A  young  coast- 
line upon  an  old  land  surface,  i.e.,  a  peneplane,  will  also  be  on 
the  whole  fairly  regular,  though  such  regularity  will  be  less  pro- 
nounced than  in  the  case  of  a  young  land.  But  a  young  coast-line 
formed  upon  a  maturely  dissected  land  surface  will  be  one  of  ex- 
treme irregularity,  such  as  is  shown,  for  example,  on  the  Maine 
coast  of  New  England  (Fig.  711),  or  the  coasts  of  Norway,  Sweden, 
and  elsewhere  (Fig.  712).  A  mature  coast-line  on  any  land  surface 
tends  toward  regularity  of  outline,  for  the  projecting  headlands  will 


The  Sculpturing  of  the  Edge  of  the  Land      807 

be  cut  back  and  the  reentrants  will  be  bridged  by  the  formation 
of  bars  and  beaches. 

Erosion  on  the  Coast  of  a  Young  Land.  —  A  newly  emerged  coastal 
plain  of  gently  seaward-dipping  strata  will  not  immediately  be 
effectively  attacked  by  the  waves,  because  the  water  near  the  shore 
is  as  a  rule  too  shallow  for  vigorous  wave  erosion.  In  such  a 
case,  an  off-shore  bar  will  be  built  first,  at  the  line  where  the  shoal- 
ing determines  the  breaking  of  the  prevailing  storm  waves  for  that 


FIG.  713. — Wave-cut  chalk  cliffs  near  Fecamp,  France.  (From  D.  W. 
Johnson,  Shore  Processes,  John  Wiley  and  Son.)  Note  the  marine  bench 
in  front  of  the  cliffs,  only  partially  covered  by  the  lighter  beach  sand. 

coast.  This  bar  is  built  by  the  eroding  of  the  bottom  by  the  break- 
ing wave,  the  hurling  forward  of  the  material  removed,  and  its  de- 
position in  shallower  waters,  from  which  it  eventually  emerges  as 
an  off-shore  bar,  as  already  described  (p.  330,  p.  539).  In  front  of 
the  bar  the  water  has  been  sufficiently  deepened  for  wave  attack, 
behind  the  bar  a  shoaling  of  the  lagoon  takes  place,  with  the  forma- 
tion of  peat  deposits,  and  eventually,  perhaps,  a  strip  of  dry  new- 
made  coastal  land. 

Meanwhile  the  waves  continue  to  attack  the  bar,  cutting  it 
away,  while  the  sand  dunes  travel  inland  over  the  lagoon  deposits. 


8o8         The  Sculpturing  of  the  Earth's  Surface 


FIG.  714.  —  Map  of  Cape  Cod,  showing  the  long  outer  erosion  cliff  of  the 
"  Forearm,"  and  the  sand  spits  built  from  the  sands  thus  eroded,  both  southward 
and  northward,  where  they  form  the  foundation  of  the  dune  headland  of  Prov- 
incetown.  (After  Coast  Survey  Chart,  from  Ratzel,  Die  Erde.) 


The  Sculpturing  of  the  Edge  of  the  Land      809 


These  deposits  of  peat,  etc.,  now  begin  to  appear  upon  the  shore, 
and  as  erosion  continues,  the  entire  new-formed  series  will  be 
removed  again.  With  the  advancing  sea,  the  water  retains  suf- 
ficient depth  for  efficient  wave  work,  and  when  the  lagoon  deposits 
have  been  wholly  removed,  the  original  shore  will  be  vigorously 
attacked,  the  water  now  being  deep  enough  for  this  process.  As  the 
sea  cuts  inland  like  a  horizontal  saw,  a  sea-cliff  is  produced,  which, 
because  of  the  regularity  of  the  coastal  plain,  will,  other  factors  re- 
maining inactive,  be  uniform  and  continuous.  This  cutting  into 
the  land  may  continue  indefinitely  if  the  tidal  and  other  currents 
are  strong  enough  to  carry  away  the  product  of  erosion.  If  not, 
a  beach  is  formed  at  the  foot  of  the  cliff,  and  erosion  eventually 


FIG.  715. — Work  of  the  waves  in  cutting  away  rocky  coasts.  A.  Suc- 
cessive stages  in  the  destruction  of  the  island  of  Helgoland.  B.  Cliffs  of  Nor- 
mandy ;  the  coast  has  been  cut  back  so  far  that  the  tributaries  of  the  old  river 
systems  now  enter  the  sea  independently.  (After  Lobeck,  in  Military  Geology.) 

becomes  checked  by  the  accumulation  of  a  protecting  belt  of 
material.  (See  Fig.  446,  p.  530,  and  Fig.  447,  p.  531.).  The  bold 
Yorkshire  coast  of  England,  the  Channel  Coast  of  France  (Fig. 
713),  and  the  long  sand  cliffs  of  the  out§r  arm  of  Cape  Cod  on 
the  Massachusetts  coast  (Fig.  714)  illustrate  the  continued  cutting 
of  the  sea  into  the  land  in  regions  which,  in  many  respects,  have 
the  characteristics  of  coastal  plain  strata.  One  striking  effect  of 
such  inward  cutting  on  the  coast  is  the  betrunking  of  the  streams 
which  formerly  ran  down  the  slope  of  the  coastal  plain  into  the 
sea.  Eventually,  the  main  stream  may  be  cut  back  so  far  that 
its  former  branches  enter  the  sea  as  independent  streams.  Good 
illustrations  of  this  are  furnished  by  the  Channel  Coast  of  France. 
(See  Fig.  715,  B.) 


8io        The  Sculpturing  of  the  Earth's  Surface 

Erosion  of  Coasts  on  Maturely  Dissected  Lands.  —  The  coast 
of  Maine  and  that  of  Sweden  present  typical  examples  of  a  relatively 
young  coastline  upon  a  maturely  dissected  drowned  land.  The 
numerous  parallel  river  valleys  have  been  converted  into  inden- 
tations, and  the  ridges  between  the  valleys  project  into  the  sea 
as  long  spurs  or  rock  tongues,  and  by  the  submergence  of  low  places 
across  them,  numerous  islands,  aligned  with  the  ridges,  are  pro- 
duced. Travel  along  such  a  coast  is  possible  only  by  boats ;  all 
roads  and  railroads  must  be  placed  far  inland.  Erosion  (Fig.  711, 
p.  805)  is  restricted  to  the  clifnng  of  the  headlands  and  islands, 


FIG.  716.  —  Marblehead  Neck,  Mass.  A  rocky  island  tied  to  the  mainland 
by  a  narrow  beach  (or  tombolo)  of  cobblestones  and  sand.  (From  D.  W.  John- 
son's Shore  Processes,  John  Wiley  and  Sons.) 

but  at  the  same  time,  the  narrower  and  shallower  indentations 
are  bridged  across  by  ^nd  and  gravel  bars  built  by  the  waves, 
and  rocky  islands  are  tied  to  the  main  land  by  sand  and  pebble 
beaches  (Fig.  716).  Thus  the  coast  is  straightened,  but  the  process 
is  necessarily  a  very  slow  one,  and  a  change  of  level  may  occur  be- 
fore it  has  proceeded  far.  If  elevation  takes  place,  there  will 
appear  abandoned  sea-scarps  inland,  as  well  as  abandoned  beaches. 
Where  a  normally  dissected  coastal  plain  is  drowned,  as  on  the 
coast  of  Maryland  and  New  Jersey,  the  sea  will  enter  the  con- 
sequent river  channels  and  wave  erosion  will  widen  the  bays  thus 
formed,  as  in  the  case  of  Chesapeake  and  other  bays  on  the 


The  Sculpturing  of  the  Edge  of  the  Land      811 

Atlantic  coastal  plain..  Where  drowning  has  gone  so  far  that  the 
inner  lowland  is  submerged,  the  cuesta  of  the  coastal  plain  may  be 
broken  into  islands,  Long  Island,  New  York,  being  an  example  of 
this.  Here  wave  attack  produces  cliffs  along  the  northern  coast, 
which  is  the  drowned  inface  of  the  cuesta,  but  on  the  southern 
coast,  where  the  submergence  has  produced  a  new  intersection  of 
the  sloping  coastal  plain  and  glacial  outwash  strata  with  the  sea, 
erosion  is  preceded  by  bar-building  and  lagoon-filling. 


FIG.  717. — Sea-caves  on  the  coast  of  California.  An  upper  cave  is 
shown  which  was  cut  when  the  land  stood  ten  feet  lower  with  reference  to  sea- 
level.  (Photo  by  G.  W.  Stose,  from  U.  S.  G.  S.) 


Special  Erosion  Features 

Stratified  and  Jointed  Rocks.  —  Vigorous  wave  erosion  on  hori- 
zontally stratified  rocks  is  apt  to  produce  vertical  cliffs,  especially 
if  the  beds  at  the  base  are  weak.  By  the  attack  of  the  waves  sea- 
caves  may  be  produced  on  softer  strata  (Figs.  717,  718),  or  by 
extensive  undermining  harder  beds  may  form  overhanging  ledges 
until  they  break  down  from  overweight.  The  formation^of  sea- 
caves  is  especially  favored  by  joints  which  traverse  the  rocks 
vertically  and  permit  the  waves  to  cut  along  them.  On  the  north 
shore  of  Lake  Superior,  an  extensive  series  of  such  wave-cut  caves 


812        The  Sculpturing  of  the  Earth's  Surface 

was  formed  in  the  soft  sandstone  of  the  Pictured  Rocks.     Many  of 
these  caves  have  since  collapsed,  but  new  ones  are  constantly  form- 


FIG.  718.  —Wave-cut  arch  known  as  the  Natural  Bridge  one  half  mile  north 
of  mouth  of  Medder  Creek,  Santa  Cruz  County,  California.     (U.  S.  G.  S.) 

/ 

ing.  Caves  of  this  kind  are  not  uncommon  on  the  British  and 
French  coasts,  and  in  the  past  some  of  these  served  as  the  haunts 
of  smugglers  (Figs.  719  a,  6).  They  are  cut  in  other  jointed  rocks 


FIG  719  a  —  Tilly  Whim  Caves.  Elevated  sea-caves  cut  by  waves  in  hori- 
zontal'  (Jurassic)  strata.  Coast  of  English  Channel,  Durlston  Head,  south  of 
Swanage,  England. 


The  Sculpturing  of  the  Edge  of  the  Land      813 

as  well,  good  examples  being  found  on  the  basaltic  coast  of  Nova 
Scotia,  while  Fingal's  Cave  on  the  Island  of  Staffa,  west  coast  of 


FIG.  719  b.  — Tilly  Whim  Caves.     Caves  due  to  marine  erosion  in  horizontal 
(Jurassic)  rocks,  coast  south  of  Swanage,  England. 

Scotland,  is  a  classic  example  of  a  wave-cut  cavern  of  consider- 
able depth  in  a  jointed  basaltic  rock  (Figs.  720,  721,  see  also  Figs. 
122  a-d,  pages  178  to  179). 
Caverns  of  this  type  are 
often  found  at  some  eleva- 
tion above  sea-level,  as  on 


FIG.  720. — Fingal's  Cave. 
Isle  of  Staffa,  off  west  coast 
of  Scotland.  Looking  seaward 
from  within  the  cave.  (Photo  by 
author.) 


FIG.  721. — Interior  view  of  Fingal's 
Cave,  a  sea-cave  in  columnar  lava  on  the 
Island  of  Staffa,  Scotland.  (From  D.  W. 
Johnson's  Shore  Processes,  John  Wiley  and 
Sons.) 


814        The  Sculpturing  of  the  Earth's  Surface 

the  coast  of  the  Island  of  Gotland  in  the  Baltic,  on  the  coast  of 
Mackinac  Island  in  Lake  Huron  (Fig.  722),  and  elsewhere.  They 
indicate  an  elevation  of  the  land  or  a  subsidence  of  the  water-level 
since  their  formation.  (See  also  Fig.  717,  p.  811.) 

When  joints  are  numerous  and  traverse  the  rock  mass  through- 
out, a  series  of  ramparts  may  be  produced,  such  a  character  being 


FIG.  722. — The  Arch  or  Natural  Bridge  cut  from  brecciated  limestone 
(Upper  Silurian)  by  the  waves  of  Lake  Huron  during  a  former  higher  level. 
The  arch  is  the  remnant  of  the  roof  of  a  wave-cut  cavern. 

especially  well  shown  in  the  jointed  sandstones  along  Cayuga 
Lake,  New  York  (Fig.  564,  p.  639).  Isolated  masses  or  sea-stacks 
may  also  be  severed  from  the  mainland  in  this  manner.  Such 
structures  are  especially  marked  on  the  coast  of  Scotland  and  the 
Orkney  Islands,  where  the  jointed  and  nearly  horizontal  Old  Red 
Sandstone  cliffs  are  rapidly  succumbing  to  wave  attack.  Similar 
sea-stacks  are  formed  (Figs.  723  a,  b)  from  the  chalk  on  the  English 
coast  and  on  the  French  coast  (see  Fig.  446,  p.  530),  and  from  the 


The  Sculpturing  of  the  Edge  of  the  Land      815 

basalt  on  the  coast  of  Nova  Scotia.  Several  ancient  stacks  are 
found  on  the  northern  shore  of  Lake  Michigan,  Lake  Huron,  and 
on  Mackinac  Island,  marking  a  former  higher  level  of  the  water. 


FIG.  723  a.  —  "Old  Harry"  and  "Old  Harry's  wife."  Sea-stacks  cut  by 
the  waves  of  the  English  Channel  from  jointed  chalk  cliffs.  The  Foreland, 
north  of  Swanage,  England. 

They  also  abound  in  places  on  the  coast  of  the  island  of  Gotland  at 
a  level  coinciding  with  that  of  the  abandoned  sea-caves. 

Where  stratified  rocks  are  steeply  inclined,  their  erosion  on  the 


FlG.  723^6.  —  "Old  Harry  Rocks"  and  chalk  cliff.     Attacked  by  storm  waves. 
Coast  north  of  Swanage,  England. 


816        The  Sculpturing  of  the  Earth's  Surface 


coast  produces  a  series  of  "  skerries  "  or  reef-like  ridges  formed  by 
the  harder  beds ;  when  submerged  they  form  dangerous  shoals. 

Igneous  Rocks.  —  Except  when  fine-grained  and  much  jointed, 
igneous  rocks  present  a  less  favorable  medium  for  cliff-cutting 
than  do  the  stratified  rocks.  Wherever  a  .fairly  coarse  granite 

forms    the  coast,   wave 

erosion  does  not  keep 
pace  with  the  weathering 
of  exposed  parts  of  the 
mass,  and  the  surface  will 
thus  often  be  rounded 
rather  than  abrupt.  A 
part  of  this  rounding 
may  also  be  due  to  pre- 
vious glaciation,  as  on 
our  northern  Atlantic 
coast,  where  the  topog- 
raphy produced  by  the 
ice  has  not  yet  been  de- 
stroyed by  wave  erosion. 
Such  masses,  if  large, 
form  a  subdued  rocky 
coast,  even  though  ex- 
posed to  very  violent 
wave  attack  (Fig.  356  a, 
•p.  429).  Basaltic  rocks, 
on  the  other  hand,  be- 
cause of  their  excellent 
jointing,  produce  frown- 


Bras. 


FIG.  724.  —  Purgatory  Chasm,  Newport, 
R.  I.  A  cleft  in  the  conglomerate  rocks  due 
to  erosion  by  the  waves  of  a  weathered  zone 
between  joint  cracks.  It  simulates  a  chasm 
left  by  the  erosion  of  a  dike,  and  was  formerly 
regarded  as  such. 


ing  cliffs,  those  of  Nova 
Scotia  and  farther  north, 
and  those  of  the  west 
Scottish  coast,  being 
typical  examples. 

Erosion  of  Dikes.  —  Along  many  of  the  cliffed  portions  of  the 
New  England  and  the  British  coasts,  basaltic  or  diabase  dikes 
which  cut  other  strata  and  are  exposed  to  wave  attack  have  been 
removed  by  the  loosening  of  the  successive  joint  blocks  into  which 
such  dikes  are  generally  divided.  As  a  result,  a  deep,  narrow 
chasm  is  formed,  with  vertical  sides  if  the  dike  was  vertical  (see 


The  Sculpturing  of  the  Edge  of  the  Land      817 


FIG.  725.  — Winthrop  Great  Head,  a  drumlin  near  Boston,  whose  eastern 
end  has  been  removed  by  wave  erosion.  In  the  foreground  is  a  beach  built 
from  the  eroded  material.  (Photo  by  D.  W.  Johnson.) 

Fig.  135,  p.  192),  and  at  the  end  of  such  a  chasm  is  often  found  a 
cavern-like  hollow  into  which  the  waves  dash,  compressing  the  air, 
whereupon  the  water  is  forced  put  again  with  great  force,  producing 
various  forms  of  waterspouts.  When  the  jointing  of  the  dike  is  not 
well  developed,  it  is  not  so 
readily  eroded,  especially 
when  the  surface  is  gently 
sloping,  in  which  case  the 
surface  of  the  dike  will  be 
even  with  that  of  the  en- 
closing rock.  Similar  fis- 
sures may,  however,  be 
eroded  in  massive  much- 
jointed  rocks,  along  a 
weak  zone,  as  is  well  il- 
lustrated by  the  chasm 
called  Purgatory  on  the 
coast  at  Newport,  R.  I., 
which  is  worn  along  a 

weak    zone    in   massive     FlG    ?26.  _Grover>s    cliff)  winthrop.    An 
conglomerate  (Fig.  724).        eroded  drumlin  on  the  Massachusetts  coast. 


8i8        The  Sculpturing  of  the  Earth's  Surface 

Erosion   of   Glacial   Deposits.  —  When   unconsolidated   glacial 
deposits  are  exposed  along  the  shore,  erosion  by  waves  progresses 

on  the  whole  very  rapidly 
until  it  becomes  checked  by 
the  accumulation  of  boul- 
ders which  are  left  behind 
during  the  removal  of  the 
finer  material.  This  is  es- 
pecially well  shown  where 
the  terminal  moraine  of  the 
last  ice  age  is  exposed  upon 
the  shore,  as  is  the  case 
along  the  south  coast  of 
Massachusetts  at  Woods 
Hole  and  the  Elizabeth 
Islands  and  on  parts  of  the 
shore  of  Long  Island.  In 
these  regions  the  beach  at 
low  tide  presents  an  accu- 
mulation of  large  boulders 
which  are  not  readily  moved 

FIG.  727. —Development  of   Nantas-  by    the    waves.     Drumlins 

ket   Beach.     (After  Johnson    and   Reed)  ,           h            t  fe     ^ 
First    stage,    original    drumlms    restored. 

Restoration  in  dotted  lines.  are  apt  to  be  fronted  by  a 

AL.     Allerton  Lost  Drumlin  boulder  pavement  (Fig.  447, 

AtL.    Atlantic  Lost  Drumlin  p     ^T\    wnich    greatly    re- 

BL.      Bayside  Lost  Drumlin  ^    ,     ,, 

BI.       Bumkin  Island  tards  further  erosion- 

G.        Great  Hill  Drumlins  cliffed  by  the 

H.       Hampton  Hill  sea  present  a  very  charac- 

LHI.  Little  Hog  Island  teristic  Profile>  which  varies 

N.  Nantasket  Hill  with  the  distance  to  which 

Q.  Quarter  Ledge  the  drumlin  has   been   cut 

Sa.  Sagamore  Head  ,      ,        „,.    ,,           TT      , 

Sk.  Skull  Head  back-     Wmthrop  Head  on 

SL.  Strawberry  Lost  Drumlin  the  Massachusetts  coast  is 

St.  Strawberry  Hill  a  drumlin  half  of  which  has 

T.  Thornbush  Hill  ,       '       , 

W.  White  Head  been  removed  by  the  sea. 

WL.  White  Head  Lost  Drumlin  The  profile  there  formed  is 

WP.  Windmill  Point  Sand  Spit  shown  in  the  illustration 
(Fig.  725).  A  near  view  of  the  profile  of  an  eroded  drumlin  cliff 
from  the  same  coast  is  shown  in  Fig.  726.  The  sands  worn  from 


The  Sculpturing  of  the  Edge  of  the  Land      819 


the  drumlins  are  partly  used  in  the  building  of  the  beach  shown 
in  the  foreground  of  Fig.  725.  Because  of  the  abundant  supply  of 
such  sands,  extensive  bars  and  spits  are  produced  by  the  long-shore 
currents  and  the  waves,  and  these  frequently  tie  rocky  islands  to 
the  main  shore,  as  in  the  case  of  Nahant,  which  is  tied  by  a  long 
sand  beach  to  the  Massachusetts  coast  at  Lynn.  Formerly  isolated 
drumlins,  more  or  less  eroded  by  the  sea,  may  thus  be  tied  together 


FIG.  7 28  a.  —  Development  of 
Nantasket  Beach.  (Johnson  and 
Reed.)  2d  stage.  Early  erosion  and 
connection  of  some  of  the  drumlins 
by  bars  or  tombolos.  (For  notation 
see  Fig.  727.) 


FIG.  728  b.  —  Development  of  Nan- 
tasket Beach.  (Johnson  and  Reed.) 
3d  stage.  Further  connection  of 
eroded  drumlins  by  beaches  (tom- 
bolos). (For  notation  see  Fig.  727.) 


by  sand  beaches,  as  has  been  the  case  on  the  Massachusetts  coast, 
in  the  formation  of  Nantasket  beach.  Here  beaches  have  in  some 
cases  been  built  so  rapidly  that  the  old  cliffs  have  been  protected 
from  further  erosion,  and  thus  drumlins  in  various  stages  of  dis- 
section, but  now  some  distance  inland  from  the  shore,  enter  into 
the  construction  of  this  remarkable  land-mass.  The  successive 
stages  in  the  formation  of  this  beach  are  shown  in  the  maps  here 
reproduced  from  the  studies  of  D.  W.  Johnson  and  W.  G.  Reed,  Jr. 


820        The  Sculpturing  of  the  Earth's  Surface 

(Figs.  727  to  730).  Extensive  sand  spits  and  bars  are  also  built 
along  the  outer  coast  of  Cape  Cod  (Fig.  714),  where  the  waves 
cut  away  the  unconsolidated  stratified  drift  of  fluvio-glacial  origin 
and  carry  it  both  southward  and  northward.  On  the  north 
they  have  built  a  succession  of  bars  on  which  the  sands  have 
been  piled  up  into  dunes  by  the  wind  to  form  the  headland  of 


FIG.  729  a.  — Development  of  Nan- 
tasket  Beach.  (Johnson  and  Reed.) 
4th  stage.  Development  of  beach- 
ridges  and  beach-plain.  (For  nota- 
tion see  Fig.  727.) 


FIG.  729  b.  —  Development  of  Nan- 
tasket  Beach.  (Johnson  and  Reed.) 
5th  stage.  The  modern  beach,  con- 
sisting of  old  cliffed  drumlins  with 
broad  beach-plain  in  front  of  it. 
(Notation  as  in  Fig.  727.) 


Provincetown.  The  seaward  face  of  the  forearm,  subject  to  con- 
stant wave  erosion,  presents  a  continuous  and  regularly  curving 
line  of  cliffs  with  scarcely  any  break  for  many  miles.  Where  clays 
are  present  in  the  sands,  as  at  Highland  Light,  the  cliffs  present 
picturesque  erosion  features  due  chiefly  to  rain- wash  (Fig.  731). 


EROSION  FORMS  PRODUCED  BY  ATMOSPHERIC  AGENCIES 

The  atmospheric  agencies,  under  which  are  classed  the  diurnal 
and  seasonal  changes  of  temperature;  the  moisture  and  gases  of 
the  air  and  the  activities  of  frost,  rain,  and  wind,  produce  erosion 


Erosion  Forms  Produced  by  Atmospheric  Agencies    821 


forms  essentially  peculiar   to   themselves.     Some  of  these  have 
already  been  mentioned,  but  they  may  again  be  briefly  referred  to, 
while  others  not  yet  noted  may  be  added.     Several  of  these  agents 
generally  act  in  conjunc- 
tion,   and    their    indi- 
vidual activities  cannot 
always  be  dissociated. 

The  Cliff  and  Talus. 
—  These  are  primarily 
the  products  of  tempera- 
ture changes  and  frost 
work.  The  fragments 
broken  by  these  agents 
from  the  face  of  the  cliff 
will  accumulate  at  its 
foot  to  form  the  talus, 
the  slope  of  which  is  in 
general  harmonious  with 
the  size  and  other  char- 
acters of  the  fragments. 
When  not  removed  by 
the  agents  of  denuda- 
tion such  a  talus  may 
accumulate  until  it 
mantles  much  if  not  the 
whole  of  the  cliff-face. 
Old  talus  surfaces  are 
generally  overgrown  with  forests  if  the  climate  is  sufficiently 
moist.  Mountains  exposed  to  much  frost  action,  but  not  covered 
by  glaciers,  are  generally  characterized  by  extensive  accumula- 
tions of  broken  fragments,  which  may  entirely  mask  the  under- 
lying ledges.  This  is  the  case  to  a  considerable  extent  in  the 
White  Mountains  of  New  Hampshire,  in  Pikes  Peak,  in  Ben  Nevis 
in  west  Scotland,  and  in  other  mountains  like  these.  In  some 
cases  these  fragments  give  to  the  summit  a  remarkably  perfect 
conical  form. 

Sculpturing  by  Rain  and  Atmospheric  Moisture.  — The  earth  pil- 
lars of  Colorado  and  of  Bozen  in  the  Tyrol  (Figs.  345  a,  b,  pp.  41 1 , 41 2) 
have  already  been  referred  to  as  products  of  rain  erosion.  Where 
cliffs  are  composed  of  soft  clays  or  shales,  or  of  soluble  material, 


FIG.  730.  —  Development  of  Nantasket 
Beach.  (Johnson  and  Reed.)  A  hypothetical 
future  stage.  The  outer  drumlins  are  all  worn 
away  and  the  beach  extends  from  the  rocky 
ridges  at  Nantasket  (Atlantic)  to  World's  End 
drumlin  (WE} ;  thence  to  Bumkin  Island 
(BI)  and  thence  to  Nantasket  Hill  (JV)  in  Hull. 
(For  notation  see  Fig.  727.) 


822         The  Sculpturing  of  the  Earth's  Surface 


fantastic  shapes  will  be  carved  by  the  rain  (Fig.  731)  or  produced 
by  a  process  of  solution,  this  being  especially  marked  in  certain 
elevated  limestone  peaks  in  which,  if  the  strata  are  inclined,  a  strik- 
ingly fretted  upland  surface  is  produced  (Fig.  346  b,  p.  413).  Re- 
markable solution  forms  are  also  produced  upon  the  surfaces  of 
salt  mountains,  such  as  those  of  Cardona,  Spain,  and  of  southern 
Persia.  The  salt  pillars  of  the  Dead  Sea,  which  have  given  rise  to  the 

legend  of  the  transfor- 
mation of  Lot's  wife  into 
salt,  are  likewise  the 
products  of  subaerial 
sculpture  upon  an  an- 
cient cliff  of  salt. 

Sculpturing  Process  of 


• 


FIG.  731.  —  Clay  cliffs  at  Highland  Light, 
Truro,  on  Cape  Cod,  showing  characteristic 
erosion  forms  due  to  rain  and  wind. 


Wind.  —  The  sculptur- 
ing process  of  wind  com- 
prises both  deflation  of 
material  loosened  under 
the  influence  of  the 
weather,  and  active  cor- 
rasion  of  the  rock  surface 
1  by  the  sand-blast.  By 

the  combined  action  of  these  processes,  wind-carved  structures, 
often  of  remarkable  form,  are  produced.  We  have  already  referred 
to  the  faceted  pebbles  or  "  dreikanter  "  which  represent  faces 
carved  by  the -sand-blast  upon  partly  exposed  pebbles  in  regions 
of  much  sand-drifting  (p.  406).  But  structures  of  a  much  larger 
type  are  also  produced  by  wind  erosion.  Such  are  the  natural 
monuments  carved  from  soft  sandstone  of  uniform  grain  in  the 
plains  about  Pikes  Peak,  Colorado,  and  which  are  generally  capped 
by  a  projecting  mass  of  more  resistant  sandstone  due  to  cementa- 
tion of  the  grains  by  iron  oxide  (Fig.  340,  p.1  407).  The  great 
sandstone  buttes  which  dominate  the  Plains  country  in  various 
regions  in  western  North  America  (Fig.  344,  p.  410,  and  Fig.  599, 
p.  701)  are  much  larger  examples  of  such  erosion  remnants  left  after 
much  of  the  rock  formerly  continuous  with  them  had  been  removed 
by  stream  and  by  eolian  erosion. 

But  by  far  the  most  striking  products  of  stream  and  eolian 
erosion,  supplemented  in  part  by  other  agencies  of  denudation, 
are  seen  in  the  wonderful  sandstone  arches  or  natural  bridges 


Erosion  Forms  Produced  by  Atmospheric  Agencies    823 

found  in  several  localities  within  the  semi-arid  district  of  southern 
Utah  and  other  regions  (Fig.  732).     One  of  these,  the  Rainbow  or 


FIG.  732.  —  Natural  Bridge,  or  sandstone  arch  formed  by  stream  and  wind 
erosion.     Utah.     (F.  J.  Pack,  photo.) 

"  Barohoini "  Natural  bridge  of  southern  Utah,  has  a  height  of 
398  feet,  a  width  between  abutments  of  278  feet,  and  is  33  feet 


FIG.  733.  • —  Looking-Glass  Rock;  a  mass  of  sandstone  carved  and  hollowed 
by  erosion.  The  size  is  indicated  by  th'at  of  the  white  horse  in  the  left  fore- 
ground, and  that  of  the  two  men  in  the  rear  opening.  Near  La  Sal  Mountains, 
Utah.  (Whitman  Cross,  photo;  from  U.  S.  G.  S.) 

wide  at  the  top.  Another  rock-form  due  chiefly  to  wind  erosion 
is  seen  in  Looking-Glass  Rock  near  La  Sal  Mountains,  Utah 
(Fig-  733)- 


824        The  Sculpturing  of  the  Earth's  Surface 

In  arid  regions  these  products  of  subaerial  erosion  are  numerous 
and  present  such  a  bewildering  series  of  fantastic  shapes  that  even 
the  most  prosaic  are  beguiled  into  comparisons  with  organic  forms, 
while  the  more  imaginative  here  find  the  prototypes  of  all  the  fanci- 
ful* creations  of  fairy-tale  and  folk-lore. 

The  sculpturing  processes  create  the  landscape  as  we  see  it; 
they  supplement  the  constructional  and  deformational  processes, 
and  it  is  they  that  are  primarily  responsible  for  the  diversified 
nature  of  the  earth's  surface,  and  for  the  great  variety  of  habitats 
in  which  the  multifarious  .types  of  plant  and  animal  life  find  a 
momentary  place  of  occupancy,  during  the  ceaseless  procession 
of  organic  forms  upon  the  earth.  Finally,  it  is  the  sculpturing 
processes  which  bring  about  the  never-ceasing  changes  in  the 
contours  of  the  face  of  the  earth,  which  like  the  changing  human 
countenance  is  never  the  same  from  day  to  day,  though  we,  with 
our  limited  power  of  vision,  are  able  to  perceive  only  the  more 
pronounced  and  abrupt  of  these  modifications. 

The  hills  are  shadows,  and  they  flow 

From  form  to  form,  and  nothing  stands ; 
They  melt  like  mist,  the  solid  lands, 

Like  clouds  they  shape  themselves  and  go. 


FIG.  734.  —  Alexander  von  Humboldt.  The  first  great  leader  in  the  study 
of  land  forms,  their  origin,  and  the  adaptation  of  organisms  to  them.  (From  the 
portrait  by  Schrader.  Guyot's  Physical  Geography.) 


INDEX 


Figures  in  italics  refer  to  pages  where  descriptions  or  definitions  are  given ;  figures 
followed  by  an  asterisk  (*)  refer  to  pages  on  which  illustrations  are  given.  Generic 
and  specific  names  are  in  italics. 


Aar  glacier,  experiments  on,  374 

Aar  massif,  203 

Abaya  Lake,  Africa,  on  map,  622* 

Ablation,  391,  392  ;  features  of  glaciers,  376 

Abrasion,  391;    by  rivers,  414;   by  waves, 

429 ;  of  glacial  material,   433 ;    of   sand 

grains,  406 

Absolute  humidity  of  air,  355 
Abyssal,  district,  515,  518;  igneous  masses, 

204 ;  igneous  rock,  84 
Abyssolith,  212 

Accessory  minerals  of  granite,  97 ;   of   ig- 
neous rocks,  94 
Acervularia,  285* 
Acid  feldspars,  91 ;  glasses,  98  ;  lavas,  122  ; 

magmas,  86,  87,  88;   taste,  50 
Acidic  rocks,  minerals  of,  94 
Acids,  40;  ionization  of,  41 
Adamantine  luster,  49 
Aden,  Gulf  of,  515 
Adirondack  Mts.,  64,   204;    anorthosites 

from,     105 ;      rock    exposures    in,    34 ; 

sections  from,  31 
Adji-darja,  see  Kara  Bugas 
Mchmina,,  318* 
^olian  Islands,  in 
Afdjada  Lake,  Africa,  on  map,  632* 
Africa,  dunes  of,  450;    marbles  of,  654; 

rift  valleys  of,  1 1 1  *,  63 1 
African  swamps,  341 
After- vibrations,  in  earthquakes,  658* 
Agassiz,  Alexander,  14 ;  on  reef  origin,  296 
Agassiz  Glacier,  383 
Agassiz,  L.,  cited  on  reefs,  300;  experiments 

by,  on  motion  of  glaciers,  374 
Agassiz,  L.,  and  Charpentier,  on  movement 

of  glaciers,  389 
Agate,  404 

Age,  relative,  of  river  and  land,  707 
Agents  of  clastation,  76 


Agglomerate,  volcanic,  432,  577* 

Aggrading  of  basins  in  arid  regions,  707 

Agnostus  limestone,  319* 

Ajusco  group,  112 

Akaba,  Gulf  of,  517*,  631,  754,  755*,  757 

Akron  dolomite,  disconformity  above,  616* 

Alabama,  coastal  plane  of,  714;  River,  715 

Alabaster,  220 

Alaskan  glaciers,  359*,  371*,  376*,  377*, 

378*,  379*,  38i*-384* 
Albert  Edward  Lake,  Africa,  on  map,  632* 
Albert  Lake,  Africa,  on  map,  632* 
Albert  Lake,  Oregon,  358,  633* 
Albite,  91 
Albertite,  351 
Alders,  in  swamps,  336 
Aletsch  Glacier,  358,  359;  map,  360*,  361*, 

364 
Aletschhorn,  358,  361,  366,  797 ;  chonolith 

of,  203 

Aletsch  neve,  364,  368 
Aleutian  Islands,  109 
Algae,  72,  272*,  273*,  274*,  323  ;  and  algous 

limestone,  271 ;   as  source  of  petroleum, 

352;   deposits  by,  333;   in  lakes,  334 
Algal  Lake,  section  of,  347* 
Algonkian  strata,  folded,  583* 
Algonquin  Lake,  map  of,  769* 
Alkaline,  lakes,  258;  taste,  50;  water,  214 
Alleghany  Plateau,  703,  728,  775 
Allier  River,  144 
Allochthonous  deposits,  345 
Alluvial  cone,  465* 
Alluvial  fan,  deposits  of,  79,  800*;    and 

plains,  464  ;   modern  examples  of,  467 
Alma,  Michigan,  731 
Alpine,  folds,  origin  of  structure  of,  607  ; 

folds,  thickness  of,  598;    geosynclines, 

619;    glaciers,  358;    overthrusts,  630*; 

system  of  folds,  603 


825 


826 


Index 


Alps,  590,  595,  607,  609,  610,  741 ;  de- 
scribed, 606 ;  gneisses  in,  653  ;  on  map, 
605  * ;  marbles  of,  653  ;  metamorphic 
rocks  of,  649 ;  rock  exposures  of,  33 ; 
shortening  by  folds  in,  619 

Alsace-Lorraine,  petroleum  of,  352 

Alsek  Glacier,  384 

Altai  Lake,  258 

Alteration  products  of  organic   slime,  349 

Alteration  stages  of  vegetal  deposits,  329 

Altitude,  criterion  of  dissection,  702 

Aluminum  minerals,  57,  $8 

Alunite,  58 

Amaltheus,  314* 

Amazon  River,  red  muds  from,  551 ;  valley 
swamps  of,  341 

Amber,  61 

America,  rock  exposures  in,  34 

American  Falls,  Niagara,  418*,  764,  773, 
774,  775;  crest  line,  774* ;  map,  763*; 
section  of,  772* 

Amethyst  Mountain,  196* 

Amiens,  on  map,  730* 

Ammonites,  3 13,. 3 14* 

Amorphous  texture,  217 

Amphibole,  62,  93  ;  distinctive  form  and 
cleavage  of,  94* 

Amphibolite,  105;  described,  652 

Amplitude  of  vibrations  of  earthquake 
waves,  66 1 

Amu-darya,  on  map,  449* 

Amundsen,  cited,  387 

Amygdaloid,  107* 

Amygdaloidal  structure,  106 

Anamorphism,  646 

Ancient  reefs,  305 

Andalusite,  62,  211 

Andes  Mts.,  674 ;  altitude  of  snow  line  in, 
357;  andesite  from,  104;  solfataric 
vents  of,  181 

Andesine,  91 

Andesite,  96,  104  ;  obsidian,  96,  104,  105  ; 
perlite,  96,  104;  porphyry,  96,  104; 
pumice,  96,  104 

Anemoclastic  rock,  77,  83 

Anemoclastics,  $68 

Angle  of  repose,  396 

Anglesite,  54 

Anhydrite,  59,  220,  221;  in  caliche,  259; 
order  of  deposition  of,  235 

Animal  life,  effects  of  condensation  of  sea 
water-on,  234 

Animal  tissues,  deposits  of  decaying,  346 


Animals  as  rock-breakers,  81 

Anorthoclase,  91 

Anorthosite,  105 

Anorthite,  90,  91 

Antarctic,  ice-sheet,  387;  mass,  510; 
Ocean,  diatom  ooze  of",  323 

Antecedent  stream,  743 

Anthracite  coal,  344,  654 

Anticlinal  valleys,  736* ;  development  of, 
733* 

Anticline,  589,  59o*~596*;  between 
basins,  603*;  erosion  cycle  on,  732 

Anticlinoria,  595* 

Anti-epicenter,  658* 

Antillean  volcanoes,  104 

Antilles,  radiolarian  ooze  of,  326 

Antimony  minerals,  54 

Antique  marble,  654 

Anti-Taurus  range,  on  map,  605* 

Antrim,  Ireland,  sections  on  coast  of,  170* 

Ants,  work  of,  437 

Apatite,  61,  222 

Apennine  Mountains,  24;  on  map,  605* 

Aphroena,  122 

Aplite  dikes,  192 

Apophyllite,  63 

Apophyses,  206 

Appalachia,  60 1 

Appalachian,  belt,  marbles  of,  653;  fold- 
ing, 602  ;  folds,  diagrams  of,  603  ;  folds, 
relations  of  to  domes  and  basins,  603; 
map,  604* 

Appalachian  Mts.,  590,  593,  596,  645,  698  ; 
described,  60 i  ;  rock  exposures  in,  34 ; 
shortening  by  folds  in,  618 ;  topography, 
development  of,  737* 

Appalachians,  609 ;  gneisses  in,  653 ; 
northern,  map  of,  738* 

Applied  geology,  20 

Apron-plain,  502 

Aqueous  elastics,  80;  precipitates,  classi- 
fication of,  217 

Aqueous  rocks,  70-73,  214;  mode  of  oc- 
currence of,  227  ;  principal  types  of,  220  ; 
textures  of,  217 

Araba,  graben,  754;  vale  of,  631,  754; 
map,  755* 

Arabia,  dunes  of,  450 

Arabian  peninsula,  Miliolitic  limestone  of, 
278 

Aragonite,  50,  221 

Area,  310*,  311 ;  at  Pozzuoli,  692 

Architecture  of  earth's  crust,  18 


Index 


827 


Arctic  Ocean,  510;  map  of,  511* 
Arctic  regions,  coral  reefs  in,  290 
Ardennes  Mts.,  731 ;  effect  of  uplift  of,  on 

Meuse,  743  ;  on  map,  730 
Aren  Glacier,  364 
Arenaceous  texture,  560 
Arenyte,  569,  577,  578 
Arete,  793,  794,  795*,  796,  800* 
Argentite,  57 
Argillaceous,  570 

Argillaceous  odor,  50 ;  sandstone,  578 
Argillite,  570,  650 
Argillutytes,  described,  580 
Argonne. Forest,  on  map,  730* 
Arid  region,  442* 
Aristotle,  29 
Arizona,  dunes  in,  451;  earthquake  jeffects 

in,  678 

Arkansas  River,  471 
Arkose  conglomerate,  577  ;  sandstone,  569, 

579 

Armenia,  soda  lakes  of,  259 
Arran  Island,  pitchstone  of,  99 
Arrowhead,  334*,  335 
Arroyos,  411 
Arsenite  minerals,  54 
Arsenopyrite,  54 
Artesian,  conditions  in  river  valley,  479 ; 

springs,    424;     system,    Dakota,    424*; 

wells,  424 
Arthur's  Seat,  169 

Artificial  classification,  66 ;   of  rocks,  67 
Arundinaria,  337* 
Arundo,  337* 

Arve,  irregular  deposits  by,  488,  489* 
Asaphiscus,  319* 
Ascending  solution,  268 
Ascutney  Mountain,  203 
Ash  rock,  described,  580 
Asia,  dunes  of,  450 ;   salt  deserts  of,  243 
Asiatic  plains,  698* 
Asphalt  rock,  351 
Asphaltum,  350,  351 
Asphalts,  349 

Assam,  India,  earthquake,  described,  683 
Asterias,  320* 
Asterism,  50 
Asthenosphere,  6,  84 ;   derivation  of  term, 

12 

Astraea,  285* 
Astringent  taste,  50 
A  sir  aides,  283* 
Astronomical  geology,  i 


Astronomy,  i,  2 

Asymmetrical  anticline,  590*,  591* 

Atacama  Desert,  259 

Atlantic  coast,  raised  beaches  of,  691 

Atlantic  coastal  plain,   714;  fall  line  on, 

722 
Atlantic  Highlands,  N.  J.,   78;    oxidized 

green  sands  of,  250 
Atlantic  ocean,  510 
Atlas  ranges,  on  map,  605* 
Atmoclastic,  568 

Atmoclastic  material,  origin  of,  77 
Atmoclastic  rocks,  77  ;  defined,  83 
Atmogenic  rocks,  70-73 
Atmology,  science  of,  13 
Atmosphere,  4,  5,  70 ;   as  rock-breaker,  77  ; 

chemical  work  of,  401 ;  contribution  of, 

to  lithosphere,    10;    corrosive   work  of, 

404  ;  derivation  of,  12  ;  materials  of,  354  ; 

operation    of,    in    rock-breaking,    391 ; 

science  of,  13 

Atmospheric  agents,  erosion  by,  820 
Atmospheric    moisture,    precipitation    of, 

356;  sculpturing  by,  821 
Atmospheric  rocks,  70,  71 
Atoll,  292,  293*;  origin  of,  295* 
Augusta,  Ga.,  722 
Australia,  dunes  of,  450 
Australian  brown-coal,  thickness  of,  343 
Autochthonous  deposits,  345 
Autoclastic  material,  81 ;  rocks,  83 
Autoclastics,  568 
Augite,   93,   105;    andesite,  96;    andesite 

porphyry,  96  ;   diorite,  104 ;  syenite,  100 
Augitite,  96,  107  ;   porphyry,  76     • 
Auvergne  volcanic  district,  map  of,  145*, 

165 

Axis  of  folding,  589 
Azores,  114;  volcanoes  of,  109 
Azov  Sea,  Bryozoa  reefs  on  border  of,  308 
Azurite,  56 

Backbones,  of  Long  Island  and  Cape  Cod, 
500 

Back  swamps,  479 

Back- wash,  523 

Bacteria,  aiding  in  decay  of  plant  tissues, 
328;  denitrifying,  270;  lime-depositing, 
270,  271;  iron-depositing,  258 

Badlands,  699*,  700 

Bahia,  Brazil,  260 

Baird  Glacier,  Alaska,  378,  map,  377* 

Bakoni  Forest  range,  on  map,  605* 


828 


Index 


Baku,  mud  volcanoes  of,  182 

Balanus,  317* 

Bald  cypress,  341 

Balkan  Mts.,  on  map,  605* 

Baltic  provinces  of  Russia,  563 

Baltic  Sea,  513,  516;   bars  of,  540;   dunes 

on,  444,  445  ;   salinity  of,  228 ;   salts  of, 

214 

Baltoro  Glacier,  371 
Bandai-San,    as   type   of   volcano,    140* ; 

explosion  of,  139 
Banded  texture,  218 
Banff,  synclinal  fold  near,  596* 
Banff  shire,  unconformity  in,  614* 
Bar,  coastal,  537 
Bar  and  barachois,  330* 
Baraboo,  Wis.,  section  beginning  at,  32 
Barachois,  330* 
Barbadoes,  326 
Barchanes,  443,  451* 
Barite,  5Q 

Barium  minerals,  59 
Barnacle,  317* 

Barohoini  Natural  Bridge,  Utah,  823 
Barrell,  J.,  cited,  6,  616 
Barrier  beach,  331,  537;  in  Scotland,  532* 
Barrier  reefs,  292  ;   origin  of,  294* 
Bars,  807  ;  wave  built,  810 ;  on  Cape  Cod, 

820 ;    produced   by   longshore  currents 

and  waves,  819 
Bar  theory  of  Ochsenius,  241 
Bartlett  Glacier,  Alaska,  472* 
Bary sphere,  16;    166,  derivation  of  term, 

12 
Basal  pinacoids,  45*,  46,  47;    monoclinic, 

47*;   triclinic,  48*. 
Basal  planes,  42,  43 
Basalt,  96,  106 ;   controversy  about  nature 

of,  168 

Basalt  porphyry,  96,  107 
Basaltic  magma,  minerals  in,  94 
Basaltic  obsidian,  96 
Basaltic  rocks,  erosion  features  on,  816 ;  of 

Kilauea,  119 

Base,  40 ;  ionization  of,  41 
Basel,  Switzerland,  630 
Base-level,  relation  of  peneplane  to,  705 ; 

local,  706 

Basement  rock,  207 
Basic  feldspars,  91 ;  glasses,  107 ;  magmas 

defined,  86,  87  ;   magma,  texture  of,  88 ; 

rocks,  minerals  of,  94 
Basin  of  Minas,  534 


Basin  Ranges,  749,  750,  751 

Basins,  599;  erosion  cycle  in,  722  ;  in  arid 

regions,  relation  to  base-level,  707;   of 

exudation,  386 ;  shallow,  602,  729 
Bastei,    Saxony,   409;    erosion    pillars  in, 

407*,  408* 
Bat  guano,  323 

Bath,  England,  warm  springs  of,  427 
Batholiths,  205 
Bathyal  district,  515,  518,  529;  zone,  511 ; 

deposits  in,  551 

Bathy metric  districts,  518,  519* 
Bauxite,  58,  403 
Bay,  509 
Bay-bar,  543* 
Bay  of  Fundy,  deposits  of,  534 ;  erosion  in, 

428 ;  tides  of,  525  ;  see  also  Fundy,  bay  of 
Beach,  530 ;  cusp,  535,  553  ;  drift,  522 
Beaches,    807 ;     abandoned,    810 ;     hard 

packed,     formation    of,     431;      raised, 

fossiliferous,  533 

Beach-plain,  545  ;  of  Nantasket,  820 
Beach-ridges,  545  ;  Nantasket,  820 
Bedford  shale,  disconformity  above,  615* 
Bed  rock  covered  by  mantle -rock,  65* 
Beheaded  streams,  721 
Beheading  in  river  capture,  720* 
Beichfirn,  364 
Belgium,  731 ;    chalk  of,   279;    deformed 

strata  of,  583 

Belt,  T.,  on  reef  origin,  297 
Ben  Nevis,  erosion  on,  821 
Bench,  marine,  807* 
Berea   sandstone,  disconformity  beneath, 

615* 

Bergschrund,  366,  368,  492,  794 
Bering  Glacier,  384 
Berkey,  C.  P.,  cited,  75 
Berlin,    oceanographic    institute    of,    14; 

age  of  formation  beneath,  64 
Bermuda   Islands,    290 ;     calcarenyte   of, 

569  ;   eolian  rock  of,  77  ;   swamp  of,  341 
Bernese  Alps,  359 
Beryl,  62  •  in  pegmatite,  207 
Betrunking  of  streams,  809  ;  map,  809* 
Beveling  of  strata  in  peneplane,  716,  717* 
Big  Pine,  Cal.,  earthquake  effects  at,  677 
Biloculina,  678* 
Binary  granite,  95 
Bioclastic  rock,  82,  83,  568 
Biogenic  rocks,  71,  72,  73,  81,  269 
Bioliths,  269 
Biology,  15,  19 


Index 


829 


Biosphere,  7,  71 ;  as  rock-breaker,  81 ;  con- 
tribution of  to  lithosphere,  10;  deriva- 
tion of  term,  12;  of  former  periods,  10; 
operation  of,  in  rock-breaking,  391 

Biotite,  63,  93;  distinction  from  horn- 
blende, 95 

Birdfoot  delta  of  Mississippi  River,  485, 
486* 

Biscay,  Bay  of,  map,  514* 

Biscayan  dune  belt,  446 

Bismite,  54 

Bismuth,  54 

Bismuth  minerals,  54 

Bitter  Seas,  of  Suez,  241 ;  map  of,  242* 

Bitter  taste,  50 

Bitumens,  origin  of,  270 

Bituminous  coal,  343,  344 ;  odor,  50 

Bivalves,  309,  310* 

Black  earth  of  Russia,  459;    map,  460* 

Black  Forest,  630,  757 ;   on  diagram,  756* 

Black  Hills,  595,  722,  723*;  dome,  725; 
origin  of  name,  723 

Black  Prairie  Valley,  715 

Black  Rock  Desert,  Nev.,  481 

Black  Sea,  230,  348,  352,  509,  516;  de- 
scribed, 231;  salinity  of,  228;  salt 
pans  of,  232 

Black  shales,  580 

Bladderwort,  336 

Blastoids,  319* 

Blister-cone,  artificial,  129* 

Block  conglomerate,  531 

Block  faulting,  diagram,  633, 634,  747,  748*, 
752*;  dissection  of ,  252*;  in  earthquakes, 
689  ;  in  Nevada,  map  and  section,  749*; 
in  Wasatch,  634* 

Block  Mts.,  634;  character  of,  747;  ero- 
sion of,  750* 

Bloody  Run,  765,  802;  on  map,  763* 

Blue  coral,  300 

Blue  ground,  of  South  African  diamond 
fields,  173 

Blue  muds,  551 

Bluestone,  74,  75,  578*,  579* 

Boghead,  349 

Bog  iron  ore,  224,  258 

Bogosloff,  109 

Bogs,  329,  341 

Bohemia,  old  volcano  of,  168 

Botany,  15- 

Bolivia,  potash  niter  of,  259 

Bombs,  volcanic,  431 

Bone-breccias,  322 


Bones,  deep  sea  accummulations  of,  552 ; 
of  vertebrates,  accumulations  of,  322 

Bonneville  Lake,  243,  254;  delta  in,  485*; 
map  of,  252*;  shore  lines  of,  253* 

Bony  coal,  345 

Borax  Lakes,  259 

Borax  of  Tuscany,  181 

Borax  salts,  223 

Bornite,  560 

Boron  minerals,  60 

Boscotrecase,  destruction  of,  129 

Bosses,  205 

Boston  Mts.,  724 

Botryoidal  texture,  218,  219* 

Bottom  feeders  in  the  sea,  347 ;  work  of, 
247 

Bottom-sets,  484 

Boulder,  giant,  501*;  movement  on  hill- 
side, 399* 

Boulder  clay,  497 

Boulder  conglomerate,  575* ;  residual,  577 ; 
moraine,  500 

Boulder  pavement,  531,  532 

Boulders,  glacial,  492 ;  formation  of,  on 
beaches,  430;  glaciated,  497*;  of  dis- 
integration, 396*,  397*;  of  Scottish 
coast,  532;  on  coast,  531;  on  moraine, 
499 

Bozen,  Tyrol,  earth  pillars  of,  412*,  821,, 

Bracciano,  Lago,  143* 

Brachiopods,  308,  309* 

Brachy-dome  (orthorhombic),  47* 

Brachy-pinacoid  (triclinic),  48* 

Brachy -prisms  (orthorhombic),  47* 

Brackish  water,  214 

Bradyseisms,  691 

Brahmaputra  River,  468 ;  effects  of  earth- 
quake on,  683 

Braided  river,  map,  477*,  480 

Brain  coral-,  287*,  299;   on  reefs,  291 

Brayman  shale,  disconformity  above,  616* 

Breaking  wave,  521*,  522* 

Breccia,  def.,  569,  576*,  577;  fault,  634; 
collapse,  635 

Breccia  ted  material,  427 

Brecciation,  427 

Bright  Angel  Canon,  789 

Brines,  214;  deposits  of,  259 

British  coast,  erosion  of  dikes  on,  816; 
sea-caves  on,  812*,  813* 

British  Isles,  organisms  in  shallow  sea 
around,  550 

Brittle,  denned,  49 


83o 


Index 


Brittle-star,  320* 

Bristol,  England,  719 

Broichhof,  on  map,  155* 

Brongniart,  A.,  25 

Bronx  Park,  glaciated  surface  in,  434* 

Bronzite,  105;  diabase,  106* 

Brown-coal,  343;  quarry,  343* 

Brownstone,  565,  579 

Bryozoa  and  Bryozoa  limestone,  307 

Bryozoa  from  Ordovician,  308* 

Buch,  Leopold  von,  25 

Buddha  temple,  790 

Buenos  Aires,  548  ;  on  map,  548* 

Buffalo,  N.  Y.,  761,  763,  764,  771 ;  ancient 
earthquake  fissure  in,  689*;  old 
coral  reefs  near,  306*;  sandstone  dike 
in.  585;  section  near,  318* 

Buffalo  River  (ancient),  on  map,  727* 

Buffon,  25 

Bulk  of  magma,  relation  of  texture  to,  89 

Bulrushes,   334*,    335;     in   upland    bogs, 

243 

Buntsandstein,  tracks  in,  482* 
Buoyancy,  due  to  seaweeds,  522 
Burlington  escarpment,  724 
Butte  of  sandstone,  410* 
Bytownite,  91 
Bysmalith,  203*,  204 

Cadiz,  Gulf  of,  map,  514*  .. 

Cairo,  rise  of  Nile  at,  480 

Calabrian  earthquake,  described,  665 ; 
map  of,  666* 

Calais,  731 

Calamine,  53 

Calcarenyte,  569,  580 

Calcareous  bioliths,  270 

Calcareous  sandstone,  578 

Calcareous  tufa,  219*,  252  ;  rate  of  deposi- 
tion of,  260 

Calcarina,  275* 

Calcilutyte,  572*,  581 

Calcirudyte,  569 

Calcite,  59,  221 

Calcium  minerals,  59 

Calderas,  extinct,  157,  158 

Calices,  286 

Caliche,  259 

California,  dunes  in,  451 ;  marbles  of,  653 

California  earthquake,  described,  684; 
displacements  in,  687* 

California,  Gulf  of,  232,  514;  map  of,  513* 

California  Valley,  467,  749 


Cambrian,  age  of  Potsdam  sandstone,  75 ; 

trilobite  limestone,   318,  319*;   of  Na- 

hant,  35 

ambridge,  Eng.,  719 
Camden,  S.  C.,  722 

amel,  foot-prints  of,  in  Sahara,  491 
Caminguin,  112;  described,  114 
Camptonite,  104 
Canada,    anorthosites   from,    105 ;   coastal 

sections  of,   35  ;    granites  and  gneisses 

of,  75 ;    mantle  rock  of,  65 ;    St.  Peter 

sandstone  of,  75 ;  upland  bogs  of,  342 
Canadian  shield,  metamorphic    rocks    of, 

648 

Canaries,  volcanoes  of,  109 
Canaveral,  Cape,  544;  map  of,  546* 
Cane,  337* 
Cannel  coal,  334,  349 
Cannon-ball  structure,  577 
Cantabrian  Mts.,  on  map,  605* 
Cantal,  145*,  149;  section  of,  150* 
Cape  Ann,  batholith  of,  205  ;    moraine  of, 

5°i 

Cape  Cod,  apron  plain  of,  503 ;  dunes  of, 
447*;  erosion  of,  809;  map  of,  808*; 
sandspits  of.  542 

Cape  Henry,  Va.,  dunes  of,  448* 

Cape  Verde  Islands,  volcanoes  of,  109 

Capri,  in 

Capucin,  148 

Caracas,  Venezuela,  destroyed  by  earth- 
quake, 670 

Carbon  minerals,  61 

Carbonaceous,  570 

Carbonaceous  shales,  580 

Carbonation,  403 

Carbonate  of  lime,  221 ;  deposited  by 
plants,  270;  deposited  from  sea  water, 
246;  from  fresh-water  lakes,  251 

Cardium  shells  in  Kara  Bugas,  240 

Cardona,  Spain,  salt  mountain  of,  220,  822 

Carex,  336* 

Caribbean  Sea,  516 

Carlsbad  twins,  97 

Carnellite,  58,  223 

Carnotite,  55 

Carolina,  coast  dunes  of,  447  ;  rock  ex- 
posures of  mountains  of,  34 

Carpathian  Mountains,  607,  609;  de- 
scribed, 603;  folding  of,  606;  marbles 
of,  653;  on  map,  605*;  rock  exposures 
in,  34 

Carpathian  oils,  352 


Index 


Carrara  marble,  653 

Caryophyllia,  284* 

Cascade  Mts.,  743 

Cascade  Pass,  glacier  of,  795* 

Cascades,  Swiss,  glacial,  369 

Caspian  Sea,  4,  230,  238,  349,  509,  516; 

characters    of,    231;     map    of,     232*; 

section  of,  240;    sub-sea  level  of,  706 
Cassel,  756 
Cassiterite,  53 
Cat-tails,  335 

Catacombs  of  Salzburg,  564* 
Catania,  lava  of  ^Etna  at,  135 
Catenary  curves  in  glacial  trough,  799* 
Cathedral  spires,  635,  637* 
Catskill    aqueduct,  geological    advice    in 

construction  of,  21 
Catskill  Mountains,  74,  583,  728 
Caucasus    Mts.,    741 ;     cross    section    of, 

608;    oils  of,  352;    rock  exposures  in, 

34 

Caustoliths,  270 
Cave  deposits,  77 
Cave-diagram,  425* 

Cave  of  the  Winds,  772* ;  origin  of,  417 
Caves,  lime  deposits  in,  262 
Cavern  breccias,  427 
Caverns,  bone  deposits  in,  322 
Cavity-filled  ore  deposits,  266 
Cawlinia,  313* 
Cayuga  Lake,  N.  Y.,  erosion  of  jointed 

rocks  on,  639*,  814 
Cedar  Breaks,'  Utah,  erosion  monuments 

in,  408* 

Cedar  swamps,  341 
Celestite,  59 
Cellulose,  269,  328 
Cementation,  induration  by,  565 
Centrosphere,  6,  16;    derivation  of  term, 

12 

Cephalopods,  313,  314*,  315* 
Cerargyrite,  57 
Cerium  minerals,  53 
Cerussite,  54 
Cette,  France,  evaporation  experiments  at, 

230 

Chain  coral,  286 
Chaine  du  Deves  section,  152 
Chaine  des  Puys,  described,  146 
Chaine  du  Velay,  145 ;  section,  152 
Chalcedony,  61,  404 
Chalcocite,  56 
Chalcopyrite,  56 


Chalk,  concretions  in,  574;   section,  278*; 

thin  section  of,  7* 
Chalk    cliffs,    279*;     of    France,    807*; 

sea-stacks  from,  815* 
Chalk  cuesta,  England,  719* 
Challenger  Expedition,  14 
Chamisso,  A.  von,  on  reef  origin,  296 
Chamonix  valley,  369 
Champagne,  730*;  lowlands,  731 
Champlain  submergence,  map,  770* 
Channel  coast,  France,  erosion  of,  809 
Chara,  272,  273*,  274*,  334,  346 
Charcoal,  328 
Charlestown   earthquake,   described,   678, 

679* 

Charpentier,  see  Agassiz  and  Charpentier 
Chary bdis,  525 
Chattahoochee  River,  715 
Checker-board  fault  structure,  757,  758* 
Chedrang  fault,  formation  of,  by  Indian 

earthquake,  684 
Chemical  ablation,  391 
Chemical  changes,  agents  producing,  402 
Chemical  combinations,  39 ;   elements,  38 ; 

precipitation  of  lime  in  lakes,  256 ;  rock 

destruction,  390;    sediments,  67;    work 

of  atmosphere,  401 
Chemically  formed  rocks,  70 
Chemistry,  38 

Cheops'  pyramid,  790*;  temple,  790 
Chert  concretions,  574 
Cherty  limestone,  225* 
Chesapeake  Bay,  715,  810 
Chicago,  111.,  earthquake  tremors  at,  678; 

rise  of  Lake  Michigan  at  695 
Chi  cot  Lake,  709;  map,  710 
Chief  Mt.,  Mont.,  599*,  627*,  629;  thrust, 

628* 
Chile,  desert  lakes  of ,  259 ;  earthquakes  of, 

672,  673*;   saltpeter  of,  223 
Chiltern  Hills,  England,  719* 
Chimneys,  glacial,  368 
China,  age  of  coal  in,  345 
Chirotherium  tracks,  482* 
Chrysocolla,  56 
Chimborazo,  132 
Chlorite,  63 ;  schists,  652 
Chlorophyll,  354 
Chonolith,  203 
Chonos  Archipelago,  earthquake  effects  in, 

675 

Chromite,  55 
Chromium  minerals,  55 


Index 


Chrysodomus,  312* 

Chrysolite,  62 

Chuar  group,  789 

Chunnemugga  Ridge,  715 

Cincinnati  Dome,  595,  602,  615,  729 

Cinder  cone,  Ariz.,  141 

Cinnabar,  56 

Cirques,  366*,  492,  793,  794*,  795,  800*; 
cutting  of,  367 ;  enlargement  of,  795, 
796* 

Cladium,  335* 

Clarke,  F.  W.,  cited,  38 

Clarke,  J.  M.,  cited,  8 

Classification  of  minerals,  50  ;  principles 
of,  66 

Clastation,  391,  392  ;  agents  of,  76 

Clastic  deposits  in  sea,  530 ;  sources  of,  527 

Clastic  material,  78;    formation  of,  390 

Clastic  rocks,  70,  73,  74,  390;  classifica- 
tion of,  566;  composition  of,  569  ;  inter- 
pretations of,  82*;  summary  of,  82; 
texture  of,  568;  varieties  of,  575  _ 

Clastics,  consolidation  of,  563  ;  structures 
of  marine,  553 

Clay,  formation  of,  403 

Clay -galls,  491 

Clay  iron-stone  concretion,  226*,  257* 

Clay,  percentage  in  blue  muds,  551 ; 
percentage  in  green  muds,  552 ;  per- 
centage in  red  muds,  551 

Clay  stones,  570,  580 

Clear  Lake,  Cal.,  andesite  obsidian  from,  105 

Cleavage,  denned,  48 

Cleveland  gulch  rock  flow,  401* 

Cliergue,  Puyde,  149 

Cliffs,  as  sources  of  geological  information, 
29;  formation  of,  821 

Clifton,  Ontario,  bend  of  Niagara  River  at, 
764 

Clinometer,  585 ;  directions  for  making, 
585  ;  fig.  of  home  made,  586* ;  use  of, 
586* 

Clino-pinacoid  (monoclinic),  47* 

Clinton  iron  ore,  225 

Clione,  312* 

Clouds,  356 

Coal,  as  evidence  of  former  life,  10;  de- 
posits, 344;  geology,  21 ;  mine,  346*; 
origin  of,  270;  seams,  345*;  shales, 
345,  580 

Coastal  dunes,  443 

Coastal  plain,  Atlantic,  714;  Alabama, 
714;  denned,  697;  drainage  system  of, 


711,    713*;  drowned,  810;  erosion  cycle 

on,  709;  first  cycle  of  erosion  on,  716*; 

regressive  strata  of,  711*;   river  capture 

in,  720*;    transgressive  strata  of,  711*; 

waterfalls  in  drainage  system  of,  721 
Coast  ranges,  467 ;   metamorphic  rocks  of, 

649 
Coasts,  straightening  of,  810;   young  and 

mature,  806 
Cobaltite,  52 
Cobalt  minerals,  52 
Cobblestone  terrace,  532* 
Cobleskill    limestone,    disconformity    be- 
neath, 616* 
Coccolithophore,  274* 
Coccoliths,   274 ;    deep-sea  accumulations 

of,  552;    in  chalk,  7*,  278* 
Cochabomba,  Bolivia,  250 
Cockle  shells  in  Kara  Bugas,  240 
Coconino  sandstone,  790 
Col,  364 

Cold  feel  of  gems,  50 
Colemanite,  60,  223,  224 
Collapse  breccia,  635 
Color,  of  minerals,  50  ;  play  of,  50 
Cologne  region,  volcanoes  of,  155 
Colorado  Canon,  relative  size  of,  3  ;  youth- 
ful condition  of,   708;    see  also  Grand 

Canon 

Colorado  delta,  mud-cracks  on,  480* 
Colorado  Desert,  252;  map  of,  232* 
Colorado,  earth  pillars  in,  411*,  821; 

marbles  of,  653  ;  volcanic  neck,  166 
Colorado    plateaus,    600,    703,    704,    791 ; 

diagram  of,  787*;   eolian  rock  of,  77; 

map  of,  787* 
Columbia  lava  plateau,   n,  64;    basaltic 

structure  of,  177;  map,  174*,  175* 
Columbia  River,  743;  dunes  of,  450,  451* 
Columbus,  Ga.,  722 
Columnar  structure,  development  of,  179; 

in  basalt,  177,  i79*;    in  obsidian,  179, 

1 80;   of  dikes,  191* 
Columnaria,  286 
Combing  wave,  522* 
Competent  beds,  618 
Complexly  folded  strata,  741 
Compound  corals,  285 
Compressive  movements,  theories  of  causes 

of,  619 

Concentric  exfoliation,  393*,  394* 
Concepcion   Bay,    earthquake   effects   in, 

674,  675 


Index 


833 


Conchoidal  fracture,  48 

Concrete  masses,  217,  218 

Concretionary  texture,  218 

Concretions,  572,  573*;  of  lime,  247 

Conductivity  of  minerals,  50 

Cone,  alluvial,  465* 

Cones,  parasitic,  Etna,  135*,  136 

Conglomerate,  569,  575*;  gneiss,  653 

Connate  water,  243,  424 

Connecticut  River,  course  of,  744;    flood 

plain  of,  474* 

Connecticut  Valley,  brownstone  of,  579 
Consequent  streams,  717 ;   downward  rate 

of  cutting  determined  by,  717 ;  examples 

of,  715  ;  radial,  722 
Constructional  forces,  19 
Contact   metamorphism,    193,    206,    643  ; 

kinds  of,  208 

Contemporaneous  veins,  206 
Continental  deposits,  439  ;    glaciers,  385  ; 

ice-sheets,  erosion  by,   805  ;    masses,  3, 

509 
Continental  shelf,  510,  512,  529;  depth  of 

water,  512  ;   etige  of,  512  ;   map  of,  511* 
Continental  water  bodies,  4 
Continents,  509 
Convection  currents,  527 
Cook  Strait,  675,  676* 
Cooling  taste,  50 
Cooper  River  delta,  467 
Copper,  55 ;  minerals,  55,  56 
Copper  River,  Alaska,  377*,  378 
Coquina,  311,  312,*  534 
Coral  conglomerate,  577 
Corallina,  272,  273*,  301 
Coral  polyps,  283* 
Coral  reefs,  83 ;    characters  and  types  of, 

290;  nullipores  on,  271 
Corals  and  coral  polyps,  282 
Coral  sands  and  muds,  552;    in  deep  sea, 

277* 

Coral  sandstone  of  Bermuda,  580 
Corals,  destroyed  by  fish,  437 
Cordilleran  region,  gneisses  in,  653 
Corndon  Hill,  section  of,  202 
Cornwall,  Eng.,  vein  of,  267* 
Corongamite  Lake,  Australia,  250 
Corrasion,  391,  392,  405,  822;    by  rivers, 

414,  421 ;    by  waves,  429 ;    glacial,  433 
Corrosion,  302  ;  by  rivers,  414 
Corrosive  work  of  atmosphere,  409 
Cortland  series,  quartz  mica  diorite  in,  103 
Corundum,  57 


Cosmic  space,  contribution  from,  to  litho- 

sphere,  n 
Cosmology,  i 

Cotswold  Hills,  England,  719* 
Cottongrass,  335* ;  in  upland  bogs,  342 
Country  grass  (see  Corallina),  302 
Country  rock,  27 
Crabs,  317 

Craspedophyllum,  of  old  reefs,  306 
Crater  Lake,   Oregon,   origin,    15;     view, 

157*;    map,  157*,  158*;    section,  158*, 

159 
Craterlets,    earthquake,    668,    669*,    672, 

678*,  679,  690 
Crayfish,  317 

Crazy  Mts.,  Mont.,  theralite  from,  105 
Creep  of  soil  on  hillsides,  401,  402* 
Cretaceous,  coal  of,  345  ;    green  sands  of, 

250,  552 
Crevasses,  marginal  on  glacier,   795;    on 

surface,  368,  377 

Crimea,  Bryozoa  reefs  in,  308 ;   mud  vol- 
canoes of,  183 

Crimo-Caucasian  chain,  605* 
Crisia,  307* 

Crinoidal  limestone,  319,  320,  321* 
Crinoids,  319,  321* 

Crooked  Creek,  Cal.,  meanders  of,  709* 
Crosby,  W.  O.,.  cited,  641;   referred  to,  51 
Cross-bedding,  455;    of  dunes,  453,  454*; 

of  river  deposits,  487,  488* ;   of  shallow 

water  deposits,  550;   river  type,  488* 
Cross  section  of  volcanoes,  construction  of, 

148 
Crustacea,  317*,  319*;  as  bottom  feeders, 

347 

Cryolite,  57 
Cryptozoon,  289 
Crystal  caves  of  Missouri,  425 
Crystalline  form,  42 
Crystalline  texture,  in  metamorphic  rocks, 

648 

Crystallization,  order  of,  94 
Crystallographic  axis,  42 
Cube,  44* 
Cuesta,  714*,  740*,  741 ;    second  cycle  of 

erosion  in,  717*;   terraced,  716*, 
Cuestas   of   Ontario   dome   region,    block 

diagram,  760* 
Cup  coral,  modern,  284* 
Cuprite,  56 

Current,  course  of,  in  rivers,  416* 
Current  ripples,  550* 


834 


Index 


Currents  of  the  sea,  519 

Cuspate  forelands,  545 

Cut  and  fill  structures,  489 

Cuttle  fish,  313,  314* 

Cuvier,  L.,  portrait,  24*,  25 

Cyanite,  62 

Cyatholiths,  274 

Cycle,  inauguration  of  new,  707 

Cycle  of  erosion,  beginning  of  second,  717  ; 

completion  of,  716;   second,  707 
Cycle  of  formation  of  clastic  rocks,  76 
Cypress,  in  swamps,  336 ;    stumps,  341 ; 

trees  in  Lake  Drummond,  339* 
Cypris,  585*,  in  playa  lakes,  483;    zone, 

3i8 

Cystoids,  319 
Cy there,  318* 
Cytherideris,  318* 

Dacite,  96,  103 ;   origin  of  name,  103 
Dacite  porphyry,  96,  103 
Dakota  artesian  system,  424*,  739 
Dakota    sandstone,    hog-back    of,    723*, 

739*,  740 
Dale  Valley,  N.  Y.,  776,  777;    on  map, 

776*  ;   relation  of,  to  Qatka,  778* 
Daly,  R.  A.,  estimate  of  change  in  sea- 
level,  298 ;   on  reef  origin,  297 
Dana,  E.  S.,  ref.,  51,  120 
Dansville,  N.  Y.,  777,  779  ;  on  map,  776* 
Dansville  Valley,  776,  777*, .780 
Danzig,  Bay  of,  549 ;   black  mud  deposits 

in  Bay  of,  348 
Darmstadt,  756 
Darton,  N.  H.,  quoted,  787 
Darwin,  C.,  cited  on  coral  reefs,  294 ;   on 

Chilean  earthquakes,  674 
Datolite,  63 
Daubree,    experiments   by,    on   origin   of 

volcanic  pipes,  164 
Davidson  Glacier,  378 
Davis,  W.  M.,  cited  on  reefs,  294  ;  quoted, 

299 

Daytona  Beach,  Fla.,  431* 
Dead  Sea,  259,  509,  633,  754,  757 ;  brine  of, 

214;    graben,    631;    map,    755*;     salt 

pillars  of,  757,  822 
Death  Valley,  Cal.,  259 
Decay  of  plants,  328 
Decomposition,  390,  392 
Dee  River,  Textularia  in,  279 
Deep  sea,  deposits  of,  552;    map,  277*; 

rocks,  exposure  of,  84 


Deflation,  404,  439,  822;  and  corrosion, 
408 

Deformation,  by  faulting,  619;  effects  of, 
582  ;  of  rocks,  582 

Deformation  of  rocks,  582 

Deformation  structures,  types  of,  582 

Degrading  of  basins  in  arid  regions,  707 

Dehna  Desert,  451 

Dekkan  Plateau,  176;  dikes  of,  190 

DekkanTrap,  176,  278 

Delaware,  coastal  plain  in,  715  ;  Bay,  715 ; 
river,  741;  Water-gap,  741* 

Delta,  deposits  in,  79 ;  glacial,  503  ;  ice 
contact  slope  of,  504*;  origin  of,  505*; 
plain,  485 ;  section  of,  484 ;  mud  flats 
of,  348,  483  ;  origin  of  name,  484 

Deltohedron,  44* 

Dendritic  glaciers,  359 

Dendritic  tufa,  255*,  256* 

Denmark  Straits,  ridge,  511* 

Denudation,  391,  392  ;  by  waves,  414 

Deposition,  agents  of,  439 

Descending  solutions,  268 

Deserts,  salts  in,  242 ;  dunes  of,  450 ; 
El-Tih,  754,  755*;  illustration  of  dy- 
namic geology  in,  36 ;  salt  deposits, 
ancient  example  of,  245" 

Desmarest,  N.,  25 

Destructional  forces,  19 

Detroit  River,  769  ;  discharge  by,  770 

Devil's  Hole,  765;  Tower,  166,  168*, 
202  ;  Wall,  Oschitz,  191 

Devitrified  old  glasses,  100 

Devonian,  flagstone  rocks,  of,  74;  reefs 
of  U.  S.,  396;  rocks  of  Lake  Erie,  31* 

Dew  point,  355 

Diabase,  96,  106,  191 ;  dikes  of,  erosion  of 
on  New  England  coast,  816  :  of  Nahant, 
35;  structure,  106*;  texture,  106 

Diagenetic  metamorphism,  642 

Diallage,  105 

Diameter  of  earth,  polar,  2 ;   equatorial,  2 

Diamond,  61,  344 ;  mine,  South  Africa,  173* 

Diamonds,  329;   in  old  volcanic  plug,  173 

Diaspore,  40 

Diastems,  616 

Diatomaceous  earth,  323 

Diatomaceous  ooze,  map  of  distribution  of, 
277* 

Diatoms,  323*,  346;  deep-sea  accumula- 
tions of,  552;  fresh  water,  334;  in 
Severn  estuary,  549  ;  silica  obtained  by, 
426 


Index 


835 


Dihexagonal  pyramid,  46* 

Dike  chasms,  191;  on  Cape  Ann,  192*; 
formations  of,  428 

Dikes,  89*,  1 88,  189*,  191*;  erosion  of, 
816;  essential  characters  of,  192;  for- 
mation of,  134  ;  in  Vesuvius,  130 ;  of 
British  Isles,  190;  on  Etna,  134 

Dinarian  Alps,  on  map,  605* 

Dinosaurs,  tracks  of,  482 

Diopside,  209 

Diorite,  96,  97,  103 

Diorite-andesite  series,  103;  glasses  of, 
104 

Diorite-gneiss,  653 

Diorite-porphyry,  96,  104 

Dip,  585  ;  diagram  of,  585*;  measurement 
of,  586*;  of  fault,  620;  relation  to 
width  of  outcrop,  588 

Dip  faults,  623,  624 

Diplopora  porosa  of  Dolomites,  10*,  272 

Disconformities,  614,  615*,  616*,  617; 
marked  by  springs,  616* 

Discrete  particles,  217 

Disintegration,  390,  392 

Dislocation  fault  or  fracture,  657 

Dismal  Swamp,  338*,  34*>  343 

Displacement  by  faulting,  621 ;  in  Cal- 
ifornia earthquake,  687 

Distributaries,  465  ;  of  deltas,  485 

Ditetragonal  prism,  45*;  pyramid,  45* 

Ditrigonal  prism,  46* 

Diurnal  temperature  changes,  820 

Dodecahedron,  44* 

Dogtown  Common,  disintegration  of 
boulders  on,  396* ;  moraine  of,  500*, 
Soi* 

Dolerite,  107 ;  decomposition  of,  404 ; 
products  of  decomposition  of,  404 

Dolomite,  59,  146,  222  ;  algal  origin  of,  272 

Dolomites,  272,  649;  of  Tyrol,  8,  9*; 
organisms  which  formed  the  rock,  10*; 
Triassic  reefs  of,  306 

Domes,  595  ;  erosion  cycle  on,  722 ;  flat, 
723 ;  low,  602 

Domoshakovo  Lake,  258 

Dover  Cliffs,  279* 

Dover  Straits,  tides  of,  524 

Drachenfels,  155*;  trachyte  of,  89,  101, 
156 

Drag  with  faulting,  635* 

Drainage  system,  form  of,'  721 

Dreieckhorn,  366,  368 

Dreikanter,  406,  822 


Drepanella,  318* 

Drift,  beach,  direction  of,  523;  types  of, 
502 

Drowned  river  valleys,  803 

Drumlins,  496,  498*;  sea  cliff  of,  531; 
wave-cut,  817 ;  erosion  on,  818 

Drummond,  Lake,  339* 

Dry  Champagne  lowland,  731 

Duckweed,  334*,  336 

Ductile,  49 

Dull,  49 

Dundas  River  (ancient),  728;  on  block 
diagram,  760*;  on  map,  727* 

Dunes,  443,  445*~453*;  arresting  of  move- 
ment of,  447  ;  rate  of  motion  of,  444, 
450;  structure  of,  453* 

Dunite,  105 

Durham,  Eng.,  concretionary  limestone 
of,  221*,  256;  Magnesian  Limestone  of, 
573  ;  rock  section  on  coast  of,  34* 

Dusky  Sound,  New  Zealand,  806* 

Dust  deposits,  457  ;    character  of,  458 

Dust  falls,  volumes  of,  443  ;  storms,  440 ; 
volcanic,  431 ;  wells  on  glaciers,  377 

Dwarfed  organisms,  in  black  mud,  549 

Dynamical  geology,  18;  regions  far  study 
of,  36 

Dynamic  metamorphism,  642,  643 

Earth,  area  of  surface,  4;  method  of  ap- 
proach in  study  of,  21 ;  movements,  655  ; 
pillars,  411*,  412*,  821 

Earthquake,  diagram,  658;  fissure,  Ariz., 
680* ;  Cal.,  686* ;  center  of  disturbance, 
657;  fossil,  689*;  path  of  particle  in, 
663*;  phenomena  summarized,  689; 
waves,  657 ;  waves,  due  to,  690 ;  with 
faulting,  622 

Earthquakes,  655;  volcanic  or  explosive, 
656 

Earth's  crust,  material  of,  38 

East  Africa,  rift  valleys  of,  632* 

East  Kaibab  monocline,  786*,  787* 

Eaton,  Amos,  27;  portrait,  28*,  31 

Eatonian  era  in  American  geology,  28 

Ebb-tide,  524 

Eccles  tower,  burial  of,  by  dunes,  445*; 
resurrection  of,  446* 

Echinarachnius,  320* 

Echinoderms,  319,  320*,  321*;  as  bottom 
feeders,  347 

Echinoids,  as  rock-destroyers,  437 

Echo  Cliff,  786*,  787*,  7Qi 


836 


Index 


Edge  of  land,  sculpturing  of,  805 

Eel-grass,  330* 

Effervescent  springs,  181,  183 

Eggishorn,  364 

Egypt,  Nummulitic  limestone  of,  280 

Eifel  district,  Maare  of,  158;  map,  159* 

Eighteen  Mile  Creek,  571* 

Einkanter,  406* 

Eld  Cleft,  Iceland,  175 

Elements,  chemical,  17,  38;  distribution  of 

important,  39 

Elevation  of  beaches,  rate  of,  694 
Elgon  Mt.,  Africa,  632* 
Elizabeth  Islands,  531 ;  erosion  on,  818 
Ellice  Island  reefs,  295 
Elmira,  section  from  Adirondacks  to,  31 
El  Tovar,  Colorado  Canon,  788 
Emergence,  sandstone  of,  559 
Emma  mine,  Utah,  266* 
Encrinus,  321* 
Endogenetic  agents,  566 
Endogenetic     rocks,     70;      defined,     71 ; 

interrelations  of,  73*;   types  of,  71 
Endomorphic  effects,  208 
Engineering  geology,  21 
Englacial  detritus,  370;  drift,  492 
England,    central,  cuesta   topography   of, 

719*;    sections  in,  32 
English  coast,  sections  of,  35;    sea-stacks 

on,  814,  815* 
Enstatite,  105 
Enterolithic  structure,  644* 
Eolian  cross-bedding,  ancient  deposits  with, 

455 ;     in    Mesozoic    sandstone,    456* ; 

in  Mississippi  limestone,  457*;    Orange 

sand,  457*;    Palaeozoic  sandstone,  456* 
Eolian  rock,  83,  567,  568,  580 ;    origin  of, 

77 

Epeiric  sea,  511,  513,  516 
Epernay,  730* 
Epicenter,  657,  658* 
Epicontinental  reefs,  298,  513,  516 
Epidote,  63  •  schist,  described,  652 
Epsomite,  60 ;  in  caliche,  259 
Epistomella,  327* 
Erie  Canal,  sections  on,  31 
Erie  lowland,  761 ;  763,  section  of,  761 
Eriophorum,  335* ;  in  upland  bogs,  342 
Erosion,  392 
Erosion  cycle,  704,  705 ;    agents  in,  705  ; 

on  coastal  plain,  709 
Erosion  features,  special,  811 
Erosion  monuments,  407* 


Erosion  plains,  700 

Erosion  processes,  391 

Erratics,  492 

Erzgebirge,  33 

Esker,  506*,  507*;  origin  of,  505 

Estheria,  in  playa  lakes,  483 

Estuaries,  mud  flats  of,  348 

Estuarine  elastics,  568 ;  deposits,  547 

Etna,  in,  189;  as  type,  141;    described, 

132 ;  map  of,  132,  133 ;  eruption  of,  128 
Eucystidium,  324* 
Eulalie  Lake,  671 

Eureka,  Nev.,  andesite  perlite  from,  105 
Europe,  age  of  coals  of,  345  ;  mantle  rock 

of,  65  ;  relief  map  of,  700 
Eutectic  mixture,  216 
Evaporate,  215 
Evaporation,  354  356  ;  of  lake  water,  252 ; 

products  from  sea  water,  227 
Even  fracture,  48 

Everglades,  300 ;  supposed  origin  of,  304 
Ewigschneefeld,  368 
Excrements,  source  of  phosphate,  322 
Exfoliation,  303 

Exogenetic  rocks,  73  ;  interrelations  of,  82 
Exomorphic  effects,  208 
Expanded  foot  of  glaciers,  377 
Explosive  type  volcanoes,  140 
Extended  consequent  streams,  712 
Extinct  volcanoes,  144 
Extrusive  igneous  rock,  84 
Eyassi  Lake,  Africa,  on  map,  632* 
Eyre  Lake,  Australia,  467 

Faceted' pebbles,  406*,  822 

Facies,  change  of,  in  marine  elastics,  554 ; 

diagram,  555* 
Fall-line,  722 

False  Cape  (Canaveral),  545* 
Fan-alluvial,  465,  466* 
Fan  glacier,  359 
Fan- shaped  fold,  590* 
Farafrah,  chalk  from,  278* 
Faroe-Iceland  ridge,  511* 
Faulberg,  368 
Fault,   81,   432,   619;    recent,    at    Muka, 

Muka  cliff,   New  Zealand,   675,    677*; 

formed   during   earthquake,   681,  681*; 

in  Wasatch  Mts.,  751* 
Fault-block,  747 
Fault-breccia,  80*,  81,  427,  432,  577,  634, 

635 
Fault-crush  elastics,  568 


Index 


837 


Faulted  region,  erosion  cycle  in,  747 

Faulting,  nature  of  movement  of,  621 

Fault-line,  620* 

Fault-line  scarp,  620*  622,  759 

Fault-line  valleys,  758,  759* 

Fault  plane,   619;    scarp,   622,  630,   759; 

9f  Rhine  Valley,  757  ;  on  diagram,  620 ; 

submarine  formed  by  earthquakes,  690 ; 

topography,  renewal  of,  759 
Fault-rubble,  432 
Faults,    developed    on    monocline,  622*; 

horizontal,  622  ;    oblique,  622 ;    rotary, 

622  ;   types  of,  621 
Fault  surface,  619 
Fault  zones,  620 

Fawsites,  286,  288* ;  of  old  reefs,  306 
Fecamp,  France,  807* 
Feel  of  minerals,  50 
Feldspar,  clouding  of  grains,  403;    effect 

of -temperature  on,  395,  397 
Feldspars,     61,     90;    separation    of,    94; 

table  of,  91 
Feldspathoids,     61,     90;     described,    92; 

separation  of,  94 ;   table  of,  92 
Felsite,  96,  100,  101,  103  ;  porphyry,  96 
Felsitic  texture,  88 
Fenlands,  330 
Ferns  in  swamps,  336 
Ferromagnesian  minerals,  as  source  of  iron, 

257  ;  table  of,  93 ;  silicates,  90 
Ferruginous  sandstone,  578 
Fetid  odor,  50 
Fibrous  texture,  218 
Field  names  of  rocks,  107 
Fiesch  Glacier,  see  Viesch  Glacier 
Fife,  Scotland,  lava  flows  of  coast  of,  196  ; 

volcanic  plugs  of  coast  of,  170 
Findhorn   River,    destruction   of   feldspar 

in,  4*5    . 
Fingal's    Cave,    178,    179*,    180*,  813*; 

origin  of,  428 
Finger   Lakes,   N.   Y.,    725,    803;     radial 

character  of,  map,  726*;  sections  on,  31 
Finland,  64;   gneisses  in,  653;    metamor- 

phic  rocks  of,  648 
Fire  bricks,  345 
Fire  clay,  345 
Fire  Island  beach,  537 
Firn,  357 
Fish,  as  rock-breakers,  81 ;-  as  source  of 

petroleum,    352;     destruction    by,    of 

corals,  etc.,  437;    sudden  killing  of,  322 
Fish-bones,  beds  of,  322,  348 


Fissile  shale,  571* 

Fissure  eruption,  174;  veins,  265* 

Fjorded  coast,  804 

Fjords,  801-803* 

Flags,  335 

Flagstone,  described,  579 

Flattop  Mt.,  599* 

Flint,  concretions  of,  224*,  574 ;  origin  of, 
327 

Flood-plain  deposits,  79 

Flood  plain,  dunes  of,  448 

Flood  plains  of  rirer,  472*,  474*,  479* 

Flood  tide,  524 

Florida,  oolites  of,  271 ;  map  of  coasf  of, 
301*,  312;  map  of  end  of  barrier  reef 
of,  303*;  reefs  of,  298,  300;  rocks  of, 
64*;  swamp,  339*;  section  of  Keys  of, 
302* 

Fluid  lava,  84 

Fluorite,  $Q 

Fluvialite,  chemical  deposits,  259;  elas- 
tics, 568;  deposits,  227 ;  rocks,  83 

Focus  (earthquake),  657 

Folding,  583;  causes  of,  617;  deforma- 
tion by,  583  ;  machine  reproducing,  617  ; 
structures  due  to,  607 

Folds,  artificially  produced,  618* ;  eroded, 
591,  592*;  reconstruction  of,  592; 
thickening  of  axes  of,  593;  types  of, 
589,  -590* 

Foot-prints,  fossil,  482  * ;  in  river  deposits, 
491 

Foot- wall,  621 ;   on  diagram,  620* 

Foraminifera,  275*,  325  ;  deep-sea  accumu- 
lations of,  552;  glauconite  in  shells  of, 
552;  in  blue  muds,  551;  in  Severn 
estuary,  549;  killed  in  estuaries,  348; 
of  chalk,  7  ;  lutytes  of,  580 

Foraminiferal  oozes,  275 

Foresets,  484*,  485*,  503*,  504* 

Fort  Hamilton,  N.  Y.,  peat  beds  on  coast 
near,  539 

Fosse,  glacial,  503 

Fossiliferous  character  of  marine  deposits, 
550 

Fossiliferous  sandstone,  80* 

Fossils,  10,  15  ;  formation  of,  illustrated 
on  sea  coast,  36 

Foster  Glacier,  Alaska,  377 

Foster's  Flats,  Niagara,  765,  774,  775*,  784 ; 
on  map,  763* 

Fracture,  denned,  48 

Fragmental  rocks,  70,  73 


838 


Index 


Fragmentation,  302 

France,  central,  64  ;  chalk  of,  279  ;  coastal 
section  of,  35  ;  joints  in  rock  on  coast  of, 
639*,  640*;  sea  caves  on  coast  of,  812  ; 
sea- stack  on  coast  of,  530*  ,  814 ;  sec- 
tions in  mountains,  of,  33  ;  volcanic  area 
of  central,  153  ;  volcanoes  of  central,  144 

Franklinite,  53 

Freestone,  565,  578 

Freiburg,  Mining  School,  25 

Freiburg  region,  section  of  old  volcano,  172 

Fresh  water,  214 

Fresh-water  lakes,  347  ;  deposits  of,  251 

Fresh- water  mollusks,  315-316* 

Fretted  upland,  796 

Fringing  reefs^292 

Frische  Nehrung  and  Haff,  444,  445 

Front  Range,  Rocky  Mts.,  738 

Frost  as  agent  of  erosion,  820 

Frost  work,  399,  821 ;  on  pebbles  and  soil, 
400 

Frustule,  323 

Fujiyama,  109,  no* 

Fulgur,  312* 

Fumarole,  86,  182 

Fumarolic  action,  181 

Funafuti  reefs,  295 

Fundy,  Bay  of,  515;  low  tide  in,  533*; 
see  also  Bay  of  Fundy 

Funnel  seas,  514,  maps,  513*,  515* 

Funnels,  volcanic,  161 

Fusulina,  282* 

Fusulina  limestone,  281* 

Gabbro,  96,  97,  104,  705 

Gabbro,  basalt  series,  105 

Gabbro-gneiss,  653  ;  porphyry,  96,  107 

Gagates,  350 

Gagatite,  350 

Galenite,  54 

Galilee,  Sea  of,  757 

Gallatin  Range,  263 

Galloway,  J.  J.,  ref.,  415 

Ganges  Delta,  peat  of,  338 

Ganges  River,  468 ;  kankar  of,  260 ;    flood 

plain  of,  470*;  land-slide  on  branch  of, 

398 

Gangue  material,  266 
Garden  of  the  Gods,  635,  636* 
Garlic  odor,  50 

Garnet,  62;  development  of,  210 
Garnierite,  52 
Gas  geology,  21 


Gases,  metamorphism  by,  646 ;  and  vapors, 

metamorphic  action  of,  211 
Gastropods,  311*,  312* 
Geant,  Glacier  du,  369 
Genesee  Falls,  at  Portage,  Lower,  784*, 

785*;     Middle,    782*;     Upper,    783*; 

at    Rochester,    Lower,    780*;    Middle, 

780* ;  Upper,  780* 
Genesee   Gorge,   hanging  valley   in,   802 ; 

youthful  condition  of,  708 
Genesee  region,   post-glacial  development 

of,  779 
Genesee  River,  726  ;   (ancient),  map,  727*; 

banks  of,  416 ;  contrasting  features  of, 

782 ;     section    of    old   valley   of,   783 ; 

geological  history  of,  775  ;  gorge  of,  77* ; 

Lower  Falls  at  Portage,  development  of, 

diagrams,  785*;    map  of  part  of,  779*; 

on  map,  776*;   sections  on,  30 
Genesee    River    region,    map    of,    776*; 

post-glacial   development  of,  779;    pre- 

glacial  character  of,  775 
Genesee  shale,  571* 

Genesee  Valley,  776,  778,  779,  780;    ma- 
ture stage  at  Portage,  783* 
Genesis,  as  basis  of  natural  classification, 

66 

Genetic  classification,  66;   of  rocks,  68 
Genie  rocks,  77 
Geogenesis,  19 
Geography,  i,  18,  20 

Geological,  engineer,   22 ;     literature,  im- 
portance of,  37  ;  observation,  field  of,  28; 

and  interpretation,  rise  of,  24 
Geology,    analytic    and    descriptive,    18; 

applied,    20;     causal   or   dynamic,    18; 

defined,  i,  2 ;  history  of,  22 ;   historical 

or    developmental,     18;     in    narrower 

sense,  14 ;   relation  of,  to  human  welfare, 

20  ;  scientific  aspect  of,  17;  scope  of,  i ; 

subdivisions  of  science,  13 
Geomorphology,  18,  19 
Geophysical  laboratories,  19 
Georgia,  swamps  in,  339 
Georgian  Bay,  726,  728;   map,  727* 
Geosyncline,  Indian,  518;    of  deposition, 

468 ;   relation  of  mountains  to,  6bo 
Geotectology,  18 
German  brown-coal,  thickness  of,  343 ;   see 

also  brown-coal 
Germany,  coast  of,  35  ;    deformed  strata 

of,    583 ;     peneplane   of   western,    702 ; 

salt  deposits  of  northern,  n 


Index 


839 


Geyser  (Iceland),  184,  186* 

Geyserite,  184,  224 

Geysers,  181,  183;  distribution  of,  184; 
effect  of  earthquakes  on,  683 

Giant's  Causeway,  n,  176*,  i77*J  erosion 
at,  428 

Giant  Geyser,  184* 

Gibbsite,  40,  57 

Gibsonite,  351 

Gilbert,  G.  K.,  cited,  200 

Glacial,  elastics,  568;  deposits,  ancient, 
508;  erosion  of,  818;  Pleistocene,  496; 
drift,  492  ;  erosion,  433  ;  geology  defined, 
20;  pond,  494,  495*;  sculpture,  land 
forms  due  to,  793  ;  streams,  383 ;  ma- 
terial, transported,  495;  tarn,  796; 
till,  497*;  transportation,  492  ;  trough, 
mature,  798  *,  799 ;  of  Norway,  799*, 
800*;  valley,  character  of,  373 

Glaciated  boulder,  435 ;  pebbles  and 
boulders,  497*;  rock  ledge,  434*; 
valley,  old,  799*;  valley,  young,  799. 

Glacier,  currents,  work  of,  492,  493  ;_  ice, 
357  ;  lakes,  802  ;  landscape,  ideal,  350* ; 
tables,  377*;  tributary,  800* 

Glaciers,  357 ;  deposits  of  modern,  493 ; 
several  types  contrasted,  805 ;  illustra- 
tion of  dynamic  geology  by,  36  ;  move- 
ment of,  374;  relation  of  parts,  375,  376 ; 
rock  destruction  by,  433;  types,  358, 
359 

Glasses,  old  de vitrified,  100 

Glassy  texture,  87,  88 

Glauberite,  223 

Glauber  salt,  223 

Glauconite,  224,  249,  552 

Glauconitic  sandstones,  character  of,  570 ; 
described,  579 

Glencoe,  742 

Glen  Roy,  parallel  roads  of,  363*-365* 

Gletscherhorn,  368 

Globigerina,  275*;  limestone,  276;  ooze, 
275,  276*;  map  of  distribution  of,  277* 

Gloucester,  Mass.,  moraine  near,  501 

Gmiinden  Maar,  160* 

Gneiss,  67;  described,  652,  647*,  648* 

Gneisses,  649,  650 ;  of  Canadian  region,  75 

Gneissic  structure,  647 

Goat  Island,  763,  764,  774 

Gobi,  Desert  of,  458 

Goderich,  Ont.,  728 

Goethe,  on  volcanic  structure  of  Kammer- 
btihl,  168 


Goethite,  40,  52,  224 

Gold,  57 ;  minerals,  57 

Goose  barnacle,  317* 

Gordon  Craters,  174* 

Gorgonias,  283,  284 ;  on  reefs,  291 

Gotland,  caves  on  coast  of,  814 ;  old  reefs 
of,  306 

Grabau,  A.  W.,  cited,  212 ;  ref.,  46,  249, 
259,  333,  415,  52i,  527,  588,  604,  623 

Graben,  630,  754 

Graben  faulting,  754 

Graham  Island,  112,  115*;  described, 
114;  section,  115* 

Grahamite,  35 

Gran'  Canaria,  oolites  of,  388 

Grand  Banks,  Newfoundland,  388 

Grand  Canon,  geological  history  of,  786; 
map  of,  786*,  788,  789*,  790;  section 
across,  791 ;  region,  character  and  de- 
velopment of,  791 ;  rocks  of,  788 

Grand  Canon  district,  600 

Grand  Canon  series,  789 

Grand  Island,  764 

Grand  Viewpoint,  Colorado  Canon,  788 

Grand  Wash,  cliffs,  map,  786*;  fault, 
792 

Granite,  95,  96 ;  cliffs,  erosion  of,  428,  429*  ; 
boss,  205*;  boulder,  disintegration  of, 
396* ;  boulders,  formed  by  disintegra- 
tion, 396 ;  dikes,  192  ;  disintegration  of, 
394 ;  gneiss,  653  ;  horizontal  joints  in, 
395*;  porphyry,  96*,  98 

Granite-rhyolite  series,  95 

Granites,  93  ;  of  Canadian  region,  75 

Granitic  texture,  88* 

Granodiorite,  103 

Granular,  disintegration,  394,  395 ;  snow, 
357  ;  texture,  88 

Graphic  granite,  97*,  207 

Graphite,  61,  67,  329,  344;  rock,  654; 
schist,  described,  652 

Graphitic  rocks,  649 

Grasses,  in  swamps,  335 

Grat,  796* 

Gravity  fault,  624 

Graywacke,  described,  579 

Graz,  Austria,  destruction  of  rock  frag- 
ments in  river  below,  414 

Greasy  luster,  defined,  49 

Great  Aletsch  Glacier,  see  Aletsch  Glacier 

Great  Balkan  Mts.,  on  map,  605* 

Great  Barrier  Reef,  Australia,  290,  291*, 
305;  described,  298 ;  section  of,  299* 


840 


Index 


Great  Basin,  749,  753;    cross  section  of, 

750* ;  ranges  of,  634 
Great  Britain,  deformed  strata  of,   583 ; 

sea-coast  section  of,   35 ;    upland  bogs 

of,  342 

Great  Fault  Line,  Cal.,  686* 
Great  Geyser  Basin,  83* 
Great  Glen,  Scotland,  759 
Great  Ice  Barrier,  387 
Great  Lakes,  4;    cobble  terraces  of,  532; 

discharge    of,     769;      dunes    of,    448; 

future  permanent  discharge  of,  696 
Great  Lakes  region,  change  of  level  in,  695 
Great  Neck,  Mt.  Taylor,  N.  M.,  166* 
Great  plains,  701,  705;   Badlands  of,  699* 
Great  Pyramid,  76*;  facing  by  Nummu- 

litic  limestone,  280 
Great    Salt    Lake,    230,    242,    259,    509 ; 

brine  of,  214;   map  of,  252*;  mirabilite 

in,  223 

Great  Whin  Sill,  197 

Grecian  Archipelago,  volcanic  field  of,  in 
Greenland,  altitude  of  snow-line  in,   357  ; 

ice-cap  of,  493 ;    ice  cover  of,  385,  386* 
Green  Mts.,  rock  exposures  in,  34 
Green  muds,  551,  552 
Green  River,  472 

Green-sand,  224,  249,  551,  552,  579 
Greenstone,  107 
Griou,  Puy  de,  149,  150 
Grit,  575 

Grooved  upland,  796 
Grossularite,  210 
Ground-mass,  denned,  88 
Ground  moraines,  496 
Ground  water,  4,  411 
Grover's  Cliff,   Mass.,   817*;    section  of, 

53i* 

Grunhorn,  Liicke,  368 
Guano,  322 

Guano  Nyiro  Lake,  Africa,  on  map,  632* 
Guettard,  £.,25 
Gulf,  509 
Gulf   Coast,   Orbitolitic  and  Nummulitic 

limestone  of,  281 
Gulf  Stream,  302 ;    influence  on  coral  reef 

growth  of,  290;   origin  of,  526;  velocity 

of,  527 
Gypsum,  59,   220;    crystals  of,  in  desert 

deposits,  244 ;    from  alterations  of  lime 

stones,   244;    from  sea-water,   229;    in 

caliche,    259 ;     order   of   deposition   of, 

235  ;  solution  of,  404 


Hackly  fracture,  49 

Hade  of  fault,  620*,  621 

Haeckel,  E.,  cited,  19 

Hail,  70,  356 

Halifax,  N.  S.,  batholith  of,  205 

Halimeda,  272*;  of  reefs,  293 

Halite,  58 ;  in  caliche,  259 

Hall,  James,  31 

Hall,  Sir  James,  26 

Halysites,  286,  288* 

Hamilton,  Ont.,  726 

Hamilton  shales,  31*,  570* 

Hammadas,  405 

Hanging  valleys,  374*,  802;  Genesee 
gorge,  797*,  800*,  802*;  formation  of, 
diagram,  801*;  on  block  diagram,  778* 

Hanging  wall,  621 ;  on  diagram,  620* 

Hardness,  defined,  49 

Hardpan,  497 

Hard  water,  214 

Harsh  feel,  50 

Haute  Loire,  volcanic  district  of,  165 

Hawaiian  Islands,  109 ;  lava  cascades  in, 
119;  map  of,  117;  tsunamis  at,  674 

Heat,  metamorphism  by,  646 

Heather,  as  peat  former,  342 

Heave,  622 

Heaven's  Peak,  Montana,  583* 

Height  of  waves,  520 

Helderberg  front,  741 ;  Mts.,  728 ;  over- 
thrusts,  626* 

Helgoland,  faults  of,  621*;  map,  showing 
erosion  of,  809* 

Heliopora,  30x3 

Helios phara,  324* 

Helix,  316*,  317*,  458 

Hellgate,  524 

Hematite,  52,  225,  402 

Hemi-brachy-dome  (triclinic),  48* 

Hemi-macrodome  (triclinic),  48* 

Hemi-prism  (triclinic),  48* 

Hemi-pyramid  (monoclinic),  47* 

Herculaneum,  126,  128;  burial  of,  127 

Herds  of  animals,  rock  destruction  by,  436 

Hereroland,  436 

Herodotus,  Nummulitic  limestone  known 
to,  280 

Hesperus  Mountain,  section  of,  201* 

Heterogeneous  conglomerate,  577 ;  tex- 
tures, 87 

Hexagonal  prism,  46*;  pyramid,  46*; 
system,  43  ;  principal  types  of,  46 

Hexahedron,  44* 


Index 


841 


Hexoctahedron,  44* 

Hextetrahedron,  44* 

High  Falls,  N.  Y.,  asymmetrical  anticline 
at,  591* 

Highland  Light,  Cape  Cod,  820 ;  erosion 
of  cliffs  at,  822* 

Highlands  of  Hudson,  64 

High  moors,  329 

Hill  peat,  342 

Hill  prairies,  715 

Hillside  springs,  423 

Himalaya  Mts.,  3,  471 ;  marbles  of,  653 

Hingham,  Mass.,  boulder  near,  501* 

Hispar  Glacier,  371 

Historical  Geology,  defined,  IQ 

History  of  geology,  22 

Hitchcock,  C.  H.,  quoted,  771 

Hoang-Ho,  459,  697  ;  material  transported 
by,  462  ;  plain  of,  467,  468* 

Hobbs,  W.  H.,  quoted,  679,  680 

Hog-back,  of  Dakota  sandstone,  739*, 
740* 

Hog-backs,  722,  734,  738;  development  of, 
diagram,  739* 

Hohenlohe  Lake,  Africa,  on  map,  632* 

Holocene  period,  20,  77 

Holocrystalline,  90 ;  texture,  87,  88* 

Holtenia,  325* 

Homogeneous  texture,  87 

Hone-stones,  581 

Honey-comb  coral,  286,  288* 

Honshiu,  northern  earthquake  in,  682 ; 
map,  682* 

Horn  (mountain),  366,  368,  793,  797,  804 

Hornblende,  93 ;  andesite,  104 ;  recogni- 
tion of,  in  granite,  95  ;  schists,  described, 
651 ;  separation  of,  from  magma,  94 

Hornblendic  granite,  97 

Hornfels,  210,  650;   described,  651 

Horse  (structural),  635* 

Horseradish  odor,  50 

Horseshoe  Falls,  764,  765,  768,  774,  775; 
map  of,  763*  ;  section  of,  771* ;  surveys 
of  crest  line,  773* 

Horse-tails,  in  swamps,  336 

Horst,  634 

Hot  springs,  181,  183 

Hubbard  Glacier,  373;  map,  of,  371* 

Hudson  Gorge,  804*  ;  Bay,  516  ;  Highlands, 
rock  exposures  in,  34 ;  thrust  in  High- 
lands, 626,  627* 

Hudson  River,  brackish  water  of,  214; 
drowned  valley  of,  695;  estuarine 


character  of,  547 ;  fjord  character  of, 
803  ;  variable  depth  of,  803  ;  subsidence 
of,  803 ;  formation,  74 ;  mud  deposits 
on,  548 ;  profiles  of,  804  * 

Hudson  River  Bluestone,  74,  578*,  579 

Humboldt,  Alexander  von,  portrait,  824* 

Humic  acid,  action  of  in  solution  of  quartz, 
426 

Humidity,  absolute  and  relative,  355 

Hungarian  soda  lake,  250 

Hungary,  603  ;  dunes  of,  451 ;  perlite  of,  99 

Huntington,  E.,  cited,  245 

Hurricane  Fault,  792;  diagram,  602*, 
787*;  map,  786* 

Hutton,  James,  portrait,  26* 

Huttonian  theory,  26 

Hydrargillite,  403 

Hydration,  403 

Hydraulic  mining,  267* 

Hydrocarbon  minerals,  61 

Hydrocarbons  from  diatoms,  324 

Hydrochloric  acid,  from  magmas,  85 

Hydroclastic,  79,  568;  rocks,  defined,  83 

Hydrocoralline,  289 

Hydrogenic  deposits,  79 ;  rocks,  70-73,  77, 
214 

Hydrogen  minerals,  60 

Hydroid  polyps,  283,  289 

Hydrology,  science  of,  19 

Hydrosphere,  4,  5,  14,  70,  77 ;  as  rock- 
breaker,  77 ;  contribution  of,  to  litho- 
sphere,  10;  derivation  of  term,  12; 
destructive  work  of,  410;  operation  of, 
in  rock-breaking,  391 

Hydroxides,  defined,  40 

Hydrozincite,  53 

Hypabyssal,  18,  83,  205 

Hypersthene,  105 

Hypnum,  334*,  336 

Hypocenter,  657,  658* 

Ice,  60,  357,  433 

Icebergs,  383,  387*;  origin  of,  387*,  388*; 
size  of,  388 ;  submerged  part  of,  388* 

Ice-caps,  384,  493  ;  erosion  by,  805 

Ice  contact  slope,  502 

Iceland,  earthquake  of,  described,  682  ; 
ice-caps  of,  384,  493  ;  fissure  eruption  in, 
84;  lava  plains  of,  175;  solfataric 
vents  of,  181 ;  volcanoes  of,  109,  174; 
volcanic  fissure  in,  171* 

Ice  movement,  causes  of,  387  ;  direction  of, 
indicated  by  scratches,  434 


842 


Index 


Ice  rafted  material  of  submarine  banks,  388 
Ice  sculpture,  800;    land  forms,   due  to, 

800* 

Ice-tables,  377 
Iddings,  J.  P.,  cited,  204 
Igneous  contact,  205,  207 ;  significance  of, 

213 
Igneous  magma,   71,   84;     character  and 

composition  of,   85 ;    order  of  minerals 

crystallizing  from,  94 
Igneous  masses,  form  and  structure  of,  188  ; 

relative  ages  of,  213 
Igneous  material,  80 
Igneous  rocks,  47,  72,  73  ;  classification  of, 

89 ;     erosion    features    on,     816 ;     es- 
sential minerals  of,   90;    formation  of, 

87  ;   outcrop  of,  84 ;   types  of,  95 
Impervious  beds,  422 
Impregnation  vein,  267 
Inclined  strata,  583  ;  continuation  of,  584  ; 

diagram  of,  584* 
Incompetent  beds,  618 
Index  fossils,  556 
India,  lava  plain  of  central,  176  ;  Miliolitic 

limestone  of,  568 
Indian,    geosyncline,    518;     Ocean,  510; 

Ocean,  atolls  of,   292 ;    radiolarian  ooze 

in,  328;  rice,  335 
Indo-Gangetic  plain,  468,  697 
Indurated  elastics,  563 
Induration,  causes  and  agents  of,  564 
Indus  River,  468 ;  kankar  of,  260 
Inface,  defined,  714 
Inglefield  Gulf,  Greenland,  385 
Inland,  desert  basins,  salts   deposited  in, 

242  ;  plain  defined,  697 
Inner  lowland,  713,  714*,  715,  716*,  717*; 

control    of   depth  of,   714;  of  English 

cuestas,  719* 
Inorganic  spheres,  4,  12 
Insect  eggs,  coated  by  lime,  248 
Insects,  trails  of  fossil,  482 
Inselberge  (island  hills),  436 
Insequent  streams,  defined,  712 
Insolation,  392 

Intercontinental  seas,  509,  510 
Interlobate  moraines,  501 
Intracontinental  seas,  513,  514 
Intrenched  meander,  706*,  707,  718* 
Intrenched  oxbow,  707,  708* 
Intrusive,     rocks,    igneous    masses,    188 ; 

rocks,  84 ;  exposure  of,  84 ;    sheets,  194 
Inverness,  Scotland,  759 


Inverted  fault,  600;  development  of,  dia- 
grams, 602* 

Inwood,"N.  Y.,  210 

Iodine  from  kelp,  271 

Ions,  17  ;  defined,  41 

Ireland,  deformed  strata  of,  583;  upland 
bogs  of,  342 ;  volcanic  plugs  of,  169, 
170* 

Iridescence,  50 

Iridosmine,  57 

Irish,  peat  bog,  342*;  Sea,   518 

Iron,  bacteria,  258 ;  carbonate  and  oxide, 
257  ;  minerals,  52  ;  nodules,  of  laterite, 
404 

Iron-stone  nodule,  257 

Irondequoit  Bay,  on  map,  776* 

Iroquois  beach  on  section,  762* 

Ischia,  eruption  in,  128;  Island  of,  map, 
no* 

Island  hills,  436 

Isoclinal  folds,  591,  592,  595 

Isometric    system,    43 ;     principal    types, 

44* 

Isoldes,  319* 
Israelites,  exodus  of,  242 
Italian  marble,  6r3 
Italy,  sections  in  mountains  of,  33 

Jaggar,  T.  A.,  cited,  202 

Jail  Rock,  Nev.,  701  *,  705 

Jamaica,     earthquake,     described,     688; 

Island,  foraminiferal  limestone  of,  281 
Japanese  earthquake,  described,  680 
Java,  solfataric  vents  of,  181 
Jaxartes  River,  dunes  of,  449;    map  of, 

449* 

Jebel  el-Tih,  754 
Jebel  Musa,  754 
Jehlam  River,  471 
Jereica,  327* 
Jet,  350 
Johnson,  D.   W.,  cited,  749;    quoted   on 

beach  cusps,  536;  ref.,  333,  521,  527, 

695 

Jointed  rocks,  erosion  features  on,  8n 
Joints,  409,  638,  63  8  "-640* 
Jones,  H.  C.,  ref.,  41 
Jordan  River,  757 
Jorullo,  112;  described,  113 
Juan  Fernandez,  674 ;   submarine  volcano 

at,  675 

Jukes-Browne,  cited,  299 
Jumna  River,  469 


Index 


843 


Junagarh  limestone,  278 

Juncus,  334* 

Jungfrau,  359,  366,  368 

Juniata  glass  sand,  578* 

Junipers  in  swamp,  341 

Jupiter  Serapis,  temple  of,  691,  692* 

Jura  Mts.,  590,  593,  596,  645,  755  ;  on  map, 

605* 

Jurassic  period,  77 

Jurassic  sandstone,  cross-bedding  of,  456 
Juvenile  waters,  185,  268,  424 

Kaibab,     Canon,     787 ;      limestone,   791 ; 

Plateau,  786*,  787*,  790* 
Kainite,  58,  223 

Kalahari  Desert,  rock  destruction  in,  436 
Kames,  502 

Kammerbtihl,  168,  169* 
Kanab  Canon,  787 
Kankar,  260,  469 
Kansas,  sections  across,  32 
Kaolin,  production  of,  403 
Kaolinite,  63 
Kaolinization,  403 
Kara   Bugas    Gulf,    231,    238,    349,    352 ; 

asphalts  of,  352;  map  of,  239*;   mirab- 

ilite  in,  223  ;  section  of,  240* 
Karakoram  Himalayas,  glaciers  of,  370 
Kara  Kum  Desert,    232*,   450;    map  of, 

449* 

Karren,  413* 
Kashmir,  Vale  of,  471 
Katamorphism,  646 

Kathiawar  Peninsula,  India,  278  \ 

Kawafune  fault-cleft,  Japan,  682*  ' 

Kelley's  Island,  glacial  flutings  on,  498,  499* 
Kelps  of  Pacific,  271 
Kemp,  J.  F.,  cited  on  shell  layers  of  Florida, 

534 ;  table  of  rocks  modified  after,  96 
Kenia,  Mt.,  Africa,  on  map,  632* 
Kerguelen  Islands,  385 
Kern  Canon,  Cal.,  798* 
Kertch,  Bryozoa  of,  308 
Kettle-hole,  494,  503  ;  origin  of,  495* 
Kettle-ponds,  503 
Kettle  topography,  499 
Keys,  300,  301,  302 ;  of  Florida,  346 
Key  West,  300 

Khyber  Pass,  Himalayas,  469* 
Kieserite,  60 
Kilauea,    87,    117,    157;     as    type,    140; 

basic  glasses  of,  107;    crater  of,  118*; 

lava  lake  of,  118*,  119* 


Kilimandjaro,    Mt.    (Kilimanjaro),    385 ; 

on  map,  632* 

Kingston,  N.  Y.,  thrust  fault  near,  626* 
Kirchheim,  Wiirttemberg,  intrenched  ox- 
bow at,  708* 

Kivu  Lake,  Africa,  on  map,  632* 
Klingstein,  102 

Knife-edge  crests  (glacial),  794,  796* 
Knox  dolomite,  597 
Kokos  Keeling  atoll,  292 
Kotsina  Glacier,  Alaska,  terminal  moraine 

of,  494* 
Krakatoa,  137 ;    air  waves  from  eruption 

of,  656 ;    as  type,   140 ;    dust  of,  439 ; 

maps  of,  137*,  138*,  139*;    Rakata  of, 

140*;  section  of,  143* 
Kunzen,  burial  of   church  of,  by  dunes, 

445 

Kurische  Nehring  and  Haff,  444*,  445* 
Kyzyl  Kum  desert,  dunes  of,  450;    map 

of,  449* 

Laacher  Lake,  159,  161*;    section  of,  143* 
Labradorite,  91 

Laccolith,  167,  199 ;    section  of,  200* 
Lacustrine,  elastics,  568 ;  deposits,  83,  227, 

25°,  439 
Lagoons,  of  atoll,   292;    salts  formed  in, 

238  ;   sapropelite  in,  347 
Lahonton  Lake,  221 ;    concretionary  lime- 
stone of,  2 19*;  map  of,  254*;    thinolite 

from,  255*;   tufa  domes  in,  256* 
Lake  Albert,  on  map,  632* 
Lake  Baikal,  516 

Lake  Bonneville,  see  Bonneville,  Lake 
Lake  district,  England,  dome  structure  of, 

744;  map  of,  745* 
Lake  Erie,  767  ;   joints  in  rocks  on,  638* ; 

outcrops  on,  29;  section  along,  30*,  31*; 

strata  on,  79 
Lake  Huron,   726,   731 ;    cross-section  of, 

728*;  map  of,  727* 
Lake  Iroquois,  779,  780;    map  of,  762*; 

section  of,  762* 

Lake  Lahonton,  see  Lahonton  Lake 
Lake  Michigan,  731;    change  of  level   in, 

695  ;  dunes  of,  448;  section  from  Bara- 

boo  to,  32 

Lake  Ontario,  30,  725,  767,  775,  780 
Lake,  P.,  and  Rastall,  R.  H.,  cited,  746 
Lake  Superior,  caves  on  shore  of,  811 
Lake  waters,  composition  of,  250 ;  table  of 

composition  of,  251 


844 


\ak 


Index 


4;    evaporation  .products  of,  250; 

origin  of,  423 ;    temporary,  formed  by 

land  slides,  398,  399 
Lakes  of  Africa,  map  of,  632 ;  see  under 

names 

Lamarck,  J.  B.,  portrait,  25* 
Lamellibranch,  309,  310*     . 
Lamina,  486 
Laminated  texture,  218 
Land,   area  of,   4;    mollusks,   315,   316*; 

plaster,  221;   surface,  original  character 

of,  697 
Land  slides,  398*;    due  to  earthquakes, 

690 

Landes,  446 
Lang  Glacier,  368 
Laon,  on  map,  730* 
Lapiaz.  413* 
Lapilli,  117,  432 

Lapland,  altitude  of  snow-line  in,  357 
La  Plata  estuary,  547,  549* ;  map  of,  548*; 

material  transported  in,  462 
La  Plata  Mountain,  201* 
Laramie  Lakes,  Wyoming,  258 
La  Sal  Mts.,  Utah,  erosion  features  near, 

823 
Lateral  moraines,  368,  370,  492;    origin 

of,  403* 

Lateral  secretion,  268 
Laterite,  404 
Lateritic  soil,  551 
Laterization,  493 
Laufen,    Wurttemberg,   intrenched  oxbow 

at,  707,  708* 
Lauterbrunnen   canon,    198*,    372*,    799, 

802 
Lava,  age  of  flows  of,  143  ;  plains,  Oregon, 

Idaho,   and  California,  175;    rock,  84; 

surface  of  sheets,  1 74 ;  tunnels,  1 20, 1 2 1  * ; 

viscous  lava  of  craterless  volcanoes,  122 
Lead  minerals,  54 
Le  Conte,  J.,  cited  on  reefs,  300 
Leda,  in  raised  beaches,  694 
Ledges,  as  sources  of  geological  information, 

29 

Lee  Lake,  709;  map,  710* 
Leith,  C.  K.,  quoted,  647,  648 
Leith,  C.  K.,  and  Mead,  W.  J.,  ref.,  646 
Lemna,  334*,  336 

Lemus  Island,  earthquake  effects  near,  675 
Length  of  waves,  520 
Leonardo  da  Vinci,  24 
Lepas,  317* 


Leperditia,  318* 

Lepidocydina,  281* 

Lepidolite,  58 

Le  Puy,  France,  166 

Leschaux,  Glacier  de,  369 

Leu  cite  Hills,  Wyoming,  166 

Levees,  natural,  479 

Lewiston,  N.  Y.,  763,  766,  771 

Libyan  Desert,  452*;  chalk  from,  278; 
origin  of  sands  of,  75,  452  ;  wind  grooves 
in,  405 

Lignite,  343 

Ligurian  Alps,  on  map,  605* 

Limbs  of  anticline,  589 

Limburgite,  96,  107 

Limburgite-porphyry,  96 

Lime,  organically  secreted,  71 

Lime  carbonate,  deposits  of,  246 ;  percent- 
age in  blue  muds,  551;  percentage  in 
green  muds,  552;  percentage  in  red 
muds,  551 

Lime  mud-rocks,  581 

Lime  sandstones,  described,  580 

Limestone,  221 ;  conglomerate,  577 ; 
formed  by  Bryozoa,  307 ;  peaks,  solu- 
tion in;  822;  solution  forms  on,  413* 

Limestones,  570;  contact  metamorphic 
effects  on,  209 

Limncea,  315,  317* 

Limonite,  52,  224,  402 

Limpets,  in  raised  beaches  of  Scotland,  533 

Lingula,  309 ;  phosphate  of  lime  in  shells 
of,  322 

Linton,  Ohio,  cannel  coal  of,  349 

Lipari  Islands,  1 1 1 ;  obsidian  of,  99 

Lisbon  earthquake,  664 

Litchfield,  Maine,  nephelite  syenite  from, 
102 

Lithium  minerals,  58 

Lithodomus,  692 

Lithogenesis,  IQ 

Lithographic  stone,  58 

Lithoidite  with  lithophysae,  100* 

Lithology,  science  of,  14;  subdivisions  of, 
16 

Lithophysae,  99,  100* 

Lithosphere,  5,  14,  15,  19,  71,  84;  contribu- 
tion to,  from  other  spheres,  10;  deri- 
vation of  term,  12  ;  movements  of,  in 
rock  breaking,  391 ;  organic  remains 
in,  7  ;  thickness  of,  5 

Lithothamnium,  272*,  301 ;  of  reef,  293 

Little  altered  rocks,  69 


Index 


845 


Little   Colorado  River,   788,   789;    mud- 
cracks  along,  479 
Littoral  district,  515,  518 
Liverpool  section,  beginning  at,  32 
Livingston  and  Lewis  ranges,  section,  599* 
Livingston  Range,  Montana,  583* 
Lobsters,  317 
Local  metamorphism,  643 
Loch  Linnhe,  Scotland,  759 
Loch  Maree,  569 

Loess,  458,  567  ;  consolidation  of,  459 
Loessmannchen  (Loesspiippcheri),  459,  572, 

573* 

Loire  River,  144 ;  sandbanks  of,  462 
Loligo,  314* 
London,   rocks  under,   64;     section  from 

Liverpool  to,  32 
London  Basin,  599 
London  coastal  lowland,  719,  720* 
Long  Branch,  destruction  by  storm  waves 

on,  541* 
Long  Island,  N.  Y.,  apron  plain  of,  503  ; 

development  of,  811;    erosion  on,  818; 

origin  of,  716 
Long  Island  Sound,  715 
Long-shore  currents,  523,  528;    bar  sand 

spits  formed  by,  819 
Long-shore  drifting,  523 
Longwood  shale,  mud-cracks  in,  491* 
Looking  Glass  Rock,  Utah,  823* 
Lop  Nor,  salt  plain  of,  243*,  249*;    types 

of  sand  ripples  in,  454 
Lorelei  Rock,  584* 
Lorraine,  minette  ores  of,  225 
Lot's  wife,  757,  822 
Lowland,  annular,   730;    inner,  see  Inner 

lowland 

Lucrinus,  Lake,  112 
Lunatia,  311* 
Luray  Cave,  263*,  264 
Luster,  49 

Lutaceous  texture,  569,  570 
Lutyte,  56 Q,  580 
Luxemburg,  minette  ores  of,  225 
Luzon,  114 
Lycra,  jet  from,  350 
Lyell,  C.,  cited,  23,  36,  37,  671 ;    quoted, 

668,  670 
Lyell,  Sir  Charles,  portrait,  27 

Maare,  of  Eifel,  158,  i59*-i6i* 
Maar,  Schalkenmehren,  160*;    Gmunden, 
1 60;  ideal  section  of,  161* 


Mackinac  Island,  Mich.,  728;    erosion  on 

coast  of,  814 ;    natural  bridge  on,  814* 
Maciurean  era  in  American  geology,  28 
Macro-domes  (orthorhombic),  47* 
Macro-pinacoids      (orthorhombic),      47*; 

(triclinic),  48* 
Madrepora,  285,  286*,  287*,  306;  on  reefs, 

291 

Maandrina,  on  reefs,  291 
Magdalena  Mt.,  N.  Mex.,  749 
Magdeburg-Halberstadt  region,  235 
Magma,  390 ;  igneous,  84 
Magmatic  origin  of  tufa-depositing  waters, 

261 ;  waters,  268,  424 
Magnesite,  60,  222 
Magnesian  Limestone,  concretions  of,.  221* ; 

of  Durham,  573 
Magnesium    carbonate,    precipitated    by 

algae,  271 ;  minerals,  59,  60 
Magnetic  character  of  minerals,  50 
Magnetite,  52,  93,  402 
Magnetite  rock,  described,  654 
Magnetitic  rocks,  649 
Maine  coast,  806,  810 ;   raised  beaches  of, 

694  ;   dikes  of,  191 ;   map  of  portion  of, 

805* 

Mainz,  see  Mayence 
Malachite,  56 
Malaspina   Glacier,   358;    map  of,   381*; 

described,  382;    moraine  on  surface  of, 

383* 

Malleable,  denned,  49 
Malta,  Globigerina  limestone  of,  276 
Maltha,  351 

Mammoth  hot  springs,  terraces  of,  261 
Man,  as  rock-breaker,  82  ;  associated  with 

glacial  deposits  in  Europe,  496 ;   destruc- 
tion of  rocks  by,  436 
Manganese-concretions,    249*;    ores,    224; 

minerals,  52 ;  nodules  in  red  clay,  553 
Mangrove,  Islands,    301 ;     swamp,    340*  ; 

trees,  341 

Manitoulin  Island,  728 
Mantle  rock,  denned,  65;    interfering  of, 

with  geological  study,  29 ;   on  bed  rock, 

65* 

Manyara  Lake,  Africa,  on  map,  632* 
Marble,  67,  209,  569 ;  described,  649,  650, 

653  ;   microsection  of,  649* 
Marble  Canon,  788,  789;    on  map,   786*; 

on  diagram,  787* 
Marble  Plateau,  on  map,  786*;    diagram, 

787* 


846 


Index 


Marblehead,  coast,  terrace  of,  532  ;  Neck, 

Mass.,  810*;  rocks  of,  206;  tombolo,  544 
Marcasite,  52 
Marengo  Cave,  264* 
Marine  bench,  530*,  807* 
Marine  elastics,  defined,  568;    structures 

of,  553 
Marine,  deposits,  227,  439;  marshes,  330; 

overlap,  diagram,  562*;    rocks  83,  567 
Maritime  Alps,  pebble  beaches  on  coast 

at  foot  of,  533 
Marjelen  See,  363,  364;    deltas  of,  362; 

section  of,  362*;  view  of,  361* 
Marl,  origin  of,  273 
Marne  River,  on  map,  730* 
Marr,  J.  E.,  cited,  746 
Marsh  gas,  329,  331 
Marsh  grasses,  331* 
Marshes,  marine,  329,  330,  331* 
Martha's  Vineyard,  coast  of,  543 
Maryland,  coast  of,  810 ;  coastal  plain  of, 

7i5 

Massa  River,  364 

Massachusetts  coast,  dikes  of,  191 ;  drum- 
lins  of,  498;  morainal  sections  of,  531; 
peat  of,  332,  333;  pebbles  of,  430*; 
tidal  falls  of,  526  ;  wave  erosion  on,  428, 
429*,  807*,  808*,  810*,  817*,  8i8*-822* 

Massif,  Central  France,  144 

Master  consequent  streams,  defined,  712 

Master  stream,  drainage  system  of,  721 

Mato  Tepee,  166,  168* 

Matterhorn  ,  797* 

Matterhorn  peak,  800* 

Mature  lands,  erosion  of  coast  on,  810 

Mature  river,  characters  of,  708 

Maturity  of  land,  705 

Mauna  Loa,  117,  119  ;  end  of  lava  flow  of, 
121*;  form  of,  '141;  height  of,  121; 
section  of,  143* 

Mayence,  754 

Mayon  volcano,  109 ;  form  of,  141 ;  view 
of,  Frontispiece* 

Mazama  Mt.,  156;    restoration  of,  159* 

McClure,  W.,  27  ;  portrait,  28* 

Meager  feel,  50 

Meandering  course  of  streams,  709 ;  on 
peneplane,  707  ;  of  river  current,  416* 

Meander  River,  417 

Meanders,  development  of,  417*;  of  Mis- 
sissippi River,  map,  710* 

Mechanical,  rock  destruction,  390;  sedi- 
ments, 67 


Medford  dike,  disintegration  boulders  of, 

397* 

Medial  moraine,  370,  492  ;  origin  of,  493* 
Medina  sandstone,  origin  of,  535 ;    fossil 

beach  cusps  in,  536* 
Mediterranean,  509 

Mediterraneans,  515  ;  importance  of,  516 
Meeteesie,  Wyoming,  unconformity  near, 

612 

Megal,  145 
Mehlem,  157 

Meissen,  pitchstone  of,  99 
Melaphyres,  107 
Melilite,  92 
Melito,   Italy,   earthquake  manifestations 

in,  668,  669 
Membranipora,  307* 
Mendelieff,  on  origin  of  petroleum,  353 
Mendenhall  Glacier,  378 
Mender,  see  Meander 
Mendoza,  Chile,  674 
Mer    de    Glace,    370*;     described,    369; 

map  of,  369* 
Merced  River  fan,  467 
Mercury  minerals,  56 
Merrill,  G.  P.,  cited,  23 
Merryman's  Lake,  section  of,  333* 
Mesozoic,  coals,  344 ;   lignites,  343 
Messina,   effects  of  earthquake  on,   670; 

earthquake     manifestations     in,     668 ; 

earthquake     described,     670;      seismo- 

gram   of,    66 1 ;     straits,    tides   of,    525  ; 

torrential  deposits  in  sea  near,  533 
Metallic,  aqueous  deposits,  224;  carbides, 

as    source    of    petroleum,    352 ;     luster 

defined,  49 
Metamorphic  rocks,  67,  68 ;    age  of,  648 ; 

types  of,  649 

Metamorphic  structures  and  textures,  647 
Metamorphism,     68,    69,    642 ;     agencies 

producing,   643 ;    classification  of,   642 ; 

contact,  193 

Meteoric,  dust,  528;   phenomena,  13 
Meteorites,  13 
Meteorology,  science  of,  13 
Meteors,  13 
Metz,  731 

Meuse  River,  743*,  744* 
Mexican  onyx,  221,  261 
Mexico,  earthquake  effects  in,  678 ;    Gulf 

of,  516 
Mezenc,  145*;    Massif  of,  section,  151*; 

on  map,  153* 


Index 


847 


Mica,  63,  90;   andesite,  104;    commercial 

source   of,    88;    diorite,    103;    syenite, 

100 

Mica  schist,  67,  651 
Michigan,    1 1 ;    old   coral   reefs   of,   306 ; 

St.  Peter  sandstone  of,  75  ;  salt  deposits 

of,  n,  245;    Sylvania  sandstone  of,  75; 

wind  grooves  in  Silurian  rocks  of,  405 
Michigan  Basin,  599,  600*,  602,  615,  729 ; 

cross-section  of,  601,  731 
Microcline,  90 
Microdiscus,  319* 
Middle  Aletsch  Glacier,  364 
Midori,  Japan,  earthquake  fault-scarp  near, 

681* 

Miers,  H.  A.,  ref.,  51 
Migration  of  geosynclines  in  Carpathians, 

607 

Miles  Glacier,  378;  map  of,  377* 
Miletus,  417 
Miliola,  278* 

Miliolitic  limestone,  278,  281,  568 
Military  geology,  22 
Millepora,  288,  289;    on  reefs,  291;    reef, 

300 

Millerite,  52 

Millet  seed  type  of  sand  grain,  440 
Milwaukee,  Wis.,  32  ;   reefs  near,  305 
Minas,  basin  of,  534 
Mineral,  coal,  61 ;  deposits,  86  ;  springs,  86  ; 

veins,  265 
Mineralizers,  209 
Mineralogy,  18,  38 
Minerals,    17,    38;     classification   of,    50; 

defined,  42  •  tables  of,  51,  52-62 
Mines,  4 

Minette  ores,  225 
Mining  camp,  268* 
Mining  geology,  20 
Minnesota,  Potsdam  sandstone  of,  75 
Mino  province,  Japan,  earthquake  in,  680 
Mirabilite,  58,  223;    in  Great  Salt  Lake, 

250 

Misenium,  127 
Mississippi    River,    amount    of    sediment 

carried  annually  by,  462  ;   delta  of,  485 ; 

flood -plain  of,  map,  463*,  475*;  levees 

of,    475,     476* ;      material    transported 

by,  462  ;   mud-flats  of,  348 ;   mud-lumps 

and    craters    on  delta  of,  "348 ;   oxbows 

and  meanders  of,  map,  710* 
Mississippi    Valley,    St.    Peter    sandstone 

of,  75 


Modified  drift,  502 

Modiola,  310*;  in  marshes,  333 

Moffettes,  181,  182* 

Mohawk  River,  728,  779 

Mon's  scale  of  hardness,  49 

Mollusks,  3io*~3i5*;    as  bottom  feeders, 

347 

Molybdenum  minerals,  55 
Monaco,  Prince  of,  14 
Monadnock,  Mt.,  741,  742* 
Monadnocks,    741 ;     described,    705 ;     on 

Great  Plains,  701  * 
Monazite,  53 
Monch,  the,  368 
Mono   Lake,    California,    258;     trona  in, 

223 

Monocline,  600;    diagram  of,  602* 
Monoclinic,    prism,    47*;     pyramid,  48*; 

system,  47 ;   principal  types  in,  47 
Monogenetic  elastics,  567 
Mont  Blanc,  369 
Mont  Dore,   145,   146;    massif  of,   147*; 

section  of,  148* 
Mont  Pelee,  87  ;  described,  136*  ;  spine  of, 

136*,  162*,  163* 
Monte  Minardo,  135 
Monte  Nuovo,  in,  112,  113*;    described, 

112  ;  eruption  of,  128,  693 
Monte  Rossi,  135 

Monte  Somma,  127*;  map  of,  125* 
Monteleone,  Italy,  earthquake  manifesta- 
tions in,  667 
Montgomery,  715 
Monticules,  135*,  136,  188 
Monument  Park,  erosion  pillars  in,  407*, 

409 
Monuments  of  erosion,  407*;  wind  carved, 

822 

Monzonite,  100 
Moon,  2 
Moorlands,  329 
Moraines,  ground,  496  ;    interlobate,  501 ; 

lateral,  368,  370/492,  493*  ;  medial,  370, 

492,   493*;     terminal,    370,   493,   494*; 

relation  of  terminal  and  interlobate,  502  ; 

sub-marginal,  493 
Morainic  dam,  803 
Moray  Firth,  614,  759 
Morse  Creek  limestone,  31* 
Morse  Glacier,  376* 

Moselle  River,  development  of,  743,  744* 
Moses,  J.  A.,  and   Parsons,   C.  L.,  ref., 


848 


Index 


Moss  Mountain,  section  and  restoration, 

201* 
Mother     liquor,     composition     of,     220; 

denned,  229 
Moulins,  glacial,  368 
Mount    Taylor,    New    Mexico,    volcanic 

necks  of,  166* 

Mountain  chains  of  Europe,  map  of,  605* 
Mountain-enclosed  basins,  deposits  in,  471 
Mountain  of  the  Law,  754 
Mountainous  countries,  outcrops  in,  33 
Mountains,     illustrations     of     dynamical 

geology  in,  36 ;  relative  size  of,  2,  3 
Movements,  metamorphism  in,  645 
Movements     of     lithosphere,    with    rock 

shattering,  80 

Mt.  Catherine,  Sinai,  754;  map,  755* 
Mt.  Desert,  batholith  of,  205 
Mt.  Grey  lock,  Mass.,  598* 
Mt.  Holmes,  Mont.,  203* 
Mt.  Monadnock,  N.  H.,  741,  742* 
Mt.  Morris,  N.  Y.,  780,  782,  784 ;  on  map, 

776*;   sections  at,  30 
Mt.  Rainier,  794*;  andesite  from,  104 
Mt.  Serbal,  754 
Mt.  Shasta,  andesite  from,  104 
Mt.  Sinai,  754 
Mt.  Sir  Donald,  797 
Mt.  St.  Elias,  382 
Mud-cracks,    480;     fossil,     482*,     490*, 

491;    in  river  deposits,  490;    of  Little 

Colorado  River,  479* 
Mud-flats,  347,  348 
Mud-line,  529 

Mud-rock,  denned,  $6g ;  contact  metamor- 
phism of,  210 

Mud-stones,  described,  580 
Mud  streams  of  Calabrian  earthquake,  669 
Mud  volcanoes,  181,  182*,  183,  680;    due 

to  earthquakes,  690 
Muir  Glacier,  376*,  378;    map  of  retreat 

of,  379* 
Muka  Muka  cliffs,  elevation  at,  675 ;    on 

map,  676*  ;  section  of,  677* 
Mull,  Island  of,   map  and  section,  193* ; 

radial  dikes  of,  192,  194* 
Munson,   Mass.,   conglomerate  gneiss  of, 

653 
Mur  River,  Austria,  destruction  of  rock 

fragments  in,  414 

Murray,  J.,  143  ;  cited,  6 ;  on  reef  origin,  296 
Muscovite,  63,  93 
Mussel,  310*,  311 


Naero  Fjord,  801* 

Nagelfluh,  563*,  564* 

Nahant,  Mass.,  819  ;  map  of,  544*  ;  sec- 
tion, on  coast  of,  35* 

Naivasha  Lake,  Africa,  on  map,  632* 

Nancy,  731 

Nantasket  Beach,  544 ;  development  of, 
diagrams,  818*,  821* 

Nantucket,  apron-plain,  503;  Sound,  on 
map,  808* 

Naples,  Italy,  691 

Naples,  N.  Y.,  on  map,  776* 

Naples  Bay,  change  of  level,  694 

Naples  volcanic  field,  in  ;  map  of,  no* 

Nashville  dome,  728,  729* 

Naters,  364 

Natrolite,  63 

Natron  Lakes,  Egypt,  259 

Natural  bridge  on  California  coast,  812*; 
Utah,  823*;  of  Virginia,  425,  426* 

Natural  bridges,  822 

Natural  classification  of  rocks,  68 

Natural  gas,  353 

Nautilus,  313,  314*,  315 

Navigator  Islands,  tsunamis  at,  674 

Neap  tide,  5,  24 

Nebraska,  dunes  of,  451 

Neckar  Valley,  on  diagram,  756* 

Necks,  volcanic,  163,  165*,  166* 

Nefud  Desert,  451 

Nephelite,  61,  92 

Nephelite  syenite,  96,  101,  102 ; .  series, 
glasses  of,  102 

Nephelite-syenite-phonolite,  series,  102 

Nephelite-syenite-porphyry,  96,  102 

Neptunea,  311* 

Neritic  zone,  518;  on  diagram,  519* 

Nero,  Bath  of,  113 

Neve,  357,  358*,  367*;  on  map,  360* 

Newberry  Butte,  790 

New  Brunswick,  raised  beaches  of,  694 

New  England,  coastal  section  of,  35,  36 ; 
deformed  strata  of,  583 ;  eskers  of, 
506*,  507*  ;  old  lavas  of,  100  ;  rock  ex- 
posures in,  34  ;  sand  plains  of,  503*,  504* 

New  England  coast,  806 ;  dike  chasms  of, 
191*;  erosion  of  dikes  on,  816 

New  England  peneplane,  702,  742*,  743 

New  England  upland,  744 

Newfoundland,  coast  sections  of,  36 

New  Jersey,  coast  of,  810 ;  coastal  plain  in, 
715  ;  meadows  of,  332  ;  rock  exposures 
of  mountains  of,  34 


Index 


849 


New  Madrid  earthquake,  670 

New  Mexico,  earthquake  effects  in,  678; 
volcanic  neck  in,  166 

New  Mountain,  Japan,  112 

Newport,  R.  I.,  wave-cut  chasm  near, 
816* 

New  world  mass,  510 

New  York  City,  flagstones  of,  74 ;  rock 
exposure  near,  34 

New  York  State,  deformed  strata  of,  583 ; 
drumlins  of,  498;  geological  map  of, 
30;  geological  system,  27;  Mass. 
boundary  line,  rocks  of,  306 ;  old  coral- 
reefs  of,  306;  salt  deposits  of,  n,  245; 
illustrations  of  rock  outcrops  from,  29 

New  Zealand,  earthquake  of,  675;  fjords 
of,  804,  806* ;  map  of,  676* ;  solfataric 
vents  of,  181 

Niagara,  779;  section  of  falls  at,  781* 

Niagara cuestas,  map,  727*;  on  block  dia- 
gram, 760*;  section  of,  761* 

Niagara  escarpment,  763,  767,  780 

Niagara  Falls,  418*,  764* ;  geological  his- 
tory of,  760;  mode  of  cutting  of,  771; 
on  map,  763*;  unequal  retreat  of,  773; 
on  map,  773* 

Niagara  Glen,  775* 

Niagara  Gorge,  802 ;  bird's  eye  view  of, 
767*;  map  of,  763*;  outcrops  in,  29; 
section  of,  776*;  youthful  condition  of, 
708 

Niagara  River,  geological  history  of,  760 ; 
peculiar  course  of,  763,  769  ;  rate  of  re- 
treat of,  771 ;  section  of,  30*  ;  volume  of, 
771 

Niagara  sea,  246 

Nicaragua,  alignment  of  volcanoes  in,  map, 
172* 

Niccolite,  52 

Nickel  minerals,  5,  52 

Nicolosi,  destruction  of,  135 

Nile,  flood  plain  of,  477*,  480 ;  mud-cracks 

on  flood  plain  of,  476*  ;  mud  flats  of,  348  ; 

delta  of,  484*,  485  ;  mud,  organic  matter 

of,  348;   region,  map  of,  755 

Nipissing,  Great  Lakes,  map,  770* 

Niter  soda,  58 

Nittany  Valley,  limestone  solution  of,  427 

Nodules  of  oxide  of  manganese,  249* 

Non-clastic  rocks,  70 

Non-fragmental  rocks,  70 

Non-marine  overlap,  562* 

Non-metallic  luster,  49 


Non-vascular  plants,  deposits  of  decaying, 

346 
Norfolk,  England,  sand  dunes  on  coast  of, 

445,  446 
^orite,  105 

Normal  faults,  621 ;  on  diagram,  620* 
Normandy  cliffs,  map  showing  erosion  of, 

509* 

North  Africa,  dunes  of,  448 
North    America,    Atlantic    coast    of,    35; 

raised  beaches  of,  694 
North  Carolina,   sand  bars  on  coast  of, 

540* ;  diorite  from,  105 
North  Island,  New  Zealand,  675 ;  on  map, 

676* 

North  Pacific,  diatom  ooze  of,  323 
North  Sea,  518 

North  Sea  coast,  England,  erosion  on,  429 
Norway,  coast  of,  806 ;    marbles  in,  654 ; 

raised  beaches  in,  694 
Norwegian  fjords,  falls  in,  802 
Nose  (topographic),  740 
Nottingham,  England,  719 
Novaculite,  581 
Nova  Scotia,  erosion  of  basaltic  rocks  in, 

813 ;    erosion  features  on  coast  of,  816 ; 

sea-stacks  on   coast   of,   815;    volcanic 

agglomerate  in,  577*;  volcanic  plugs  in, 

173 
Nubian  sandstone,  75 ;  as  source  of  sands  of 

Libyan  Desert,  452 
Nullipores,    271;    of  Triassic  reefs,   306; 

on  coral  reefs,  291 
Nummulites,  280* 
Nummulitic  limestone,  76,  280* 
Nunataks,  386,  493,  804 

Oatka  creek,  on  map,  776* 
Oatka  valley,  776,  779 
Oban,  Scotland,  759 
Oblique  faults,  623,  624 
Obolus,  309 
Obsidian,  95,  96,  98 

Ocean,  salinity  of  water  of,  228 ;    tides  of 
ideal,  526;   velocity  of  currents  of,  526 
Oceanic  coral  reefs,  292 
Oceanic  currents,  529 
Oceanic  reefs,  organisms  of,  293 
Oceanography,  14 
Oceanographic  institutes,  14 
Oceans,  4,  509;  the  four,  510 
Oceanic  depressions,  3 
Ochsenius,  Bar  theory  of,  241 


850 


Index 


Octahedron,  44* 
Octopus,  313 
Odor,  50 

Offlap  and  overlap,  559,  560* 
Offlapping,  progressive,  558,  559* 
Offset,  620*;  in  faulting,  625* 
Off-shore  shallow  water,  deposits  in,  549 
Ohio,   old  coral  reefs  of,   306 ;    Sylvania 

sandstone     of,     75  ;      freestone,     578; 

River,  in  flood,  474* 
Oil,  field,  351*;    producers,  352;     sands, 

352;   shales,  334,  353,  580;   storers,  352 
Okefenoke  Swamp,  339 
Oklahoma,  section  in,  32 
Obsequent  streams,  714 
Old  age  of  land,  705 
Old  Faithful  geyser,  184,  185* 
Old  Harry,  sea-stack,  815 
Old  Man  of  the  Mountain,  339* 
Old  plains,  699,  744 
Old  Red  Sandstone,  575* 
Old  Stone  Age,  265 
Old  world  man,  510 
Older  igneous  masses,  188 
Older  vegetal  deposits,  343 
Oldland  of  Wales,  719* 
Olean  conglomerate,  569 
Oligoclase,  9 
Olivine,  93 ;    gabbro,  96.    105  ;    porphyry, 

96 ;   norite,  105 

Omori,  Japan,  earthquake  rifts  near,  68 1 
Onondaga,  cuesta,  760,   761*,  763;    lime- 
stone, disconformity  beneath,  616* 
Onondaga-Marcellus  oil,  352 
Ontario,  Lake,  see  Lake  Ontario 
Ontario,    salt   deposits   of,  245 ;    western, 

Sylvania  sandston     ^,  75 
Ontario  dome,  725,  728,  761,  775;  block, 

diagram  of,  760*;   map  of,  727;   rate  of 

rising  of,  696 

Ontario  lowland,  761,  775;  section  of,  761* 
Ontario  Valley,  779 
Onyx, -Mexican,  221,  261 
Oolites,    217*,    574;     of   bacterial   origin, 

271*;    deposited   by   springs,    262;    of 

great  Salt  Lake,  origin  of,  274 
Oolite  cuesta,  England,  719* 
Oolitic  limestone,   217*;    precipitated  by 

bacteria,  302 
Oolitic  texture,  217* 
Opal,  61 
Opalescence,  50 
Opaque,  50 


Opheopholis,  320* 

Ophicalcites,  649,  654 

Ophitic  textures,  106 

Oppido,  Italy,  earthquake  manifestations 
in,  666,  668 

Orbitoides,  from  Cuba,  281* 

Orbitolites,  300 

Orbulina,  275* 

Ordinario  (marble),  654 

Ordovician,  Bryozoa,  308* ;  elastics,  75 

Ore  chamber,  266* 

Organ  pipe  reef,  300 

Organic,  elastics,  568;  matter  from  Nile 
mud,  348;  precipitates,  269*;  remains, 
in  marine  elastics,  554;  rocks,  i,  72, 
73,  269;  rocks,  as  source  of  clastic 
material,  528;  salts,  215;  sediments,  67 ; 
separation  of  lime  in  lakes,  257 ;  silica, 
322;  spheres,  12;  tissues,  269 

Organisms,  as  rock  producers,  71;  com- 
mingling of  remains,  in  delta,  486; 
rock  destruction  by,  436 

Origin  of  rivers,  411 

Oriskany  sandstone,  80*,  578* 

Orkney  Islands,  sea-stacks  on,  814 

Orpiment,  54 

Orthoceran  type,  315 

Orthoceras,  315*'  limestone,  315 

Orthoclase,  61,  ^  j,  91 ;  recognition  of,  in 
granite,  95 

Orthopinacoid  (Monoclinic),  47* 

Orthorhombic,  prism,  47*;  pyramid,  47*; 
system,  47  ;  principal  types  in,  47 

Osar,  see  Esker 

Oscillation  waves,  521 

Osorno,  eruption  of,  674,  675 

Ostend,  dunes  at,  sections,  459* 

Ostracods,  317,  318* 

Ottawa  outlet,  on  map,  770* 

Outcrop,  2Q;  deflection  of,  diagram,  587*, 
588*;  relation  between  width  of,  and 
dip,  588 ;  width  of,  diagram,  589* 

Outcrops,  in  mountainous  countries,  33; 
of  igneous  rocks,  84 

Outer  lowland,  England,  71.9* 

Outliers,  740*;  origin  of,  714 

Outwash  plain,  503 

Overdeepened  subsequent,  721* 

Overdeepening,  801 

Overlap,  marine,  562*;  non-marine,  562*; 
of  beds  of  alluvial  fan,  466 ;  progressive, 
555*,  556*,  557*;  replacing,  560*, 
561* 


Index 


851 


Overturned  anticline,  590* 

Owari   province,    Japan,    earthquake    in, 

680 
Owen's  Lake,  Cal.,  250,  258,  259 ;  trona  in, 

223 

Owen's  Valley  earthquake,  677 
Oxbow,     709;      intrenched,     707,     708*; 

formation  of,  417* ;  of  Mississippi  River, 

map,  710* 

Oxford,  England,  719 
Oxidation,  402 
Oxides,  39 

Oxus  River,  map,  449* 
Oxyhydroxides,  40 

Ozark  dome,  map,  724*;  plateau,  723 
Ozokerite,  61,  351 

Pacific  Ocean,  510 ;  coral  sands  and  muds 
of,  552 ;  radiolarian  ooze  in,  326 

Pahoehoe,  120 

Pailleret,  Puy  de,  149 

Paint  Pots,  Yellowstone,  183 

Palaeogeography,  20 

Palaeolithic  time,  265 

Palaeontology,  19 ;  science  of,  15 

Palaeozoic,  coal,  345 ;  crinoidal  limestone, 
321*;  petroleum,  352 

Palestine  rift  valley,  631,  633,  757 

Palisades,  197*,  198;  columnar  structure 
of,  199 ;  contact  metamorphism  in, 
207 ;  diabase,  base  of,  200  ;  section  of, 
199*;  trap  rock  of,  73 

Pallas,  P.  S.,  26 

Paludina.  315* 

Panama  Canal,  geological  advice  in  con- 
struction of,  21 

Paradoxides,  319* 

Paraffin,  351 

Parallel  Roads  of  Glen  Roy,  363*,  364*; 
map,  365* 

Parana,  547 

Parasitic  cones,  188 

Parian  marble,  654 

Paris,  731;  Oceanographic  Institute  of, 
14;  on  map,  730;  rocks  of,  64;  sec- 
tions near,  32,  33 

Park  City,  Utah,  268* 

Pattie  Basin,  map  of,  232* 

Pearlite,  98 

Pearly  nautilus,  314* 

Pearly  luster,  49 

Peat,  3^9*;  beds  in  alluvial  plain,  469; 
bog,  Scottish,  341* 


Peat  deposits,  formation  of,  329;  on  coast, 

807  ;  on  shore,  809 ;  thickness  of,  332 
Peat  marsh,  538 
Peat-moss,  334*,  336 
Pebble  beaches,  532 
Pebbles,  form  of,  430* ;  formation  by  wave 

erosion,  430;   glaciated,  497 
Pechuel-Loesche,  quoted,  436 
Pecten,  310* 
Peekskill,  N.  Y.,  quartz-mica  diorite  from, 

103 
Pegmatite,   93,   97,    207;  dikes,    88,    192, 

193*;  veins,  206 
Pegmatitic  structure,  207 
Pekin,  see  Peking 
Peking,  467 
Pelagic  district,  519 
Pelecypods,  309,  310* 
Pelee,  spine  of,  161  ;  view,  162*,  163* 
Pele's  hair,  96,  107,  118 
Peloponnesia  Mts.,  on  map,  605* 
Pelvoux,  Massif  de,  366* 
Penck,  A.,  74 ;  on  reef  origin,  297 
Peneplane,  148,  701,  705,  806;    formation 

of,  716;    New  England,   702;    relation 

to  base-level,  705 
Pennine  chain,  section,  198* 
Pennine  escarpment,  197 
Pennsylvania,    old    coral    reefs    of,    306; 

rock  exposures  of  mountains  of,  34 
Pennsylvanian  coals,  344 
Pentelic  marble,  654 
Pentlandite,  52 
Periades,  Glacier  des,  369 
Peridotite,  96,  105 ;   change  to  serpentine, 

106 

Peridotitic-gneiss,  653 
Period  of  waves,  521 
Perlite,  96 ;  thin  section  of,  99 
Permeability  of  rock,  422 
Permian  age  of  Helgoland  rocks,  79 
Permian  glaciated  rock  fragment,  435* 
Permillage,  228 
Permille,  227 

Persia,  salt  mountain  of,  822 
Persian  salinas,  245 
Peruvian  coast,  guano  of,  322 
Pervious  beds,  422 
Petrifactions,  19 
Petrifactology,  19 
Petrified  bird's  nest,  219 
Petrography,  18 
Petroleum,  351;   chemical  origin  of,  353; 


Index 


formation  diagram,  352* ;  from  diatoms, 
324;  geology,  21 ;  origin,  313;  source, 
352 

Petrology,  18 

Petrosilex,-ioo 

Phacolith,  202,  203 

Phenocrysts,  88,  89 

Philadelphia,  Pa.,  722 

Phlegraean  field,  map  of,  no*;  solfataric 
action  in,  181 

Phlogopite,  63 

Phonolite,  96,  102 ;  felsite,  96 ;  obsidian, 
102  ;  porphyry,  06,  102 

Phosphate  of  lime,  in  sea,  248;  organic, 
322  ;  secreted  by  brachiopods,  309 

Phosphate  rock,  222 

Phosphorus  minerals,  61 

Phragmites,  335,  337*;  in  upland  bogs,  342 

Phyllites,  649,  650,  651 

Physa,  315,  317* 

Physical  geography,  18 

Physiography,  18,  20 

Phytology,  15 

Picrites,  105 

Pictured  Rocks,  Lake  Superior,  812 

Piedmont  glaciers,  359,  378,  382,  384 

Pikes  Peak,  64,  396,  636*;  contact  at, 
21 1 ;  disintegration  of  granite  on,  395*; 
erosion  on,  821 ;  wind-carved  monu- 
ments near,  822 

Phillips,  A.  H.,  ref.,  51 

Pillow  lava  of  Hawaii,  119 

Pillowy  lava,  195 

Pinacoidal  planes,  42,  43 

Pindus  Mts.,  on  map,  605* 

Pine  Barrens,  715* 

Pinelands,  715 

Pinus,  337* 

Pipes,  volcanic,  161,  163 

Pisolite,  218*,  574;  deposited  by  springs, 
262 

Pisolitic  texture,  218 

Pitch  Lake  of  Trinidad,  351 

Pitch  of  axis  of  anticline,  594 

Pitching  folds,  594*;  eroded,  594*,  595* 

Pitchstone,  96,  98 

Place  of  cooling,  relation  of  texture  to,  89 

Placer  deposits,  267 

Plagioclase,  61,  go,  91 

Plain,  607 ;  alluvial,  465 ;  definition  lim- 
ited, /or 

Plains,  mature,  699 ;  of  construction,  697  ; 
young,  697 


Plains  country,  sandstone  buttes  of,  822 
Plane,    707 ;   high-level,    709 ;    of   marine 

erosion,  706 
Planetary  currents,  529 
Plankton,  279,  348 
Planktonic  organisms,  352 
Planorbis,  315*,  317 
Plant  types  in  lake,  347* 
Plant  zones  in  lake,  333  * 
Plants,   carbonate  of  lime  deposited  by, 

270;     as   rock-breakers,    81 ;    of    salt 

marshes,  334* 
Plaster  of  Paris,  221 
Plateau  basalts,  149 
Plateaus,  703 

Platinum,  57;  minerals,  57 
Platte  River,  braided  character  of,  map, 

477* 

Plattenkalke,  572 
Platy  rocks,  572  ;  structure,  572 
Plauen'sche  Grund,  syenite  from,  101 
Play  fair,  John,  portrait,  26* 
Playa  deposits,   79,   480;    characters  of, 

483  ;  lakes,  244*,  480 
Pleistocene,  20;    glacial  deposits  of,  496; 

gravel  covered  by  lava,  143* 
Pliny,  on  eruption  of  Vesuvius  in  79  A.  D., 

127 

Plomb-du-Cantal,  149 
Plucking,  glacial,  433 
Plug,  lava,  of  Mt.  Pelee,  163 
Plugs,  volcanic,  163 
Plutarch,    on   early   condition   of   Monte 

Somma,  127 
Plutonic  plug,  204 
Pochutla,  112 
Point  of  Rocks,  Md.,  569 
Polarized  light,  examination  of  rocks  by, 

9i 

Polistena,    Italy,    earthquake    manifesta- 
tions in,  669 

Pollen  grains  in  lake  deposits,  334 
Polygenetic  elastics,  567 
Poly  halite,  236 
Polyps,  283 
Polystomella,  275* 
Pompeii,   126;    burial  of,   127;    ruins  of, 

127* 

Pond,  plant-filled,  section,  338* 
Pond-lilies,  335 
Ponds,  4 

Popocatepetl,  109 
Porcelanite,  210,  650,  651 


Index 


853 


Pore  spaces  of  rocks,  4 

Porites,  285,  287*,  299,  301;  on  reefs,  291 

Porphyries,  89 

Porphyritic,  andesite,  96 ;  augite  andesite, 
96 ;  augitite,  96 ;  basalt,  96 ;  dacite, 
96;  diorite,  96;  gabbro,  96;  granite, 
96;  limburgite,  96;  olivine  gabbro, 
96 ;  peridotite,  96  ;  phonolite,  96  ;  pyrox- 
enite,  96 ;  quartz  diorite,  96 ;  rhyolite, 
96;  syenite,  96;  texture,  87*,  88,  89; 
trachyte,  96 

Porphyry,  87* 

Port  Nicholson,  New  Zealand,  earthquake 
effects  at,  677 

Port  Royal,  Jamaica,  earthquake  effects, 
at,  688 

Portage-Chemung  cuesta,  760;  on  block 
diagram,  760*;  section  of,  761* 

Portage,  gorge  at,  784,  78* ;  Lower  Falls  of 
Genesee  at,  783*,  784,  785;  Middle 
Falls  of  Genesee  at,  782*;  sections  at, 
30;  Upper  Falls  of  Genesee  at,  783* 

Portageville,  N.  Y.,  782,  784 ;  moraines, 
780;  on  map,  776*,  779*;  region,  map 
of  ancient  river  valleys  in,  778* 

Potamogeton,  347 

Potash  from  kelps,  271 

Potash  salts,  223  ;  .in  desert  deposits,  244  ; 
of  Germany,  235 

Potassium  minerals,  58 

Pot-holes,  4i9*~42i*;  elevated,  419; 
glacial,  368 ;  in  boulders  of  ancient 
Niagara,  775 

Potsdam  quartzite,  578;  sandstore,  75, 
579 

Poughquag  quartzite,  627 

Pozzuoli,  in,  113,  691 

Prairie,  origin  of,  702* 

Prairie  Plains,  701 

Pre-Cambrian  of  Manhattan  Island,  75 

Precious  coral,  284 

Precipitate,  215 

Precipitation,  methods  of,  216;  of  car- 
bonate of  lime,  methods  of,  252 

Prehnite,  63 

Pressure,  induration  by,  564 ;  metamor- 
phism  by,  643 

Primary  textures,  87 

Primitia,  318* 

Primitiopsis,  318* 

Prismatic  structure  in  basalts,  obsidian, 
etc.,  179 

Prisms,  43 


Prochlorite,  63 

Progressive  offlapping,  558*,  559* 
Progressive  overlap,  555*,  556*,  557*, 
Protozoa,  275,  325  ;  as  source  of  petroleum, 

352 

Proustite,  57 
Provincetown,  820;    dune  lands  of,  447; 

headland,  543  ;  on  map,  808* 
Pseudomonas,  271* 
Pseudopodia,  275 
Psilomilane,  52 
Pteropod  ooze,  313* 
Pteropods,    311,    312*,    313*;     deep-sea 

accumulations  of,  552  ;   killed  in  estua- 
ries, 348 

Ptychoparia,  319* 
Pudding-stone,  576*,  577 
Pulpit  Rock,  Nahant,  35* 
Pumice,  95,  96,  99* 
Pungent  taste,  50 

Purgatory  chasm,  Newport,  R.  I.,  816*,  817 
Puy,  Bassin  de,  section,  152*;  de  Cliergue, 

149;    de  Dome,  146*;    de  Grioce,  149; 

de  Pailleret,  149 
Puys,  chaine  des,  145, 146, 147* ;  of  France, 

64 

Pyramid  Lake,  tufa  dams  of,  256* 
Pyramids,  42 
Pyrenees   Mts.,    marbles  of,   653 ;     num- 

mulitic  limestone  of,  280;  on  map,  605* 
Pyrite,  52 ;   in  marsh  deposits, "3 3 3  ;  oxida- 
tion of,  403 

Pyritiferous  shales,  581 
Pyritohedron,  44* 

Pyroclastic  material,  80,  85 ;  rocks,  80,  83 
Pyroclastics,  568 
Pyrogenic  material,  80;    rocks,  70,  71,  72, 

73  ;  principal  types,  84 
Pyrolusite,  52,  224 
Pyrosphere,  5,  70,  84;     as  rock-breaker, 

80 ;     contribution    to    lithosphere,    1 1 ; 

derivation    of    term,    12 ;      destructive 

work  of,  431 ;  operation  in  rock-breaking, 

391 
Pyroxene,  62,  93 ;    distinctive  form  and 

cleavage  of,  94* 

Pyroxenes,  93  ;  separation  of,  94 
Pyroxenite,    96,    105,    106 ;    gneiss,    653 ;' 

porphyry,  96 
Pyrrhotite,  52 

Quahaug,  310*,  311 
Quaking  bog,  336 


854 


Index 


Quarries  as  sources  of  geological  informa- 
tion, 29 

Quarrying,  391,  392 ;  by  rivers,  414,  415 ; 
glacial,  433 

Quartz,  39,  61,  go  ;  cause  of  imperfect 
crystallization  of  —  in  igneous  rocks, 
95 ;  conglomerate,  577 ;  dacite  series, 
103 ;  diorite,  96,  97,  107 ;  diorite 
porphyry,  96,  103  ;  diorite  series,  glasses 
of,  103  ;  felsite,  95,  96,  97 ;  felsite  por- 
phyry, 96  ;  flour  rock,  581 ;  mica  diorite, 
103 ;  rocks,  contact  metamorphism  of, 
211 ;  sandstone,  570,  578;  solution  of, 
425  ;  syenite,  101 

Quartzite,  211,  565,  578,  649,  650;  gneiss, 
653 

Queensland,  467 

Quiet  type  of  volcano,  140 

Quiriquina  Island,  earthquake  effects  on, 
674 

Radial  consequents,  722 
Radial  dikes,  192,  194*,  195* 
Radiolaria,   323,   324,   325 ;    deep-sea  ac- 
cumulations of,   552;    flint  from,   280; 

in  red  clay,  553  ;   silica  obtained  by,  426 
Radiolarian  ooze,  326 ;  map  of  distribution 

of,  277 

Radium  minerals,  55 
Rafted  material  in  sea,  530 
Rain,  as  agent  of  erosion,  820 ;  sculpturing 

by,  821 

Rainbow  Natural  Bridge,  823 
Raindrop  impressions,  490,  491* 
Raindrops,  fossil,  482 
Rain-water,   disposition  of,   411;    erosive 

work  of,  410 
Raised    beaches,    694;     in    Great    Lakes 

.region,  695 

Rakata  of  Krakatoa,  140* 
Raleigh,  N.  C.,  722 
Ran  of  Cutch,  232 
Rantum,    Church    of,    overwhelmed    by 

dunes,  444 

Rare  earth  minerals,  53 
Rascles,  413* 
Rate  of  cooling  of  igneous  rock,  influence 

on  texture,  87 
Reade,  T.  Mellard,  cited  on  solution  of 

rock,  427 
Realgar,  54 

Reconstructional  forces,  19 
Recumbent  anticline,  590* 


Red  Bank,  N.  J.,  oxidized  greensands  of, 
250 

Red  beds,  gypsum  of,  221 ;  of  loessic  origin, 
459 ;  origin  of,  492  ;  origin  of,  in  Bay  of 
Fundy,  535 

Red  clay,  553 

Red  Cliff  Peninsula,  ice  mantle  of,  385 

Red  muds,  551 

Red  Sea,  513,  515;  map  of,  517*;  salinity 
of,  228  ;  salt  pans  of,  232 

Red  shales,  581 

Red  Valley,  722,  723* 

Red  Wall  limestone,  787,  789,  790 

Reeds,  335,  337*  ;  in  upland  bogs,  342 

Reef,  290 

Reef-building  animals,  282 

Reef-section  of  Palaeozoic,  304* 

Reefs,  common  structures  of,  304 

Reelfoot  Lake,  671 

Regelation  theory  of  glacier  movement, 
389 

Reggio,  Italy,  effects  of  earthquake  in, 
670 

Regional  metamorphism,  643 

Regur  of  India,  459 

Reid,  H.  F.,  cited,  657 

Relative  humidity  of  air,  355 

Relief,  stages  of,  705  ;  impression  of  tracks, 
etc.,  origin  of,  482 

Replacement  deposits,  266 

Replacing  overlap,  560*,  561* 

Resequent  fault-line  scarp,  759;  fault 
scarps,  792 

Residual,  boulder  conglomerate,  577  ;  rocks, 
568;  soil,  consolidated,  567 

Resina,  destruction  of,  128 

Resinous  luster,  40 

Retreat  of  glacier  front,  378,  382 

Reverse  faults,  622 

Reversible  Fall,  St.  John,  524*,  525* 

Rhabdoliths,  274 

Rhabdosphere,  274* 

Rheims,  on  map,  730* 

Rheims-Epernay-Sezanne  escarpment,  731 

Rhine,  gorge  and  peneplane  of,  702,  703*; 
graben,  630,  756,  757 ;  diagrams  of 
development  of,  756 ;  region,  extinct 
volcanoes  of,  154;  rift  valley  of,  631*; 
sandbanks  of,  462  ;"•  trough,  754;  vol- 
canic district  of,  156* 

Rhizocrinus,  321* 

Rhodochrosite,  52 

Rhodonite,  52 


Index 


855 


Rhombohedron,  46* 

Rhone     Glacier,     map,     375*;    views    of 

change  in  past,  380 
Rhone  River,  364,  376 ;   irregular  deposits 

by,  488,  489* 
Rhone  Valley,  361 
Rhyolite,  95,  96,  97 ;   origin  of  name,  98  ; 

porphyry,  96,  98 
Ribbon  limestone,  572* 
Richmond,  Va.,  722 
Ridge  Road,  763 

Rift  valley,  630,  754;  of  Rhine,  map,  631* 
Rift  valleys  of  Africa,  631  ;   map  of,  632* 
Rikwa  Lake,  Africa,  on  map,  632* 
Rill-marks,  489,  553,  554* ',   fossil,  490* 
Rimutaka    Mts.,    earthquake    effects    in, 

675  ;  fault  cliff  in,  677  ;  on  map,  676* 
Ripple  marks,  553;    current,  489*;    dia- 
grams of  types  of,   550;    fossil,   490*; 

in  shallow  water  deposits,  550 ;  on  dunes, 

453*;   on  sea  shore,  535 
Ripples,  types  of,  in  desert,  454 
Rise  of  ocean  surface,  theory  of  coral  reef 

origin,  297 

River-bottom  dunes,  448 
River  capture,  734*,  735*  ;  in  coastal  plain, 

720* 
\  River  deposits.  464 ;    characters  of,  486 ; 

red  color  of  ancient,  491;    variation  of, 

section,  488* 

*  River  flood-plain  deposits,  472 

-  River  sediments,  sorting  of,  462  ;    varia- 

tion in,  sections,  478* 

River  terraces,  Columbia  River,  472*; 
West  River,  473* 

River  valleys  as  sources  of  geological  in- 
formation, 29  ;  illustrations  of  dynamical 
.^     geology  in,  36 

River  water,  erosive  work  of,  412 
^  Rivers,    4 ;     chemical    deposits    of,    259 ; 
erosion  products  of,  411 

Roba-el-Khali,  451 

'Rocca  Morifina,  section,  143* 

Rocher  Corneille,  165,  166;  St.  Michel, 
165*,  1 66 

Roches  moutonnees,  373*,  374 

Rochester,  N.  Y.,  775;  Lower  Falls  of 
Genesee  at,  780* ;  Middle  Falls  of  Gene- 
see  at,  780* ;  section  near,  30 ;  sections 
of  falls  at,  781* ;  Upper  Falls  of  Genesee 
at,  780* 

Rock,  38,  64  ;  avalanche,  398 ;  destruction, 
390;  destruction  by  rivers,  variation 


of,  415;  exposures  in  flat  countries, 
28;  flour,  of  glaciers,  499;  flour,  of 
ground  moraine,  496 ;  origin  of,  434  ; 
flowage,  646  ;  fracture,  646 ;  glaciers,  401 ; 
salt,  220;  shattered  by  plant  growth, 
81*;  step,  800*;  structures,  field  for 
study  of,  28 

Rockaway  Beach,  538 ;  map  of  change  in, 
542  ;  rill-marks  on,  554* 

Rocks,  17  ;  age  relations  of,  64  ;  classifica- 
tion of,  64,  66 ;  principal  types  of,  64 
shattering  of  heated  rock  by  cold  rail 
water,  410 

Rocky  Mts.,  583,  759 ;  Front  Range  of, 
738 ;  rock  exposures  of,  35  ;  reduction  in 
width  by  folding  of,  619 

Rodderberg,  157* 

Rogers,  A.  T.,  ref.,  51 

Rolandseck,  157 

Roman  Mediterranean,  516 

Rome  outlet,  779 

Rondout,  unconformity  near,  609*;  dia- 
grams, 610*,  6n* 

Roofing  slate,  581,  638 

Roofshale,  345  ;  fern  in,  344* 

Ropy  lava,  100,  195 

Rosenlaue  Glacier,  polished  rocks  of,  433* 

Rotalia,  7*,  275*;  in  chalk,  278* 

Rothliegendes,  236 

Rothpletz,  A.,  cited,  274 

Rouen,  in  trenched,  meanders  near,  718* 

Round  Top,  Md.,  arch  at,  591* 

Roxbury  Pudding-stone,  576* 

Royal  Gorge  of  Arkansas,  471 

Rubble  concrete,  73,  74,  82,  83,  577 ;  rock, 
569,  575 

Rubbly  material,  569 

Rudaceous  texture,  569 

Rudyte,  560,  575 

Rukwa  Lake,  Africa,  on  map,  632* 

Run-off,  356,  411 

Rush  salt  grass,  334* 

Rushes,  334* 

Russia,  great  plains  of  central,  700 ;  rock 
exposures  of,  34 

Russian  peneplane,  701 

Rutile,  53 

Sabrina  Island,  112;  described,  114* 
Saco  River,  flood-plain  of,  473* 
Safnoni,  181,  224 

Saginaw  Bay,  726;  map,  727*;  river 
(ancient),  map,  727* 


856 


Index 


Sagittaria,  334* 

Sahara  Desert,  442*;   foot-prints  in,  491 ; 

sub-sea  level  of  floor  of,  706 
Sahara  dust,  transportation  of,  443 
St.  Abb's  Head,    Scotland,    unconformity 

near,  613* 
St.  David's  gorge  (ancient),  767,  768,  769 ; 

diagram,  767* 
St.  Francis  Mts.,  723,  724 
St.  Gotthard  massif,  section  of,  606* 
St.  Helena,  N.  Y.,  on  map,  776*,  779* 
St.  John,  N.  B.,  reversible  fall  at,  524*, 

525* 
St.  Monans,  Scotland,  ripples  on  coast  of, 

537* 
St.  Patrick's  Cathedral,  N.  Y.,  building 

stone  of,  653 
Saint  Peter  sandstone,  75 
St.  Pierre,  destruction  of,  137 
Salem  upland,  723 

Salina  lowland,  761 ;  section  of,  761* 
Salina  salt  beds,  shafts  sunk  to,  31 
Saline  lagoons,  348;  water,  214 
Saline  lakes,  480 ;  deposits  of,  250 
Salinity    currents,     527 ;      defined,     228 ; 

of  water  favoring  coral  reef  growth,  291 
Salisbury  Crags,  198;    section  of,  199 
Salt,  17,  38,  40;  amount  in  sea  water,  227 ; 

formation  of,  40,  41 ;    from  sea  water, 

229;    in  ocean  water,  24;   solution  of, 

404  ;  use  of  term,  215 
Salt,    deposits,    origin    of,    1 1 ;     gardens, 

220;    geology,    21 ;     lick,    322;    marsh, 

map,  332*;    meadow,  331,  332;    mines, 

220 ;  mines,  contortion  of  strata  in,  645* ; 

mountain,     solution     forms    of,     822  ; 

pans,  229  ;  pans,  of  Black  Sea,  232  ;  peat, 

silt  in,  333  ;  Range,  India,  overthrust  in, 

629 ;  River,  Arizona,  section,  478* 
Salton  Desert,  252;   Sink,   232;    map  of, 

232* 

Saltpeter,  259 
Salts,  ionization  of,  41 ;  order  of  deposition 

of,  235 

Salty  water,  214;   taste,  50 
Salzburg,  563 
San   Bernardino,    Pass,    wind   erosion  in, 

406 ;  Valley,  change  of  water  courses  in, 

679 

Sancy  vent,  148,  149 

Sand  beaches,  533  ;  firm  character  of,  529 
Sand  blast,  822  ;  work  of,  391,  405 
Sand-craters,  680 


Sand-dollar,  320* 

Sand  dunes,  77,  44S*~453*;   types  of,  443 

Sand,  garnet  and  magnetite,  529;  grains 
of  Libyan  Desert,  452  ;  grains,  rounding 
of,  in  rivers,  415  ;  grains,  rounding  of,  on 
beach,  43 1 ;  grains,  secondarily  enlarged, 
455*,  456;  plain,  glacial,  503,  504*; 
ridges,  553 ;  spit,  540 ;  spits  on  Cape 
Cod,  820;  storms,  440;  storms,  in 
Sahara  Desert,  441 

Sand  Hills,  Neb.,  451 

Sandia  Mt.,  N.  Mex.,  749,  750* 

Sandstone,  569,  577  ;  buttes,  822  ;  dike  as 
earthquake  record,  689*,  690;  of  emer- 
gence and  of  submergence,  559 

San  Filippo,  Sicily,  tufa  deposits  at,  260 

San  Francisco  earthquake,  684;  map  of 
fault  trace,  685*;  seismogram  of,  662 

Sanidine,  101 

San  Juan  Canon,  706* 

San  Rafael  piedmont  glacier,  384 

San  Sebastian  Vizcaino,  513 

Santa  Maria  Island,  Chile,  earthquake 
effects  on,  674 

Santiago,  Chile,  damaged  by  earthquake, 
672 

Santorin,  in  ;  map,  122*;   view  of,  123* 

San  Vignone,  Baths  of,  in  Tuscany,  rate  of 
tufa  deposits  in,  260 

Sapping,  795  ;  glacial,  433 

Sapropel,  335,  351 

Sapropelite,  334,  338,  346,  347,  348,  349, 
351  ;  in  lakes  and  lagoons,  347 

Sarcoui,  Grand  Puy  of,  124*,  146 

Sargasso  Sea,  527 

Sargassum,  527 

Sassolite,  60 

Satin  spar,  220 

Saturated  solution,  215 

Saturation,  of  air,  355  ;  point,  215 

Saussure,  H.  B.  de,  26 

Saville  Kent,  cited,  290 

Saxicava  arctica,   in  raised   beaches,   694, 

695* 

Scale  of  hardness,  49 

Scalenohedron,  46* 

Scallop,  310*,  311 

Scaly  texture,  218 

Scandinavia,  eskers  of,  507 ;  gneisses  in, 
653;  ice-caps  of,  384,  493 ;  metamor- 
phic  rocks  of,  648 ;  raised  beaches  in, 
694 ;  rock  exposures  of,  33  ',  upland  bogs 
of,  342 


Index 


857 


Schalkenmehren  Maar,  160* 
Scheelite,  55 

Schist,  649,  650 ;  hornblende,  650 ;  mica,  650 
Schistosity,  647 
Schwagerina,  282* 
Schwarzwald,  see  Black  Forest,  756 
Sceria,  107 

Scirpus,  334*  ;  in  upland  bogs,  342 
Scotland,    block    conglomerate    of,    S3i ', 
raised  beaches  of,  694;   rock  exposures 
of  Highlands  of,  33 

Scottish  coast,  erosion  features  on,  816 ; 

sea   stacks   on,    814;     sections   of,    35  J 

glens,    deposits    in,    71;     lochs,    803; 

rivers,  sorting  of  sediment  by,  464 

Scottish    Highlands,     64,     743 ;    gneisses 

of,  653 ;  thrust  in,  629* 
Sculpturing  processes  of  wind,  822 
Scylla  rock,  525  ;  effects  of  earthquake  on, 

670 

Sea,  area  of,  4  ;  subdivisions  of,  509 
Sea-anemones  on  shore  boulders,  532 
Sea-caves,  California  coast,  811*;   forma- 
tion of,    811;    on   British   coast,    812; 
on  French  coast,  812 
Sea-cliffs,  35 

Sea  coast,  rock  exposures  on,  35 
Sea  fans,  284 
Sea  salinas,  220 
Sea    salts,     composition    of,     228,     229; 

conditions  favoring  deposition  of,  231 
Sea  scarps,  abandoned,  810 
Sea     shore,     illustrations     of     dynamical 

geology  on,  36 
Sea  stacks,  530*,  814  ;   ancient,  on  Macki- 

nac  Island,  815 ;   on  Gotland,  815 
Sea  urchins,  319,  320* 
Sea  water,  amount  of  salt  in,  227  ;  experi- 
ments in  evaporation  of,  230;    salinity 
of,  228 

Seaweeds,  buoyancy  given  by,  522  ;  illus- 
tration of  buoyant  power  of,  531 ;  on 
deltas,  348 

Seasonal  temperature  changes,  820 
Searle's  Lake,  Cal.,  259;   trona  in,  223 
Searle's  Marsh,  see  Searle's  Lake 
Secondary    enlargement    of    sand    grains, 

565* 

Secondary  minerals  of  igneous  rocks,  94 
Secondary  structures,  582 
Secondary  textures,  88 
Sectile,  49 
Sedan,  on  map,  730* 


Sedges,  336*;  in  swamps,  336 
Sedimentary  contact,  211;  significance  of, 

213 

Sedimentary  rocks,  47 
Seine,  intrenched  meanders  of,  718* 
Seismic  center,  657 
Seismic  disturbances,  types  of,  656 
Seismogram,  659,  660*,  661*,  662* 
Seismograph,  659*,  660*,  661* 
Seismology,  15 
Selkirk  Mts.,  797 
Selma,  715 

Seminara,    Italy,    earthquake    manifesta- 
tions in,  669 

Semper,  C.,  on  reef  origin,  296 
Sentis,  erosion  forms  in,  413 
Senya  fault  cleft,  Japan,  682* 
Separation  of  mineral  matter  from  water, 

215 

Sepia,  314* 
Septaria,  222*,  574* 
Sericite,  403  ;  in  bluestone,  579 
Serpentine,  63,  106,  649,  650,  654 
Serpula,  316*,  317;  at  Pozzuoli,  692 
Seven  Mts.,  154 

Severn  estuary,  549  ;  map  of,  547* 
Sevier  fault,  map  of,  786*;    diagram  of, 

787* 

Sevier  Lake,  Utah,  258 
Seward  Glacier,  358,  382*,  383 
Shafts  (salt,  etc.),  as  sources  of  geological 

information,  29 
Shale,  565,  570*,  571,  580 
Shales,  210;  bituminous  and  oil,  353 
Shallow    water    deposits,    characters    of, 

550 ;  foraminiferal  deposits,  276 
Shaly  structure,  571 
Shantung,  467 

Shawangunk  conglomerate,  569 
Sheba  Temple,  790 
Sheet  lava,  174 
Shell  conglomerate,  577;    layers  on  coast 

of  Florida,  534 
Shimer,  H.  W.,  cited,  549 
Shinumo  basin,  789 
Ship  barnacle,  317* 
Shivwits  Plateau,   map,    786*;    diagram, 

787* 
Shonai  River,  Japan,   earthquake  fissure 

on,  680 

Shore  zone,  518;  deposits  in,  530 
Shortening  by  folding,   in  Appalachians, 

618 ;  in  Swiss  Alps,  619 


858 


Index 


Shove  in  faulting,  623 ;   on  diagram,  620* 
Siberia,  great  plains  of  western,  698 
Siberian  islands,  coral  reefs  of,  290 
Siccar  Point,   Scotland,  unconformity  at, 

612*,  613* 

Sicily,  volcanic  field  of,  1 1 1 
Siderite,  52,  225 

Siebengebirge,  map  of,  155*;    view,  154* 
Sierra  Nevada  Mts.,  Cal.,  467,  677,  749, 

750 

Sierra  Nevada  Mts.,  Spain,  on  map,  605* 
Sierra  Teras,  uplift  of,  679 
Silesian  coal  fields,    strata  of,  overridden 

by  Carpathians,  606 
Silica,    224;     organic,    323;     organically 

secreted,  71 ;  minerals,  61 
Silicate  minerals,  61-63 ;  from  magmas,  86 
Silicilutyte,  581 
Silicious,  bioliths,  270;  shales,  581 ;  sinter, 

224;  sponge,  325* 
Silky  luster,  49 
Sill  at  Nahant,  35 
Sillimanite,  62 

Sills,  89,  194 ;  origin  of  term,  197 
Silurian,   division,   75 ;    reefs,  of  Gotland, 

306  ;  of  Wisconsin,  305  ;   salt  beds,  246 
Silver,  56;  minerals,  56,  57 
Silver  Cliff,  Col.,  pitchstone  of,  99 
Silver  Peak  Marsh,  Nev.,  244* 
Simple  corals,  285 
Sinter  terraces,  formation  of,  261* 
Simplon  massif,  608* 
Sinai  fault-block,  754;  map,  755* 
Sink- holes,  425 
Sinks,  extinct,  157 
Skerries,  816 
Slates,  581,  637,  649,  6 so 
Slaty  cleavage,  636,  637,  638*,  647 
Slaty  structure,  571 
Slickensides,  620,  634 
Slime,  organic,  alteration  products  of,  349 
Slip  (in  fault),  623 
Slumgullion  rock  flow,  400* 
Smaltite,  52 

Smith,  William,  26,  35;  portrait,  27* 
Smithsonite,  53 
Smooth  feel,  50 
Snake  River  lava  plateau,  64 ;    canon  in, 

175*;  map  of,  174*,  175* 
Snow,    70,    356;     compacting    of,    357; 

drifts,  356 ;    evaporation  of,   404 ;    ice, 

357  ;  line,  356 
Soapstone,  654 


Soapy  feel,  50 

Society  Islands,  tsunamis  at,  674 

Soda  Lake,  Cal.,  258 

Soda  Lake,  Nev.,  259 ;  trona  in,  223 

Sodalite,  102 

Soda  niter,  58,  223,  259 

Sodium  carbonate  lakes,  259 

Sodium  minerals,  58 

Sodium  sulphate  lakes,  258 

Solfatara,    in,    181*;    eruption  of,   127; 

material  due  to  eruption  of,  693 
Solfataric,  action,  181 ;    vents,  181 
Solid  red  coral,  283 
Solnhofen,  Bavaria,  572,  581 ;    limestone, 

solution  of,  427  ;  quarry,  572* 
Solomon's  Temple,  790 
Solution,    by    underground    water,    424; 

of    rocks,    77 ;     annual    amounts,    427 ; 

of  salt,   cavings  due  to,  427 ; .  surface 

forms  due  to,  822 
Sonora  earthquake,  678 
Sonora  province,  Mex.,  earthquake  effects 

in,  678 

Sorkul  Lake,  481* 
Sorting,    agents   of,    438 ;     conditions   of 

sorting  by  waves,  529 
South  Africa,  diamond  mines  of,  173* 
Spalling  of  granite,  393  * 
Spanish  Peaks,  radial  dikes  of,  192,  195* 
Spartacus,    gladiators    of,    encamped    on 

floor  of  Monte  Somma,  127 
Spartina,  331,  334* 
Spatter  cones,  volcanic,  121* 
Specific  gravity,  50;    average  of  rocks  of 

earth's  crust,  6  ;  of  earth,  6 
Spencer  Glacier,  378* 
Spey  River,   destruction  of  feldspar    in, 

4i5 

Sphagnum,  336,  341,  342 
Sphalerite,  53 
Spheres  of  the  earth,  relationships  of,  5*, 

6*;  summary  of,  n 
Spherosiderite,  226 
Spherulites,  99 
Spherulitic  obsidian,  99* 
Sphinx,  76*,  452  ;  rock  of,  75 
Spicules  of  lime,  283 
Spike  Geyser,  187* 
Spillway,  760 
Spine,  volcanic,  163 
Spinel,  5Q 
Spirifer,  309* 
Spiroloculina,  278* 


Index 


859 


Spits,  produced  by  long-shore  currents  and 

waves,  819 
Splintery  fracture,  49 
Spodumene,  58 ;  in  pegmatite,  207 
Sponge,    Cretaceous,  326*;     spicules,  323, 

326,  327*;   flint  from,  280* 
Sponges,  in  Severn  estuary,  549 
Springfield  Plain,  724 
Spring  tides,  524 
Springs,  4;   artesian,  424;   as  illustrations 

of    dynamical    geology,    36;     deposits 

by,  260 ;  hill  side,  423 
Squid,  313,  314* 
Staffa,  n,  176,  178*,  179,  180;  erosion  at, 

428;   Fingal's  Cave  on,  813* 
Staggering  course  of  streams,  721 
Stagnant  seas,  348 

Stalactites,  77,  263,  425;   of  lava,  121 
Stalagmites,  264*,  425;   of  lava,  121 
Star  coral,  283,  284 
Starfish,  319,  320* 
Stassfurt  deposits,  235 
Static  metamorphism,  643 
Stationary  level  theory  of  reefs,  296 
Statuario  (marble),  654 
Statuary  marble,  654 
Staurolite,  62 
Steam  holes,  195 
Stegocephalians,  tracks  of,  482 
Stefanie  Lake,  Africa,  on  map,  632* 
Step  (topographic),  740*,  741 ;   fault,  620 
Stephanite,  57 
Sternberg,  Count  Caspar  von,  on  Kammer- 

biihl,  169 
Stibnite,  54 
Stillbite,  63 
Stocks,  193 
Stomatodaeum,  283 
Stonewort,  273,  334 
Stone  reefs,  538 
Stools,  in  swamps,  337 
Stoss-strahl,  658*,  661 
Strassburg,  630 
Strata,  78 

Stratification,  79,  486,  487* 
Stratified,    character   of   beds   of   alluvial 

plain,  467;    deposits  in  shallow  water, 

550 ;     drift,    502 ;     rocks,    78 ;     erosion 

features  on,  811 
Stratigraphy,  IQ 
Stratum,    486;     relation    to    surface,    of 

beveled,  700 
Streak,  50 


Streams,  rejuvenated,  717 

Strike,  585*,  586* ;  deflection  of,  587 ;  dia- 
gram of,  587*,  588*;  faults,  623,  624* 

Stripped  belt,  713 

Strokr  Geyser,  186*;  extinction  of,  187, 
683  ;  origin  of,  187 

Stromatopora,  289*;  of  Devonian  reefs, 
306 ;  of  Wisconsin  reefs,  305 

Stromboli,  in* 

Strongylocentrotus,  320* 

Strontianite,  59 

Strontium  minerals,  59 

Structural  geology,  18 ;  field  for  study  of, 
28;  in  broader  sense,  17;  in  narrower 
sense,  18 

Structures,  secondary  and  primary,  com- 
pared, 582  ;  types  of  deformation,  582 

Styliola,  313* 

Styliolina  fissuretta,  7* 

Stylolite,  644*,  645* 

Subcrustal  source  of  clastic  material,  528 

Subglacial,  detritus,  370;  drift,  492; 
streams,  370 

Sub-marginal  moraines,  493 

Submarine  banks,  388 

Submergence,  sandstone  of,  559 

Submergence  of  islands,  variation  of,  296* 

Sub-metallic,  40 

Subsequent  streams,  713;  overdeepened, 
721* 

Subsidence  theory  of  coral  reefs,  294 

Subsidences  due  to  earthquakes,  690 

Subsurface  waters,  solution  by,  425 

Sub-translucent,  50 

Sub-transparent,  50 

Sub-vitreous,  49 

Suez,  Gulf  of,  754;  map  of,  517*,  755* 

Suez  Canal,  241 ;  map  of,  242* 

Sugar  Loaf,  Portage,  784,  785* 

Sulphur,  60  ;  minerals,  60;  odor,  50 

Sulphur  Springs  Valley,  artesian  condi- 
tions in,  479 

Sulphureted  hydrogen  in  marshes,  333 

Sulphurous  vapors  from  magmas,  85 

Sumatra,  tropical  swamp  of,  341 

Sunda  straits,  map  of,  137* 

Sunsets,  brilliant,  due  to  dust  of  Krakatoa, 
441 

Supai  formation,  790 

Superglacial  drift,  492 

Superimposed  streams,  744 

Super-saline  water,  214 

Susquehanna  River,  726,  776 


86o 


Index 


Swabian  Alp,  755,  756*;    wind  work  on, 

405 

Swallow-holes,  425 
Swamp  plants,  334*,  337* 
Swamps,  4,  333  ;   fresh  water,  3  29 ;   origin 

of,  423 
Swanage,   Eng.,    sea-stacks  on   coast   of, 

815 

Swash,  521,  523 
Sweden,  coast  of,  806,  810 ;  fault  structure 

of,   757,   758*;    trilobite  limestone  of, 

318,  319* 
Swells,  519,  520 
Switzerland,  606 
Syene,  syenite  from,  101 
Syenite,   96,    100 ;    origin  of  name,    101  ; 

porphyry,   96,    101 ;    series,   glasses  of, 

101 ;    trachyte  series,    100 ;   gneiss,   653 
Sylt,  Island  of,  coast  destruction  at,  444 
Sylvania  sandstone,  75,  440*,  453,  569,  578, 

580 ;  cross-bedding  of,  455* 
Sylvanite,  57 
Sylvite,  58,  223 

Symmetrical  anticline,  590*,  591* 
Synclinal,   mountain,   599*;    ridges,   598; 

valley,  732* 
Synclines,    596*;     between    domes,    603; 

erosion  cycle  on,  732 ;    limb  or  arm  of, 

596  ;   thickening  of  axis  of,  597* 
Synclinorium,  598* 
Syr-darya,  map  of,  449* 
Syringopora  of  old  reefs,  306 

Table  mountains,  origin  of,  714 

Table  Rock  in  Genesee  River  at  Portage, 

784* 
Table    Rock,    Niagara,    on    map,    773*; 

rock  falls  of,  418 
Tachylites,  96,  107 
Taconic  mountains,  75 
Tagus  River,  664 
Talc,  63;  schist,  652 
Talefre,  Cascade  du,  369 
Talus,     breccia,    577;     formation,    821; 

slope,  398 

Tamarack  in  swamps,  336 
Tampa,  Fla.,  reefs,  300 
Tanganyika  Lake,  516,  631,  633*,  757 
Tarapaca  Desert,  259 
Tarn,  796,  800*,  802,  803 
Tarnish,  50 
Tasman  Glacier,  373 
Tasmanian  Sea,  518 


Taste,  50 

Tchernozom,  see  Tschernosem 

Tectonic  metamorphism,  643 

Tectosphere,  6,  84 

Teeth,  deep-sea  accumulations  of,  552 

Temperature    change,  effect   of,    in    rock 

destruction,  392 

Temperature  of  center  of  earth,  6 
Tenacity,  49 

Teneriffe,  Peak  of,   143*;    phonolite  ob- 
sidian from,  102 
Tennessee,  central  basin  of,  729*;  River, 

729 

Tepetate,  260 
Terebratulina,  309* 
Terminal  moraines,  370,  382,  493,  494*, 

499* 

Termites,  work  of,  437 
Terra  Nuova,  Italy,  earthquake  manifesta- 
tions in,  667 

Terrapin  Rocks,  Niagara,  on  map,  773* 
Terrestrial  deposits,  227, 439 ;  of  Foraminif- 

era,  276 

Terrigenous  deposits  in  deep  sea,  map,  277* 
Terrigenous  material,  527 
Tertiary,  brown-coal,  343,  345;    lignites, 

343;   oils,  352;   rocks  formed  by  Char  a, 

273 
Tetragonal,  prism,   45*;    pyramid,    45*; 

system,  43 ;    principal  forms,  45 ;    tris- 

octahedron,  44* 
Tetrahedrite,  56 
Tetrahedron,  44* 
Tetrahexahedron,  44* 
Teufelsmauer,  191* 
Texas,  section  in,  32 
Textularia,  7*;  in  chalk,  278*,  279 
Textures,  of  aqueous  rocks,  217 ;  of  elastics, 

566,  568 ;  of  igneous  rocks,  87 ;  relation 

to  magma,  88 
Theories  of  reef  origin,  293 
Theralite,  105 

Thermal  metamorphism,  642,  643 
Thinolitic  tufa,  255 
Thousand  Islands,  762 
Three  Sister  Peaks,  Oregon,  367* 
Throw,  622 
Thrust   fault,    625 ;    magnitude   of,    625 ; 

on  diagram,   620*;    passing  into  fold, 

619* 
Thrusting,    duplicating   and   inversion   of 

strata  by,  625 
Tibet,  borax  of,  224;  deserts  of,  706 


Index 


861 


Tidal,    bore,    534*;    currents,   523,    529; 

lagoon,  545* 
Tierra  del  Fuego,  109 
Tillite,  508,  577 

Tilly  Whim  caves,  British  coast,  812*,  813* 
Tind,  804 
Tinkal,  223 
Tin  minerals,  53 
Titanite,  63 
Titanium  minerals,  53 
Toledo,   O.,   glass-making  from   Sylvania 

Sand  at,  75 

Tombigbee  River,  715 
Tombolo,  810*,  819*;  Nahant,  544 
Tonto  beds,  788 
Topaz,  62 

Topographic  features  due  to  fault,  630 
Topsets,  484 
Torbanite,  350 
Toroweep    fault,    map,    786*;     diagram, 

787* 
Torrential  currents,  cross-bedding  due  to, 

487* 

Torridon  breccia,  569 

Torsion  joints,  640*,  641* 

Tough,  4Q 

Tourmaline,  62 ;  in  pegmatite,  207 

Trachyte,  89,  96,  100,  101 ;  of  Sarcoui, 
124;  origin  of  name,  101 

Trachyte-phonolite,  102*;  -porphyry,  96, 
101 

Trachytic  texture,  102* 

Transportation,  agents  of,  438;  and  sort- 
ing of  clastic  material  in  the  sea,  528; 
by  streams,  460;  of  sand  and  dust,  dis- 
tances of,  441 

Transporting  power  of  streams,  variation 
of,  460,  461 

Trans-Caspian  deserts,  wind  erosion  in, 
406 

Translation,  waves  of,  521 

Translucency,  50 

Translucent,  50 

Transparent,  50 

Transylvania,  603 

Transylvanian  Alps,  on  map,  605* 

Trapezohedron,  44* 

Trap  rock,  73,  96,  107 

Traverse  Bay,  sand  spit  in,  541* 

Travertine,  rate  of  deposition  of,  260 

Trend  of  fault,  620 

Trenton,  N.  J.,  722 

Trenton-Utica  oils,  352 


Trent  River  outlet  on  map,  769* 

Triassic,  coal,  345 ;  reefs  of  Dolomites,  306 

Triclinic,  hemi-prism,  48*;  pyramid,  48; 
system,  48;  system,  fundamental  forms 
of,  48* 

Trigonal,  prism,  46*;  trisoctahedron,  44* 

Trilobites,  318,  319* 

Trinidad  asphalt  lake,  351 

Tripoli,  Africa,  323 

Tripolite,  323 

Trisoctahedron,  44 

Tristetrahedron,  44* 

Trist  Glacier,  364 

Trona,  223 

True  glaciers,  358 

Trugberg,  368 

Tschard-schui,  thickness  of  dune  deposits 
at,  449 

Tschernosem,  459,  549;  map  of  distribu- 
tion, 460* 

Tsunamis,  519,  684;  defined,  656;  due  to 
earthquakes,  690;  during  eruption  of 
Krakatoa,  138 

Tubipora,  300 

Tufa  dams  and  terraces,  256*,  261 

Tufaceous  texture,  218 

Tundras,  arctic,  342 

Tungsten  mineral,  5$ 

Tuolumne  River,  pot-holes  on,  419*    < 

Turquois,  5# 

Turtle  stone,  222*,  574* 

Tuscany,  borax  of,  181,  224 

Twig-rush,  335* 

Twinning  of  plagioclase,  91 

Tyndall  Glacier,  383 

Types  of  peat,  329 

Tyrol,  606 ;  marbles  in,  654 ;   reefs  in,  306 

Uinkaret  Plateau,  map,   786*;    diagram, 

787* 

Uinta  Mountains,  472 
Uintaite,  351 
Ulexite,  60,  223 
Unaltered  rocks,  69 
Unconformities,  611,  612*,  613*,  614* 
Under-clay,  345 
Underground    water,    deposits    by,    260; 

destructive  work  of,  424 ;  rate  of  flow  of, 

423 ;  types  of,  422 
Undermining  by  waterfalls,  417 
Undertow,  522,  523,  528 
Uneven  fracture,  40 
Unglaciated  summit,  800*;    valley,  800* 


862 


Index 


Uniclinal  ridges,  diagram,  593*,  722,  733, 

738 

Unicline,  594,  595*,  598,  732*,  734 
Unio,  315* 
United  States,  mantle  rock  of  the  northern, 

65 

Unkar  group,  789 
Unstratified  drift,  502 
Upham,  W.,  on  reef  origin,  297 
Upland  bogs,  plants  of,  342 
Upper  Aletsch  Glacier,  364 
Ural  Mountains,  237 ;  rock  exposures  in,  34 
Uraninite,  55 
Uranium  minerals,  55 
Uruguay,  547 
U-shaped  trough,  803* 
U-shaped  valley,  797,  799,  800* 
Usiglio,  J.,  experiments  by,  230 
Utah  Basin,  753 
Utah  erosion  monument,  409* 

Valdivia,  Chile,  destroyed  by  earthquake, 
675 

Vale,  of  Eden,  197;  of  Kashmir,  471 

Val-del-Bove,  134,  189;  map  of,  133* 

Valle-del-Bue,  see  Val-del-Bove 

Valley  profiles,  river  and  glacier,  802* 

Valleys,  circling,  722  ;  relative  size  of,  2 

Valparaiso,  Chile,  damaged  by  earthquake, 
672  ;  destruction  by  earthquake,  675 

Vanadinite,  55 

Vanadium  minerals,  55 

Vascular  plants,  333,  335;  deposits  from, 
328 

Vatna  Jokull,  384;  map,  385* 

Vaughan,  T.  W.,  cited,  290 

Vegetal  deposits,  types  of,  329 

Vein  deposits,  86 

Vein  minerals,  sources  of,  268 

Veins,  265;  contemporaneous,  206;  rela- 
tive age  of,  266 

Velocity  of  waves,  520 

Venezuela,  soda  lakes  of,  259 

Vent,  volcanic,  116,  163 

Ventriculites,  326* 

Venus,  310* 

Verdun,  731 ;  -on  map,  730* 

Vermilion  Cliffs,  791 

Versailles,  on  map,  730* 

Vertical  movements  in  earthquakes,  689 

Vesuvianite,  62 

Vesuvius,  87,  112;  as  type,  141;  cinder 
cone  of,  142*;  described,  125;  1872 


eruption  of,  128;  furrowed  cone,  131*; 
map  of,  125*;  map  of  vicinity  of,  no*; 
section  of,  143*;  view  in  crater,  129*, 
130*;  view  of,  127* 

Vicksburg  limestone,  281 

Victoria  Land,  ice  barrier  of,  387 

Vienna,  rocks  of,  64 

Viesch  Glacier,  363,  364;   section  of,  362* 

Virginia,  coast  dunes  of,  447 

Vistula  River,  black  mud  of  the,  549; 
organic  matter  of  mud  in,  348 

Vitreous  luster,  49 

Vivipara,  315 

Vlightberg,  unconformity  on,  609*;  dia- 
grams, 610*,  611* 

Volcanic,  agglomerate,  80,  431,  577*; 
area,  central  France,  map,  153*;  ashes 
and  cinders,  117;  bombs,  116*,  117, 
431;  breccia,  427,  577;  conduit,  167; 
elastics,  denned,  568;  centers  on  frac- 
ture lines,  153;  district  of  Rhine,  map 
of,  156*;  dome,  124;  dust,  431;  dust, 
transportation  of,  443  ;  eruption,  84 ;  fun- 
nels, pipes,  spines  and  necks,  161 ;  glass 
in  red  clay,  553  ;  lapilli,  431 ;  material  of 
old  vents,  171;  neck,  166*,  167*; 
phenomena,  modern,  109;  pipe,  167; 
plug,  164, 167,  204 ;  plug  of  Fife,  170, 171 ; 
sands  and  muds,  551,  552;  sandstone 
described,  580;  tuff,  80,  580;  vents, 
materials  ejected  from,  117 

Volcano,  section  of  ancient,  in  Freiburg 
region,  172* 

Volcano  in  eruption,  86* 

Volcanoes,  active,  dormant  and  extinct, 
112;  age  of,  143;  alignment  of,  171; 
characteristic  form  and  activity  of 
modern,  116;  classification  of,  140; 
comparison  of  form  of,  141 ;  craterless, 
122;  distribution  of,  109,  map  of,  108; 
extinct,  of  Rhine  region,  154*,  155*; 
formation  of  new,  in  historic  period,  112 ; 
illustrations  of  dynamic  geology  by,  36; 
old  necks  and  plugs  of,  164;  sections 
of  various  types  of,  143*;  structural 
characters  of,  144;  types  of,  140-141 

Volcanology,  16,  144 

Volga  River,  231 

Volume  of  river  transported  material,  462 

Vosges  Mountains,  630,  757 ;  on  diagram, 
756* 

V-shaped  valley,  797,  800* 

Vulcano,  in 


Index 


863 


Wad,  224 

Wadis,  411 

Wales,  oldland  of,  719*;  rock  exposures  of, 

33 

Walther,  Johannes,  portrait,  245* 

Wanakah  shales,  31* 

Wannehorn,  366 

Wappinger  limestone,  627 

Warsaw,  N.  Y.,  777;  on  map,  776* 

Wasatch  Mts.,  749,  750,  753 ;  recent  fault 
in,  75i* 

Washington,  D.  C.,  722 

Watchung  Mts.,  180;  columnar  structure 
of  basalt  of,  177 

Water,  60;  amount  of,  in  air,  355 ;  mechan- 
ical work  of,  77  ;  types  of,  214 

Waterfalls,  characteristic  of  young  streams, 
708;  in  coastal  plain  drainage  system, 
721 

Water-gap,  723,  734,  741 

Water  laid  elastics,  79 ;  denned,  568 

Waterlime,  581* 

Water  plantain,  335 

Water-table,  422 

Water  vapor,  from  magma,  85 ;  sources  of, 

354 

Watkins  Glen  gorge,  420*,  421* 

Wave-built  beaches,  raised,  694 

Wave  currents,  522 

Wave-cut,  cliff,  807 ;   terraces,  raised,  694 

Wave-marks,  553 

Wave  normal,  658* 

Wave  quarrying,  428 

Waverly  Oaks,  esker  of,  507  * 

Waves,  519;  diagram,  520*,  521*,  522*; 
bars  and  spits  formed  by,  819 ;  destruc- 
tive work  of,  427 ;  force  of,  428 

Wave-worn  bricks,  430* 

Wayland,  N.  Y.,  on  map,  776* 

Way  land  Valley  on  block  diagram,  777* 

Wealden  district,  595 

Weathering,  390,  392 

Wellington,  New  Zealand,  earthquake 
effects  near,  675;  on  map,  676* 

Wells,  4 ;  oil,  gas,  etc.,  as  sources  of  geo- 
logical information,  29 

Werner,  A.  G.,  25,  33,  36;  on  basalt,  168; 
portrait,  25* 

Wernerite,  62 

West  Alps,  cross  section  of,  607* 

Westchester  dolomite,  653 

West  Indies,  fora'miniferal  limestones  of, 
281 


West  Kaibab  fault,  map,  786* ;  diagram, 

787* 

West  River,  terraces  of,  473* 
West  Spanish  Peaks,  189*,  190* 
Wet  Champagne  lowland,  731 
Whin  Sill,  197 ;  section  of,  199* 
Whirlpool,  766*,  767,  768,  770,  774;   dia- 
gram of  development  of,  768*;  on  map, 

763* 
Whirlpool  Rapids,  765*,  766,  768;  on  map, 

763* 

Whirlpool  sandstone,  767 
Whitby,  jet-bearing  cliffs  of,  350 
White    Cliffs,     Colorado    Plateau,     791; 

eolian  rock  of,  77 
White  Mts.,  741;    erosion  in,  821;    rock 

exposures  in,  34 
Whitsunday  Island,  293* 
Wick,  Scotland,  wave  destruction  at,  428 
Willemite,  53 

Williams  Canon,  contacts  in,  212* 
Willis,  Bailey,  machine  for  producing  folds, 

617*;  folds  produced  by,  618* 
Wind,  as  agent  of  erosion,  820 ;  destructive 

work  of,  404;  gaps,  734,  736;  grooves 

in    Libyan    Desert,    405 ;     sculpturing, 

409*;     sculpturing    processes    of,    822; 

transportion  by,  439 
Windom  shales,  31* 

Winthrop  Great  Head,  Mass.,  817*,  818 
Wisconsin,  drumlins  of,  498 ;  old  coral  reefs 

of,  305 ;   Potsdam  sandstone  of,  75 
Witherite,  59 
Woevre  lowland,  731 
Wolframite,  55 
Wollastonite,  62,  209 
Wood  in  old  lavas,  169 
Woods  Hole,  531;   erosion  at,  818;   tides 

at,  525 
Worms,   316*,   317;    as   bottom   feeders, 

347  ;    work  of,  437 
Wrack,  of  sea,  522 
Wulfenite,  55 

Wyoming,  waste  basin  in,  471 
Wyville-Thompson  ridge,  511* 

Yakutat   Bay,    Alaska,  .  382 ;   earthquake 

of,  684;  map  of,  381 
Yangtse-Kiang,    467;     red    muds    from, 

55i 

Yardangs,  405,  406* 
Yellow  River,  China,  459,  467 
Yellow  Sea,  509 ;  origin  of  color  of,  459 


864 


Index 


Yellowstone  National  Park,  203 ;  algae  of 
hot  springs  of,  274;  geysers  of,  184 ;  lava 
of,  196;  lithophysae  in  lithoidite  of,  100; 
obsidian  of,  99 ;  quartz-diorite  from,  103 

York,  Eng.,  719 

Yorkshire  coast,  erosion  of,  809 

Yosemite  Valley,  797  *,  799,  802 

Young  land,  erosion  on  coast  of,  807 

Youth  of  land,  705 


Zacanthoides,  319* 

Zechstein,  236 ;  Sea,  map  of,  238* 

Zeolites,  63 

Zinc  minerals,  53 

Zircon,  53 

Zirconium  minerals,  53 

Zittel,  C.  von,  cited,  23 

Zones,  bathymetric  diagram,  519* 

Zoology,  15 


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